Adatta Dolomite Stability Sinkhole Investigation.pdf

MERAFONG CITY LOCAL MUNICIPALITY: ADATTA PIPELINE, CARLETONVILLE: DOLOMITE STABILITY SINKHOLE INVESTIGATION VGI3737 S21

Compiled by: VGIconsult Projects P.O. BOX 604 FOURWAYS 2055 TEL : (011) 469 0854 FAX : (011) 469 0961 FAX : 0866892847 e-mail: [email protected]

Client: Merafong City Local Municipality No 3 Halite Street Carletonville 2500

Consulting Engineering Geologists & Engineers

VGIconsult VGIconsult Projects (Pty.) Ltd. Registration number 2003/015042/07

MERAFONG CITY LOCAL MUNICIPALITY No 3 Halite Street CARLETONVILLE 2500

Telephone Direct : (011) 469 0854 Fax : (011) 469 0961 Fax : 0866892847 E-mail [email protected]

ATTENTION: L. MAJA

Your reference

P.O. Box 604 Fourways 2055 Gauteng

Our reference

Date

VGI3737 S21

26 SEPTEMBER 2016

MERAFONG CITY LOCAL MUNICIPALITY: ADATTA PIPELINE: DOLOMITE STABILITY SINKHOLE INVESTIGATION SUMMARY This report presents the results of dolomite stability investigations and recommendations on improvements to the subsurface and the affected wet services infrastructure on the Adatta Pipeline where the water supply line from the Ada Reservoirs to Welverdiend and Khutsong South Extensions collapsed into two sinkholes, within the Merafong City Local Municipality. A 20m diameter size sinkhole extending to a depth of approximately 4m and a second sinkhole (6m in diameter extending to a depth of approximately 6m) occurred within a radius of 30m on the 90 degree bend of a 1m diameter water supply steel pipe, located above ground and approximately 170m west of the Ada Reservoirs, on 31 July 2016. It is presumed that subsurface erosion of highly erodible dolomite residuum (wad) material was initially triggered by a leak on the water supply line over an extended period of time. As a result of the initial leak, the water supply lines settled and failure of the wet services occurred, leading to the formation of two sinkholes. The investigation area covers approximately 4 ha (200m by 200m). The regional topography of the area falls from 1475m AMSL in the east to 1465m AMSL in the west. Locally surface water runoff takes place as sheetwash down the gradient in a westerly direction. Existing subsurface services located in the affected area including two damaged 300mm diameter north to south aligned HDPE water pipes and an unknown 100mm diameter size pipe. The site is underlain by dolomite and chert of the Eccles Formation, Malmani Subgroup of the Chuniespoort Group of the Transvaal Supergroup. The dolomite bedrock over most of the site is blanketed by weathered soil derivatives. Dolomite and chert outcrop is however observed towards the west, north and north-east of the sinkholes. The investigated area can be divided into two dolomite Hazard Zones: Dolomite Hazard Zone 1: The geotechnical data gathered during this investigation permits the dolomite hazard of the area of the sinkholes and the directly surrounding area as largely reflecting a high susceptibility of large to very large size sinkhole and subsidence formation with respect to ingress of water and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. Composite Inherent Hazard Class 7/8//1. In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged.

Directors: D.B. Buttrick Ph. D. (Eng. Geol.), MSAIEG, Pr. Sci. Nat. ; A.A. Gerber B. Eng., Pr. Eng. ASB: N.Y.G. Trollip M. Sc. (Eng. &Env.), Pr. Sci. Nat., AMSAIEG ; I. Kleinhans M. Sc. (Eng. &Env.), Pr. Sci. Nat.

2 Dolomite Hazard Zone 2: The area is characterised as largely reflecting a medium susceptibility of medium to large-size sinkhole and subsidence formation with respect to ingress water, and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. Composite Inherent Hazard Class 3/4//1. In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged. This report documents recommendations and procedures on the improvement of subsurface conditions, improvements and stabilisation, replacement of wet services, improvement of surface water run-off, minimum standards of precautionary measures and monitoring actions. The following option dealing with the affected area is outlined in this report, namely: 

The use of the Inverted Filter Method and a Compaction Grouting (i.e. backfilling) programme to rehabilitate the affected area.

MERAFONG CITY LOCAL MUNICIPALITY: ADATTA DOLOMITE STABILITY SINKHOLE INVESTIGATION SUMMARY TABLE OF CONTENTS 1. 2. 3.

4. 5.

6.

7.

8.

9.

10.

PIPELINE,

CARLETONVILLE:

Preface PAGE NUMBER

INTRODUCTION ............................................................................................................. 1 TERMS OF REFERENCE AND SCOPE OF WORK ....................................................... 1 AVAILABLE INFORMATION .......................................................................................... 1 3.1. Topographic Data................................................................................................... 1 3.2. Geological Information ........................................................................................... 1 3.3. Industry Standards ................................................................................................. 1 3.4. Geotechnical Reports ............................................................................................. 2 3.5. Geohydrological Information .................................................................................. 2 DESCRIPTION OF THE STUDY AREA .......................................................................... 3 PROCEDURES USED IN THIS STUDY .......................................................................... 3 5.1. Site Inspection by VGIconsult................................................................................. 3 5.2. Assimilation of Available Data -Information sources ............................................... 4 5.3. Gravity Surveys ...................................................................................................... 4 5.4. Rotary Percussion Boreholes ................................................................................. 4 5.5. Thickness and depth concepts ............................................................................... 6 5.6. Coordinate System................................................................................................. 6 5.7. Map production (projection, co-ordinate system and datum) .................................. 6 5.8. Hazard Characterisation Procedure ....................................................................... 7 5.9. Dolomite Area Designation ................................................................................... 12 5.10. Monitoring Designations ....................................................................................... 12 5.11. SANS 1936-1: Table 1, SANS 1936 Part 1 (2012) ............................................... 15 GEOLOGY AND GEOHYDROLOGY ............................................................................ 17 6.1. General Geology .................................................................................................. 17 6.2. Local Geology ...................................................................................................... 17 6.3. Geohydrology....................................................................................................... 19 6.4. Past Sinkholes and Subsidences ......................................................................... 19 DOLOMITE HAZARD CHARACTERISATION .............................................................. 20 7.1. Site investigation and Dolomite Hazard Assessment Procedures ......................... 20 7.2. Hazard Characterisation of the site ...................................................................... 20 CONCLUSIONS ............................................................................................................ 24 8.1. Results of the borehole drilling programme .......................................................... 24 8.2. Dolomite Hazard Characterisation and Suitability of the site for the current land use ....................................................................................................................... 25 RECOMMENDATIONS ................................................................................................. 26 9.1. Rehabilitation of Sinkhole Areas and related wet services.................................... 26 9.2. Precautionary measures ...................................................................................... 29 9.3. Stormwater Management ..................................................................................... 29 9.4. Monitoring Actions................................................................................................ 29 9.5. Database of ground movement and stability conditions ........................................ 30 GENERAL ..................................................................................................................... 31

TABLES SUMMARISED BOREHOLE INFORMATION AND INHERENT HAZARD CHARACTERISATION

TABLE 1

FIGURES LOCALITY PLAN TOPOGRAPHICAL SHEET LEGEND SITE LAYOUT WITH GROUND ELEVATION CONTOURS REGIONAL GEOLOGY GEOLOGY LEGEND REGIONAL GEOHYDROLOGY

FIGURE 1 FIGURE 1A FIGURE 2 FIGURE 3 FIGURE 3A FIGURE 4

DRAWINGS RESIDUAL GRAVITY, BOREHOLE POSITIONS AND DOLOMITE HAZARD ZONATION PROPOSED REHABILITATION AREA

VGI3737 S21/1 VGI3737 S21/2

APPENDICES GRAVITY SURVEY REPORT BOREHOLE PROFILES

APPENDIX 1 APPENDIX 2

1

1.

INTRODUCTION This report presents the results of a dolomite stability investigation carried out in the area where the water supply line from the Ada Reservoirs to Welverdien and Khutsong South Extensions collapsed into two sinkholes, within the Merafong City Local Municipality. A 20m diameter size sinkhole extending to a depth of approximately 4m and a second sinkhole (6m in diameter extending to a depth of approximately 6m) occurred within a radius of 30m on the 90 degree bend of a 1m diameter water supply steel pipe, located above ground and approximately 170m west of the Ada Reservoirs, on 31 July 2016. The purpose of the investigation is to determine the extent of poor subsoil conditions or erosion and to provide recommendations and procedures on subsurface, wet service and structural improvements. It is presumed that subsurface erosion of highly erodible dolomite residuum (wad) material was initially triggered by a leak on the water supply line over an extended period of time. As a result of the initial leak, the water supply lines settled and failure of the wet services occurred, leading to the formation of two sinkholes. These investigations involved field inspections, gravity survey, borehole drilling programme, analysis, reporting and recommendations on subsurface soil improvements, wet services and structural improvements and stabilisation procedures to be followed.

2.

TERMS OF REFERENCE AND SCOPE OF WORK VGIconsult was appointed by the Merafong City Local Municipality to conduct a dolomite stability investigation for the sinkhole on the Adatta Pipeline, as part of an Emergency investigation programme of five sinkhole areas in the Merafong City Local Municipality, under Reference Letter ID (WS), dated August 2016. VGIconsult presented technical and budget cost proposals on the investigation of the sinkholes on the Adatta Pipeline in letter reference VGI3737 S21, dated 4 August 2016.

3.

AVAILABLE INFORMATION Information sources include: 3.1.

Topographic Data Topographic Map, 1: 50 000 Scale Series: issued by the Chief Directorate: Surveys and Mapping, Department of Land Affairs: Sheet Name Carletonville

3.2.

Reference 2627 AD

Geological Information Geological Map, 1: 250 000 Scale Series: issued by the Geological Survey of South Africa (Council for Geoscience): Sheet Name West Rand

3.3.

Reference 2626

Industry Standards o

South African National Standard SANS 1936 Parts 1 – 4 (2012).

o

South African National Standard SANS 2001-BE3: 2012 (DSS), Construction works

2

Part BE3: Repair of sinkholes and subsidences in dolomite land.

3.4.

o

South African National Standard SANS 633: 2012 (DSS), Soil profiling and rotary percussion borehole logging on dolomite land in Southern Africa for engineering purposes.

o

Environmental Earth Sciences, Springer-Report: “A Performance Based Approach to Dolomite Risk Management” by D Buttrick, N Trollip, R Watermeyer, N Pieterse, A Gerber, Volume 64, Issue 4, p1127 – p1138, dated 2011.

o

South African National Standards: The Application of the National Building Code: SANS 10400, Part A: General principles and requirements and Part B: Structural Design, (2004).

o

Council for Geoscience/South African Institute of Engineering and Environmental Geologists (2003): Guideline for engineering-geological characterisation and development of dolomitic land.

o

Council for Geoscience (2007): Consultants Guide: Approach to Site on Dolomite Land.

o

Department of Public Works (PW344), September 2010: Appropriate Development of Infrastructure on Dolomite. Manual for Consultants.

o

Annals of the Geological Survey of South Africa: “Subsurface subsidences and sinkholes caused by lowering of the dolomitic water-table on the Far West Rand Gold Field of South Africa” by RJ Kleywegt and DR Pike, Volume 16, p77 – p105, dated 1982.

Geotechnical Reports Geotechnical data pertaining to the Merafong City Local Municipality area of jurisdiction is housed at the West Rand District Municipality (WRDM) offices and/or the Council for Geoscience (CGS). o

3.5.

Intraconsult report, referenced IR 111 BW, dated September 1995: “A Report to the Western Services Council on the dolomite stability risk characterization of the proposed bulk water supply alignment and reservoir site: Greater Carletonville”.

Geohydrological Information Groundwater information is made available by the Department of Water Affairs through the National Groundwater Information System (NGIS) which offers read-only access to data from: o o o

National Groundwater Archive (NGA); Water Management System (WMS); and Hydstra [http://www.dwa.gov.za/chart/]).

In addition, the report “Geohydrology Guideline Development: Implementation of Dolomite Guideline – Phase 1, Activities 19 & 28: Desktop development of a Dolomite Hydrogeological Compartment Map and explanation booklet (Report), dated November 2009, Project Number: 14/14/5/2, Authors: Martin Holland: Water Geosciences Consulting, Frans Wiegmans: Golder Associates”, was consulted.

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4.

DESCRIPTION OF THE STUDY AREA The two sinkholes under investigation is located on the 90 degree bend (approximately 170m west of the Ada Reservoirs) of the above ground 1m diameter (steel pipe) Adatta water supply line to Welverdiend and Khutsong South Extensions. The area under investigation is located approximately 700m north of the R501 Provincial Road and 2km to the south-west of the town Carletonville, within the Merafong City Local Municipality. The location of the site is displayed on Figure 1, superimposed on the topographic map. The site layout and ground elevation contours and known wet services are displayed on Figure 2. The investigation area covers approximately 4ha (200m by 200m). The regional topography of the area falls from 1475m AMSL in the east to 1465m AMSL in the west. Locally surface water runoff takes place as sheetwash down the gradient towards a south to north aligned canal located approximately 130m west of the affected area. The canal is bordered to the west by tailings dams. Existing subsurface services located in the affected area including two north to south aligned damaged 300mm diameter size HDPE water pipes and an unknown 100mm diameter size pipe. Electrical cables connected to the pump of the Ada Reservoirs may possibly also run in closeproximity to the affected area. Existing wet service and electrical site layout plans was not made available from the Merafong City Local Municipality for the area under investigation. A 20m diameter size sinkhole extending to a depth of approximately 4m and a second sinkhole (6m in diameter extending to a depth of approximately 6m) occurred within a radius of 30m on the 90 degree bend of a 1m diameter water supply steel pipe, located above ground and approximately 170m west of the Ada Reservoirs, on 31 July 2016. It is presumed that subsurface erosion of highly erodible dolomite residuum (wad) material was initially triggered by a leak on the water supply line over an extended period of time. As a result of the initial leak, the water supply lines settled and failure of the wet services occurred, leading to the formation of two sinkholes. The area in a radius of 750m of the current two sinkholes is earmarked by the occurrence of a number of sinkholes in the past. The various sinkholes and subsidences recorded will be discussed in Section 6.4 of the report.

5.

PROCEDURES USED IN THIS STUDY 5.1.

Site Inspection by VGIconsult VGIconsult (Mr Jacques Meintjes) inspected the above mentioned two new sinkholes on 1 August 2016. The following was observed: 

Affected area of 30m diameter, with two sinkholes approximately 170m west of the Ada Reservoirs, on the 90 degree bend of a 1m diameter size steel water pipe.



A 20m diameter size sinkhole extending to an approximate depth of 4m and a second sinkhole of 6m diameter extending to an approximate depth of 6m.



Numerous damaged water pipes: 1m diameter steel water line, 2 x 300mm diameter HDPE pipes and an unknown 100mm diameter size wet service.

It is presumed that subsurface erosion of highly erodible dolomite residuum (wad) material was initially triggered by a leak on the water supply line over an extended period of time. As a result of the initial leak, the water supply lines settled and failure of the wet services occurred, leading to the formation of two sinkholes. The following recommendations were given in the Interim Report VGI 3737 S21, dated 4 August 2016, documenting the site inspection:

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5.2.



The affected area should be fenced off immediately and a soil berm placed around the affected area to prevent any run-off surface water entering the affected area causing additional subsurface soil erosion.



As the Welverdiend, Khutsong South Extensions 4 and 5 areas are currently without any water, it is recommended that a water line is constructed at ground surface approximately 40m to the north of the affected area to provide emergency water to these affected townships in the period before and during sinkhole investigation and rehabilitation.



A detailed dolomite stability investigation including a gravity survey and the drilling of approximately 16 percussion boreholes is urgently required to evaluate subsurface conditions, determine the extent of the affected area and to provide recommendations in terms of appropriate rehabilitation.

Assimilation of Available Data -Information sources This investigation is initiated by gathering, assimilating and reviewing existing, available geological, geotechnical, geophysical and geohydrological data pertaining to the site.

5.3.

Gravity Surveys The gravity method is the most widely used remote sensing technique applied on dolomite land. Variations in the earth’s structure and composition give rise to variations in density. Indirectly the density variations are determined by measuring the gravity field which allows the determination of location, form and distribution of causative geological factors. The gravity survey of a dolomitic terrain can be used to help determine dolomite bedrock configuration (bedrock topography). The gravity survey of the area under investigation was conducted on a 10m grid spacing by Geo Focus Geophysical Services on 13 and 14 September 2016. The gravity survey report is presented in Appendix 1. A relative Bouguer gravity anomaly contour map is produced from field surveys presenting anomalies of gravity high and low fields and gradient. Once depth of dolomite bedrock on the Bouguer gravity high, low and gradients are confirmed by drilling, the relative Bouguer field is adjusted by subtracting a regional field so that the map becomes a better representation of depth to dolomite bedrock. Removal of the estimated regional field results in the creation of a residual data set. The residual gravity is displayed on Drawing VGI3737 S21/1. The gravity survey revealed a prominent west to east trending gravity high field (shallow dolomite bedrock) directly north of the two sinkholes. The gravity high field is bordered by a steep gravity gradient, with a gravity low field (deep dolomite bedrock) located in the southern portion of the investigated area and a second gravity low field in the north-eastern portion of the investigated area. All of the recorded sinkholes and subsidences appears to have formed on the steep gravity gradient areas.

5.4.

Rotary Percussion Boreholes A total of nine percussion boreholes (3737 S21-01 to 3737 S21-09) were drilled in the area surrounding the sinkholes and on gravity anomalies. Two existing boreholes (BH7 and BH7A) from the Intraconsult Report No. IR 111 BW (dated September 1995) is incorporated into the current study. The nine boreholes were drilled on 13 and 21 September 2016 by JK Developments Drilling Contractor. The positions of the boreholes are displayed on Drawing VGI3737 S21/1. The borehole logs

5

are presented in Appendix 2 and summarised in Table 1. 5.4.1. Borehole Information JK Developments Drilling Contractor was instructed by VGIconsult to undertake the drilling programme. Current practice requires that a number of rotary percussion boreholes should be drilled according to set specifications as outlined below: 5.4.2. Drilling equipment should comprise of the following mobile unit: a)

b)

compressor unit with measured and calibrated constant air delivery rating at (21,2m³/s (750 cfm) and 1600KPa (16 Bar) minimum or 26,9m³/s (950 cfm) with 2100KPa (21 Bar) maximum; and pneumatic percussion drilling rig with 165mm nominal diameter button bit capable of drilling in all soil and rock types.

5.4.3. Drilling a) b) c) d)

Representative samples shall be retrieved for every 1m drilled. Boreholes shall be drilled to 60m or 6 m into dolomite bedrock, whichever occurs first. The depth of groundwater strikes shall be recorded. Hammer rate, sample and air recovery must be recorded, as well as hardness of the formations drilled.

5.4.4. Sampling a) b)

c)

Samples should be taken representatively and sufficiently to allow appropriate visual and tactile analysis. Tags should be included in every bag, recording depth of sample, penetration time and borehole number. Metres drilled where no sample return is recorded should also be tagged and bagged. Samples should be filed sequentially in plastic sleeves, with the borehole number clearly marked on the sleeve.

5.4.5. Drilling depths Current practice requires that: a) b) c)

d)

Boreholes be drilled at least 6 meters into hard rock dolomite; Boreholes be drilled to at least 60 m if dolomite bedrock is not confirmed at shallower depth; A selection of boreholes are to be drilled into dolomite bedrock on gravity high anomalies to determine the shallowest bedrock depth (in instances where dolomite bedrock is anticipated to be very deep (deeper than 60 m), In de-watered areas, a representative selection of boreholes shall be drilled to 100 m or into dolomite bedrock to obtain a perspective of subsurface conditions below the OWL.

5.4.6. Drill contract details Drilling work is undertaken using a down-the-hole rotary percussion rig. The drilling machine is a Thor 5000. The compressor used is an Atlas COPCO (XRVS 476 CD) and delivers 27,7 m³/min at a pressure of 1900 kPa to a 165 mm diameter hammer (button bit). The drill operator was J.G. Maluleke. The boreholes drilled during this investigation were terminated 6m into dolomite bedrock, with the exception of one borehole (3737 S21-02) where drilling was terminated before that due to difficult

6

drilling conditions. The boreholes were backfilled according to industry guidelines. 5.5.

Thickness and depth concepts In the context of this report the concepts of thickness and depths are used as follows: Depths Range in m 0-2/4 2/4-8/12 8/12-16/24 16/24-36/44 More than 40

Appellation Near-surface Shallow Intermediate Great Depth Very Great Depth Thicknesses

Range in m 0-8/12 8/12-16/24 16/24-36/44 36/44 and greater 5.6.

Appellation Thin Intermediate Thick Very Thick

Coordinate System The X-and Y-coordinates (values) for the boreholes, as reflected in Appendix 2, conform to the South African Coordinate System as set in the national control survey network maintained by the Chief Directorate: Surveys and Mapping of the Department of Land Affairs. The X-and Y-values are given in metres latitude (7 digit value) and longitude (5 or 6 digit value). These coordinates are projected using the Gauss Conform Projection which is the Transverse aspect of the Mercator projection. The reference ellipsoid is the WGS84 (Hartebeeshoek 1994) ellipsoid. The Central Meridian (longitude of origin or Lo) for this site is 27ºE, with the site as a whole located west of the Central Meridian. In the South African coordinate system the X coordinates are measured southwards from the equator (where x = 0) towards the South Pole which is positive. Y coordinates are measured from the Central Meridian (CM), increasing from the CM in a westerly direction so that Y is positive west of the CM and negative east of the CM.

5.7.

Map production (projection, co-ordinate system and datum) The projection information of the figures and drawings in this report are reflected on each individual figure and drawing and listed below: Projection surface:

Mercator (cylinder)

Projection orientation:

Transverse aspect

Datum or reference ellipsoid (model for the shape of the earth):

World Geodetic System 84 (as updated in 2004 and valid to 2010)

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Central Meridian:

In degrees (29 for this study)

False easting and northing:

Zero degrees

Scale factor:

1

A geographic coordinate system (GCS) uses a three-dimensional spherical surface to define locations on the earth. A GCS includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). Latitude and longitude values are traditionally measured either in decimal degrees or in degrees, minutes, and seconds (DMS). Latitude values are measured relative to the equator. Longitude values are measured relative to the prime meridian. Data defined on a geographic coordinate system is displayed as if a degree is a linear unit of measure. Although longitude and latitude can locate exact positions on the surface of the globe, it is not a uniform unit of measure. The drawings and figures are therefore presented (coordinated) in metres latitude (7 digit value) and longitude (5 or 6 digit value). 5.8.

Hazard Characterisation Procedure New national standards (Draft SANS 1936) require use of internationally accepted terminology. The applicable terminology and its definition (with previous term used) are given below: 1.

Hazard Source of potential harm. Hazard is the function of magnitude (of the events), area, and frequency.

2.

Inherent hazard (Inherent Risk) A reflection of the geological susceptibility of a karst area to an event (sinkhole or subsidence). Inherent Hazard is expressed in terms of three broad categories, namely low, medium and high, typically, but not exclusively, denoting anticipated number of events per area over time.

3.

Inherent hazard class (Inherent Risk Class) A site is characterised in terms of eight standard inherent hazard classes, denoting the likelihood of an event (sinkhole or subsidence) occurring as well as its likely size (diameter). The larger the Inherent Hazard Class (IHC) number, the greater the likelihood of the event occurring and the larger its potential size should it occur.

4.

Hazard rating The number of events that have occurred over a 20 year period due to human impact.

5.

Tolerable hazard rating The hazard rating is expressed as tolerable where the number of events experienced is less than and including 0.1 events per hectare per 20 years (preferably tending to 0 per hectare) that is exceeding the return period of 200 years and intolerable where the number of events experienced exceeds 0.1 events per hectare per 20 years (return period less than 200 years).

6.

Return period Known as a recurrence interval and is an estimate of the interval of time between

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events of a certain size. 7.

Subsidence Shallow, enclosed depression. In past South African literature subsidence, as defined above, is synonymous with doline. The term subsidence is substituted to prevent contradiction with international literature.

8.

Sinkhole A feature that occurs suddenly and manifests itself as a hole in the ground. In international literature the term sinkhole is often synonymous with doline.

9.

Dolomite land Land underlain by dolomite or limestone rock directly or at a shallow depth, typically no more than:

10.

a)

60 m in areas where no de-watering has taken place and the local authority has jurisdiction, is monitoring and has control over the groundwater levels over the areas under consideration; or

b)

100 m in areas where de-watering has taken place or where the local authority has no jurisdiction or control over groundwater levels

Event Occurrence [or change of a particular set of circumstances], in the context of this standard referring to a sinkhole or subsidence. An event can be one or more occurrences, and can have several causes. An event can consist of something not happening. An event can sometimes be referred to as an “incident” or “accident”.

11.

Potential loss of support Refers to the removal of support below the foundation due to a nominal sinkhole or subsidence event. In proposing suitable foundation types in D3 areas, consideration should be given to the potential loss of support which could be anticipated for the designated Inherent Hazard Class based on the nascent1sinkhole size. The philosophy to be applied to the design of the foundations is that in the event of catastrophic loss of support, there is sufficient time for occupants to safely escape after the occurrence of the sinkhole, and the level of expected damage associated with soil movements unrelated to sinkhole formation in near surface horizons is kept within reasonable limits.

The hazard characterisation procedure (previously referred to as “risk characterisation procedure in South African literature) is in accordance with the peer reviewed paper referenced in Section 3.3 of this report: The available information, geophysical data, borehole data and geohydrological information gathered during the investigation has been pooled and reviewed permitting the formulation of a perspective concerning the characterisation of the stability of the site. The predominant mobilising agencies considered in this investigation are major groundwater level fluctuations (>6m), ingress water, ground vibrations and gravity. Use is 1

Adj. beginning to develop

9

made of a generalised list of evaluation factors to evaluate the hazard. These factors are as follows: -

Receptacle development; Mobilising agencies, particularly ingress water from leaking services; Potential sinkhole development space; Nature of the blanketing layer; Mobilisation potential of the blanketing layer; Bedrock morphology.

Receptacles or disseminated receptacles refer to any voids or cavities in the dolomite bedrock or in the overburden capable of receiving mobilised materials. Receptacles are assumed to be present as no reliable geophysical tool exists to determine the location of these features. Consequently the information gathered from boreholes, including penetration times, air loss, hammer action, etc., combined with geophysical and geological information are used to formulate an impression of the degree of voids. The potential sinkhole development space, where used, refers to the expected maximum size sinkhole that conservatively may be anticipated to be generated if sustained ingress of water were to occur. This factor is related to the depth of the receptacles or disseminated receptacles. The gravity survey combined with borehole information strongly guides the appraisal of this factor. The nature of the material covering the receptacles, be they above or in the bedrock, determines the susceptibility of the subsurface material to erosion by ingress water. The presence of materials such as shales or intrusives, which can act as aquitards, serve to reduce the mobilisation potential and enhance the stability. In the case of dramatic groundwater level fluctuations the susceptibility of the soil material to mobilisation (i.e. consolidation settlement -subsidence formation, or ravelling and arch failure -sinkhole formation, due to pore pressure changes in soils), is strongly influenced by the position of the original groundwater level in the subsurface profile. In assessing the susceptibility of a subsurface profile to sinkholes and subsidences due to groundwater level drawdown, attention is given to the nature and extent of the material below the groundwater level. For example in the case of compressible dolomite residuum, as the groundwater recedes, pore pressures in the residual dolomite soils, typically characterised by high void ratios, gradually dissipate and the effective stress on the soil increases causing consolidation of the compressible material. A surface depression may occur gradually due to the load of the near-surface materials on the deeper lower density materials that settle into a denser state. The general maximum magnitude of natural groundwater fluctuation in the Gauteng dolomites is in the order of 5m (Hobbs, 2004; Holland, 2007). Hence artificial groundwater level drawdown is generally defined as drawdown exceeding 6m. Experience shows that groundwater level drawdown (beyond seasonal variations) has the greatest negative impact on dolomite stability in areas of shallow groundwater levels (30m or shallower) i.e. deeper (>30m) groundwater levels, pose less of a negative impact on stability, in the event of groundwater level drawdown or dewatering. However, where groundwater drawdown occurs in areas of deeper groundwater, the size of instability is typically anticipated to be large to very large. In view of the factors discussed above the following characteristics have been extracted from the gathered information during the assessment process: -

borehole position relative to the gravity data.

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-

collar elevation. depth to dolomite bedrock. depth to potential receptacles. depth to present groundwater level. nature and thickness of blanketing layer i.e. material type, penetration times, etc. position of the bedrock with respect to the present and original groundwater level. thickness and nature of the soil materials above and below the present and original groundwater level.

Inherent hazard is a reflection of the geological susceptibility of karst area to an event (sinkhole or subsidence formation) and is expressed in three broad categories, namely low, medium and high. The following reference to incidences, gives a perspective of the magnitude of problems encountered in each of the of hazard zones in research areas. It is important to note that these figures are largely derived from developments not effectively and appropriately designed or maintained. Inherent Hazard

Anticipated events per hectare over time (magnitude of problem)*

LOW

0 up to and including 0.1 events per hectare anticipated but occurrence of events cannot be excluded. Return Period is greater than 200 years.

MEDIUM

Greater than 0.1 and less than and equal to 1.0 events per hectare. Return period is between 200 and 20 years.

HIGH

Greater than 1.0 events anticipated per hectare. Return period is less than 20 years.

* that have occurred per hectare in a 20 year period in the "type" areas (statistics based on poor service design and maintenance)

The study area is characterised in terms of eight standard Inherent Hazard Classes. These classes denote the chance of a sinkhole or subsidence occurring as well as its likely size (diameter). The terminology used in terms of likely size of an event (sinkhole or subsidence) is defined as follows: Maximum diameter of surface manifestation (in metres) <2 2-5 5-15 > 15

Terminology Small-size Medium-size Large-size Very large-size

The larger the Inherent Hazard Class number, the greater the chance of a sinkhole or subsidence occurring and the larger its potential size should it occur. The meaning/definition of each Inherent Hazard Class is as follows: Inherent Hazard Class Class 1 Areas Class 2 Areas Class 3 Areas Class 4 Areas

Characterisation of Area Areas characterised as reflecting a low inherent susceptibility of all sizes of events occurring. Areas characterised as reflecting a medium inherent susceptibility of small-size events occurring. Areas characterised as reflecting a medium inherent susceptibility of medium-size events occurring. Areas characterised as reflecting a medium inherent susceptibility of large-size events occurring.

11

Inherent Hazard Class

Characterisation of Area

Areas characterised as reflecting a high inherent susceptibility of smallsize events occurring. Areas characterised as reflecting a high inherent susceptibility of Class 6 Areas medium-size events. Areas characterised as reflecting a high inherent susceptibility of largeClass 7 Areas size events occurring. Areas characterised as reflecting a high inherent susceptibility of very Class 8 Areas large-size events occurring. NOTE: The event size reflects the predominant anticipated nascent event size. Class 5 Areas

The definitions above are summaries of the Inherent Hazard Class table presented in the 2001 paper referenced in Section 3. Inherent Hazard is defined in terms of ingress water and groundwater level drawdown reflected by two Inherent Hazard Class designations separated by a double forward slash, i.e.Inherent Hazard Class (Ingress water) // Inherent Hazard Class (groundwater level drawdown) As an example, a designation of 1//8 indicates that the zone displays a low Inherent Hazard with respect to water ingress but a high Inherent Hazard with respect to groundwater level drawdown. Further combinations may be appropriate: As an example, a designation of Inherent Hazard Class 1//1/4/8 indicates that the zone displays a low Inherent Hazard with respect to water ingress but a low to high Inherent Hazard with respect to groundwater level drawdown. This definition may, for example, be necessary in cases where groundwater was not encountered or the original groundwater level is not known and dolomite bedrock could not be confirmed. Zones delineated on a site may be combinations of the above. In some instances, the Inherent Hazard Classes are indicated with the primary zone description given first followed by a suffix in brackets. The primary Inherent Hazard Class describes the predominant characterisation of the zone and the suffix describes the characterisation of anticipated pockets or small subareas within the zone: As an example, a designation of Inherent Hazard Class 8(4) indicates that the zone predominantly displays a high Inherent Hazard for up to very large-size sinkhole and subsidence formation with anticipated pockets or small sub-areas of Class 4 i.e. displaying a medium hazard for up to large-size sinkhole and subsidence formation. Specific commentary should be provided on the impact that the action of ingress water may have on the soil profile upon lowering of the ground water level or base level of erosion. Does the susceptibility of the subsurface profile remain unchanged from an ingress of water perspective or not, as the the groundwater level is lowered and the previously “protected” profile is exposed? . Example: The lowering of the groundwater level and exposure of a poor subsurface profile in an area of previously shallow groundwater level designated as Inherent Hazard Class 3//7, results in a change in susceptibility from medium to high and the Inherent Hazard Class from 3 to 6 i.e. the Inherent Hazard Class 3//7 will change to

12

Inherent Hazard Class 6//7 once groundwater level drawdown has occurred. 5.9.

Dolomite Area Designation Dolomite Area Designations must be identified on sites located on or near dolomite land (land where dolomite is located at or near [less than 100 m] ground surface). The definitions of the Dolomite Area Designations as defined in SANS 1936 Part 1 (2012) are as follows: Dolomite area designation

Description

D1

No precautionary measures are required.

D2

General precautionary measures, in accordance with the requirements of SANS 1936-3, that are intended to prevent the concentrated ingress of water into the ground, are required.

D3

Precautionary measures in addition to those pertaining to the prevention of concentrated ingress of water into the ground, in accordance with the relevant requirements of SANS 1936-3, are required.

D4

Development may only be considered provided the following requirements are met:  Involvement of Competence Level 4 geo-practitioner in all the categories of the geotechnical engineering work, i.e. site characterization, analysis and design, supervision and review, supervision of execution and management (primary geo-practitioner).  Review and acceptance of all the categories of the geotechnical engineering work by a Competence Level 4 peer. This peer reviewer may not be a business associate of the primary geo-practitioner(s) and may not have a vested interest in the project.  All the categories of the geotechnical engineering work to be reviewed and accepted by the Authority who may request a further review by an Authority designated Competence Level 4 peer, if required.  The responsible Local Authority must indicate its commitment to maintain dolomite risk management principles in accordance with SANS 1936-4.

5.10. Monitoring Designations According to SANS 1936 Part 4 (2012) Monitoring Designations must be identified and delineated according to the Inherent Hazard characterisation of the site and knowledge of problems which could impact on the infrastructure on site. The generic Monitoring Activities considered appropriate for dolomite site are as follows: Annotation

A

Activity* Visual inspections of ground, structures and above ground infrastructure(e.g. buildings, taps, gardens, private and public open space etc.):  Examine buildings for cracks.  Visual checks for outside dripping taps and pressure valves.  Visual checks for damp or moss grown areas.  Visual check for over-wetting of gardens.  Visual check for blocked drainage ports in garden walls.  Visual check for cracks in the ground.

Reaction

Any evidence of cracking or ground settlement should immediately be reported and investigated. Any evidence of blockages should be reported and cleared immediately.

Purpose

Monitor, control and prevention of concentrated ingress of water

13 Annotation

Activity*

B

Visual inspection of stormwater systems:  Visual checks for debris in open storm water channels at, for example, the start of the rainy season and after heavy storms.  Visual checks for water flowing out of stormwater manholes at the start of the rainy season and after heavy storms.  Search for ground cracks and cracks in lined and unlined channels.

C

Testing of wet-services for leaks:  Regular air and water tests on wetservices.  In waste water and stormwater pipes with a diameter greater than 100 mm wet-services to be inspected by camera.

D

E

Monitoring of structures and ground levels: In many instances visual inspections may not be sufficient: It may be necessary to undertake precision ground-surface levelling, particularly in areas that have been rehabilitated after a sinkhole or subsidence event. Such levelling must be undertaken by a surveyor, recorded and stored in the databank and appropriate actions taken when excessive settlement continues. Monitoring of the groundwater level: This activity not only entails the measuring and recording of the groundwater level, but also the analysis and understanding of groundwater level trends over time and the control of excessive [beyond seasonal fluctuations] groundwater level drawdown:  Drill and equip strategic boreholes with the necessary equipment to measure the groundwater level.  Recorded groundwater level, monitoring well number, date and other relevant observations.    

Analyse groundwater level trend over time on a regular basis. Report artificial/excessive groundwater level drawdown to Council and the Department of Water Affairs. Investigate cause of artificial/excessive groundwater level drawdown. Arrest artificial/excessive groundwater level drawdown.

Reaction

Purpose

Any leaks to be reported and repaired immediately.

Any evidence of movement must be reported and investigated.

Monitor the effects of concentrated ingress of water or groundwater level drawdown

Evidence of lowering must be reported to Council and the Department of Water Affairs.

Monitor, control and prevention of groundwater level drawdown

*If there is no evidence of a particular problem, this result should be recorded.

The Frequency with which each Activity is to be performed is selected from the following categories:

14

Annotation ()DAILY ()WEEKLY ()¹ ()³ ()6 ()¹² ()24 ()NA ()tbd

Frequency Activities to be undertaken daily. Activities to be undertaken weekly. Activities to be undertaken once a month. Activities to be undertaken quarterly. Activities to be undertaken bi-annually. Activities to be undertaken annually. Activities to be undertaken once every two years. NO ACTION REQUIRED TO BE DETERMINED

Areas of ‘no dolomite hazard’ require no monitoring from a dolomite risk management perspective. No action is required to lower the risk of dolomite-related instability, as these areas are not located on dolomite land. For example, portions of sites located on granite rock or Witwatersrand Supergroup rocks. Such areas may be designated as (ABCDE)0. Areas of ‘low hazard’, for example Inherent Hazard Class 1 areas, are assigned a low priority and require basic monitoring and maintenance activities at long intervals, for example, where a site straddles very thick Karoo Supergroup rocks (in excess of 40 metres). The site or portion thereof may, for example, be designated as (ABC)24D0E12 indicating that all identified activities which control ingress water need only be undertaken once every two years, precision structure-and ground levelling not being required and groundwater level monitoring being required at long intervals. However, where such rocks overlie dolomite residuum below the original groundwater level a designation of (ABC)24D0E3 may apply, indicating that activities which control the ingress of concentrated water remain necessary once every two years but groundwater level monitoring is critical and should be undertaken quarterly. Areas of ‘high hazard’, for example Inherent Hazard Class 5, 3(5), 3/6 and therefore high priority in terms of monitoring and maintenance, should receive attention more frequently. These areas require stringent monitoring and maintenance activities at short intervals. Such areas are typically characterised by: o o o o o o o o o o

Metastable subsurface conditions or latent sinkhole formation. High Inherent Hazard conditions. Poor subsurface conditions e.g. cavities, cavernous conditions, sample or air loss. Previous sinkhole or subsidence formation. Palaeo-sinkhole or palaeo-subsidence structures. Geological contact areas. Fault zones. Shallow dolomite groundwater above dolomite bedrock. Anticipated ground settlement or Ponding of water, etc.

For example, an area in which various sinkholes have already been reported and where the area is designated as high hazard or even medium to high hazard from an ingress of water perspective a ABC3 or even (AB)daily (D)3 designation may apply, indicating the need to undertake activities controlling ingress of water quarterly, or even daily. A further example may be, an area in which various sinkholes have already been reported and where the area is designated as high hazard from a groundwater level drawdown perspective. In such a case a (ABC)12D0E3 designation may apply, indicating the need to undertake activities to monitor groundwater fluctuations and drawdown quarterly.

15

5.11. SANS 1936-1: Table 1, SANS 1936 Part 1 (2012) According to SANS 1936 Part 1 (2012) the development types suitable for the eight standard Inherent Hazard Classes are summarised as follows: Inherent Hazard Class

Land Usage Type

Commercial and miscellaneous non-residential usage 1

High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage

2

3

4

5

6

7

8

High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage High rise dwelling units Low rise dwelling units Dwelling houses Commercial and miscellaneous non-residential usage High rise dwelling units Low rise dwelling units Dwelling houses

Land Usage Permitted with Dolomite Area Designation and footprint investigation requirement in terms of Deemed-To-Satisfy C1 (D3 + FPI), C2 (D3 + FPI), C3 (D2 + FPI), C4 (D2 + FPI), C5 (D2 + DLI), C6 (D2 + DLI), C7 (D2), C8 (D2) RH2 (D2 + FPI), RH3 (D2 + FPI) RL1 (D2 + FPI), RL2 (D2 + FPI) RN1 (D2), RN2 (D2), RN3 (D2) C1 (D3 + FPI), C2 (D3 + FPI), C3 (D3 + FPI), C4(D3 + FPI), C5 (D3 + DLI), C6 (D3 + DLI), C7 (D3+ FPI), C8 (D3) RH3 (D3 + FPI) RL1 (D2 + FPI), RL2 (D2 + FPI) RN1 (D3), RN2 (D3), RN3 (D3) C1 (D3 + FPI), C2 (D3 + FPI), C3 (D3 + FPI), C5 (D3 + DLI), C6 (D3 + DLI), C7 (D3 + FPI), C8 (D3) RH3 (D3 + FPI) RL2 (D3 + FPI) RN2 (D3), RN3 (D3) C1 (D3 + FPI), C2 (D3 + FPI), C3 (D3 + FPI), C5 (D3 + DLI), C6 (D3 + DLI), C7 (D3 + FPI), C8 (D3) RH3 (D3 + FPI) RL2 (D3 + FPI) RN2 (D3), RN3 (D3) C1 (D3 + FPI), C2 (D3 + FPI), C3 (D3 + FPI), C5 (D3 + DLI), C6 (D3 + DLI), C7 (D3 + FPI), C8 (D3) RH3 (D3 + FPI) RL2 (D3 + FPI) RN3 (D3 + FPI) C2 (D3 + FPI), C3 (D3 + FPI), C6 (D3 + DLI) , C7 (D3 + FPI), C8 (D3) NA NA NA C6 (D3 + DLI) NA NA NA NA NA NA NA

16 Land Use Class

Definitions Commercial and miscellaneous non-residential usage

C1

Places of detention, police stations, and institutional homes for the handicapped or aged

C2

Hospitals, hostels, hotels

C3

Commercial developments ≤ 3 storeys, including railway stations, shops, wholesale stores, offices, places of worship, theatrical, indoor sports or public assembly venues, other institutional land uses, such as universities, schools, colleges, libraries, exhibition halls and museums, light (dry) industrial developments, dry manufacturing, commercial uses such as warehousing, packaging, electrical sub-stations, filling stations

C4

Commercial developments > 3 storeys, including railway stations, shops, wholesale stores, offices, places of worship, theatrical, indoor sports or public assembly venues, other institutional land uses, such as universities, schools, colleges, libraries, exhibition halls and museums, light (dry) industrial developments, dry manufacturing, commercial uses such as warehousing, packaging, electrical sub-stations

C5 C6 C7 C8

Fuel depots, processing plants or any other areas for the storage of liquids, waste sites Outdoor storage facilities, stock yards, container depots Parking garages Parking areas High rise dwelling units

RH1 RH2

> 10 storeys > 3 storeys with a population of ≤ 1 500 people per hectare

RH3

> 3 storeys with a residential coverage ratio of ≤ 0,4, no higher than 10 storeys, and a population of ≤ 800 people per hectare Low rise dwelling units

RL1

≤ 3 storeys with 80 to 120 units per hectare and a population not exceeding 600 people per hectare

RL2

≤ 3 storeys with up to 80 units per hectare and a population not exceeding 400 people per hectare Dwelling houses

RN1

Up to 60 dwelling houses per hectare with stands larger than 150 m2, and a population of ≤ 300 people per hectare

RN2

Up to 25 dwelling houses per hectare with stands no smaller than 300 m 2, and a population of ≤ 200 people per hectare

RN3

Up to 10 dwelling houses per hectare with 1 000 to 4 000 m 2 stands, and a population of ≤ 60 people per hectare

Infrastructure and social facilities Designation IN1

Description Trunk roads (national and regional roads which facilitate intercity travel) and primary distributor roads (major arterial roads forming the primary network for an urban area as a whole), railway lines, power lines, runways, bulk pipelines, including water, sewer, fuel and gas lines, pump stations

Inherent hazard class determined in accordance with the requirements of SANS 1936-2 1

2

3

4

5

6

7

8

Dolomite area designation

D2

D3

D4

17 IN2

IN3

Cemeteries

IN4

Dams, slimes dams

IN5

Solid waste disposal facilities

D2

D3

D4

D3

D4

D3

D4 D3

D4

GEOLOGY AND GEOHYDROLOGY 6.1.

General Geology The site is underlain by chert and dolomite of the Eccles Formation, Malmani Subgroup of the Chuniespoort Group, Transvaal Supergroup with the possibility of intrusive materials in the form of dykes. The various lithological units and their weathered derivatives, recorded on the site are as follows: Lithostratigraphic Unit

Lithology

Intrusive

Syenite and associated soil derivatives.

Eccles Formation, Malmani Subgroup, Chuniespoort Group, Transvaal Supergroup

Dolomite, chert and associated soil derivatives.

Unconsolidated materials deemed to be various recent deposits (most likely 24 Ma [Miocene Epoch] and younger) are anticipated to mantle the hard rock geology (and residual product thereof). The material varies in thickness, sedimentological- and geotechnical properties: Post-Gondwana Deposits Colluvium

Local Geology A summary of the material intercepted in boreholes drilled on the site is reported here for ease of reference (explanations of letters, symbols and abbreviations are given in Table 1): Blanketing Layer (m) – (m) Chert Residuum

3737 S21/1 (1570) 3737 S21/2 (1569) 3737 S21/3 (1568) 3737 S21/4 (1568)

Fines subordinate

Dolomite Residuum

Residual Syenite Highly weathered dolomite

Manganiferous Soils

BH No. (Collar Elev. m AMSL)

Ferroan Soils

6.2.

Soils of sandy, silty or clayey composition or gravels

Colluvium

6.

Reservoirs and public swimming pools, water care works, attenuation and retention ponds for stormwater management and artificial lakes

(m) – (m)

-

0-1

1-6

6-15

-

15-25

-

0-30

-

30-46

-

-

-

0-4

13-15

-

-

4-13 15-24

-

0-1

-

-

-

1-24

Dolomite Bedrock (m) – (m) (m AMSL)

25-30 (1545) >46 (<1523) 24-30 (1544) 24-30 (1544)

18 Blanketing Layer (m) – (m)

Highly weathered dolomite

Manganiferous Soils

Fines subordinate

Residual Syenite

(m) – (m)

Dolomite Bedrock (m) – (m) (m AMSL)

3737 S21/5 (1568)

-

-

-

-

-

0-14

14-20 (1554)

3737 S21/6 (1570)

-

0-1

1-3

-

-

3-9

9-15 (1561)

-

0-8

-

8-21

-

0-5

5-7 18-23

7-18

-

0-2

-

-

0-3

-

-

3->30

0-2

-

2-16 17->30

-

3737 S21/7 (1568) 3737 S21/8 (1568) 3737 S21/9 (1570) BH 7 (1565) BH 7A (1568)

o

Dolomite Residuum

Ferroan Soils

Colluvium

Chert Residuum BH No. (Collar Elev. m AMSL)

-

16-17

21-29 23-34 2-14 -

29-35 (1539) 34-40 (1534) 14-20 (1556) >30 (<1535) >30 (<1538)

Colluvium Colluvium is intercepted from near surface, only in Boreholes BH7 and BH7A. The horizon is thin (2m to 3m) or absent.

o

Residual syenite Residual syenite is intercepted from an intermediate depth, only in Borehole BH 7A. The horizon is thin (1m) or absent.

o

Chert residuum Chert residuum (fines subordinate) is typically intercepted from near surface. The horizons are thin (1m to 8m), the exception is Borehole 3737 S21-02 comprising of a thick (30m) horizon.

o

Dolomite residuum (ferroan and manganiferous material) Dolomite residuum (ferroan material) is intercepted from near surface to intermediate depth. The horizons are thin to intermediate thick (2m to 14m). Dolomite residuum (manganiferous material /wad) is intercepted from near surface to a great depth. The horizons are thin to very thick (9m to >27m).

o

Problematic conditions in the overburden above the groundwater level: The OWL of 145m (or 1424m AMSL) is located within dolomite bedrock. Problematic conditions (sample and/or air loss) were encountered during the drilling programme within the chert residuum, dolomite residuum, highly weathered dolomite and dolomite bedrock. Cavernous conditions including disseminated voids are intercepted in four of the boreholes drilled, i.e. 3737 S21/01 (6m to 12m), 3737 S21/02 (>46m), 3737 S21/07 (9m to 18m) and 3737 S21/08 (12m to 18m) all within dolomite residuum (manganiferous soils).

o

Dolomite bedrock conditions:

19

Dolomite bedrock is considered to be the depth at which hard, unweathered dolomite rock is confirmed (i.e. no less than 6 m of unweathered rock). Dolomite bedrock is confirmed at shallow to very great depths (9m to >46m). Highly weathered soft rock dolomite is intercepted from near surface to intermediate depths (from ground surface to 34m) typically above hard rock dolomite. The horizons are thin to intermediate thick (6m to 23m). 6.3.

Geohydrology A desktop hydrogeological investigation and situation assessment was undertaken by Water Geoscience Consulting (WGC) on a number of dolomitic areas in 2009. According to the WGC Dolomite Hydrogeological Compartment Map (2009) the site is located in the Boskop-Turffontein Dolomite Groundwater Management Area. The regional groundwater level (OWL), as recorded in the SCTC archives, is anticipated at a depth of 1424m AMSL to 1425m AMSL (or 145m) within this portion of the BoskopTurffontein Dolomite Groundwater Compartment, taking the average ground elevation on the site as 1570m AMSL. The regional dolomite groundwater information is presented in Figure 4. A groundwater level of 1424m AMSL (or 145m) is considered for the dolomite hazard assessment of the site. All the boreholes were recorded as “dry” 24 hours after drilling and the groundwater level is located within dolomite bedrock (<1523m ASML to 1561m AMSL).

6.4.

Past Sinkholes and Subsidences The area in a radius of 750m of the current two sinkholes is earmarked by the occurrence of a number of sinkholes in the past. The following sinkholes had been reported: 

Sinkhole located 200m north of the two current sinkholes, reported in 1989. The size and depth of the sinkhole is unknown.



Two sinkholes (dimensions unknown) reported in 1993, located approximately 700m south of the current sinkholes.



Sinkhole of 11m diameter size extending to a depth of 8m directly west of the Eastern Ada Reservoir, reported on 3 July 2008.



Small sinkhole (< 2m diameter) extending to an unknown depth, located 30m north of the Western Ada Reservoir on the supply line to Carletonville, reported on 3 July 2008.



Sinkhole of 14m diameter size extending to a depth of 6m, located approximately 80m north of the Ada Reservoirs, reported on 3 July 2008.



Two sinkholes, each measuring approximately 14m in diameter (unknown depth) and two subsidences, measuring between 12m to 18m in diameter is observed on Google Map. The four instability features is located approximately 100m west of the current two sinkholes within the area of the canal along the tailings dam.

The location of previous sinkholes and subsidences in relation to the current sinkholes are presented in Figure 2.

20

DOLOMITE HAZARD CHARACTERISATION 7.1.

Site investigation and Dolomite Hazard Assessment Procedures The site investigation procedures are presented in Sections 5.1 to 5.4 of this report. The dolomite hazard assessment procedures are presented in Section 5.8. Summarised information concerning the hazard characterisation of the site is provided in Table 1 of this report and the dolomite hazard zonation is displayed on Drawing VGI3737 S21/1. Hazard Characterisation of the site Based on the current data gathered, the site is characterised in terms of two primary Inherent Hazard Class area, namely: 

Dolomite Hazard Zone 1: Dolomite Inherent Hazard Class 7/8//1 defined as an area characterised as largely reflecting a high susceptibility of large to very large-size sinkhole and subsidence formation with respect to ingress water and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged.



Dolomite Hazard Zone 2: Dolomite Inherent Hazard Class 3/4//1 defined as an area characterised as largely reflecting a medium susceptibility of medium to large-size sinkhole and subsidence formation with respect to ingress water and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged.

a)

Blanketing layer Dolomite Hazard Zone 1 Blanketing Layer (m) – (m) Dolomite Residuum

BH No. (Collar Elev. m AMSL)

3737 S21/1 (1570) 3737 S21/2 (1569) 3737 S21/7 (1568) 3737 S21/8 (1568) BH 7 (1565)

Fines subordinate

Residual Syenite Highly weathered dolomite

Manganiferous Soils

Chert Residuum

Ferroan Soils

7.2.

Colluvium

7.

(m) – (m)

-

0-1

1-6

6-15

-

15-25

-

0-30

-

30-46

-

-

-

0-8

-

8-21

-

0-5

5-7 18-23

7-18

0-3

-

-

3->30

-

-

21-29 23-34 -

Dolomite Bedrock (m) – (m) (m AMSL)

25-30 (1545) >46 (<1523) 29-35 (1539) 34-40 (1534) >30 (<1535)

o

Colluvium: Intercepted from near surface, only in Borehole BH7. The horizon is thin (3m) or absent.

o

Chert residuum: Is typically intercepted from near surface. The horizons are thin (1m to 8m), the exception is Borehole 3737 S21-02 comprising of a thick (30m) horizon.

o

Dolomite residuum (ferroan and manganiferous material)

21

Dolomite residuum (ferroan material) is intercepted from near surface to intermediate depth. The horizons are thin (2m to 5m). Dolomite residuum (manganiferous material /wad) is intercepted from shallow to great depths. The horizons are thin to very thick (9m to >27m). o

Problematic conditions in the overburden above the groundwater level: The OWL of 145m (or 1424m AMSL) is located within dolomite bedrock. Problematic conditions (sample and/or air loss) were encountered during the drilling programme within the chert residuum, dolomite residuum, highly weathered dolomite and dolomite bedrock. Cavernous conditions including disseminated voids are intercepted in four of the boreholes drilled, i.e. 3737 S21/01 (6m to 12m), 3737 S21/02 (>46m), 3737 S21/07 (9m to 18m) and 3737 S21/08 (12m to 18m) all within dolomite residuum (manganiferous soils). Dolomite Hazard Zone 2 Blanketing Layer (m) – (m)

3737 S21/3 (1568)

-

0-4

13-15

-

3737 S21/4 (1568)

-

0-1

-

3737 S21/5 (1568)

-

-

3737 S21/6 (1570)

-

3737 S21/9 (1570) BH 7A (1568)

Residual Syenite Highly weathered dolomite

Manganiferous Soils

Fines subordinate

Dolomite Residuum

Ferroan Soils

Colluvium

Chert Residuum BH No. (Collar Elev. m AMSL)

(m) – (m)

Dolomite Bedrock (m) – (m) (m AMSL)

-

4-13 15-24

24-30 (1544)

-

-

1-24

24-30 (1544)

-

-

-

0-14

14-20 (1554)

0-1

1-3

-

-

3-9

9-15 (1561)

-

0-2

-

-

0-2

-

2-16 17->30

-

16-17

2-14 -

14-20 (1556) >30 (<1538)

o

Colluvium: Intercepted from near surface, only in Borehole BH7A. The horizon is thin (2m) or absent.

o

Residual syenite: Intercepted from an intermediate depth, only in Borehole BH 7A. The horizon is thin (1m).

o

Chert residuum: Is typically intercepted from near surface. The horizons are thin (1m to 4m).

o

Dolomite residuum (ferroan and manganiferous material) Dolomite residuum (ferroan material) is intercepted from near surface to intermediate depth. The horizons are thin to intermediate thick (2m to 14m). Dolomite residuum (manganiferous material /wad) is not encountered in any of the boreholes drilled in this zone.

o

Problematic conditions in the overburden above the groundwater level:

22

The OWL of 145m (or 1424m AMSL) is located within dolomite bedrock. Problematic conditions (sample and/or air loss) were only encountered in Borehole 3737 S21/09 within the chert residuum (0m to 2m). No cavernous conditions or disseminated voids are intercepted in any of the boreholes drilling in this zone. b)

Dolomite Bedrock Dolomite bedrock is considered to be the depth at which hard, unweathered dolomite rock is confirmed (i.e. no less than 6 m of unweathered rock). Dolomite Hazard Zone 1: Dolomite bedrock is confirmed at intermediate to very great depths at <1523m AMSL to 1545m AMSL (or 25m to >46m below ground surface). It should be noted that dolomite bedrock was not encountered in Boreholes 3737 S21/02 at 46m (or 1523m AMSL) and BH7 at 30m (or 1535m AMSL) below ground surface. Thin horizons (8m to 11m) highly weathered soft rock dolomite is encountered from an intermediate depth of 15m to 23m. Dolomite Hazard Zone 2: Dolomite bedrock is encountered at 1544m AMSL to 1561m AMSL (or 9m to 24m below ground surface). It should be noted that dolomite bedrock was not encountered in Borehole BH7A at 30m (or 1538m AMSL). Thin to intermediate thick horizons (6m to 23m) highly weathered soft rock dolomite is encountered from ground surface to an intermediate depth of 15m.

c)

Hazard Characterisation The site is characterized by the following conditions: i.

Blanketing layer Dolomite Hazard Zone 1: The blanketing layer consists typically of chert residuum underlain by dolomite residuum (ferroan and manganiferous soils) and highly weathered soft rock dolomite. It should be noted that colluvium was encountered above the chert residuum in Borehole BH7 to the west of the sinkhole. The chert residuum and dolomite residuum horizons exhibit rapid penetration times during drilling and a high manganese content (wad) within the dolomite residuum horizon with sample and air loss. Disseminated receptacles are recorded above the OWL within dolomite residuum in Boreholes 3737 S21/01 (between 6m to 12m), 3737S21/02 (>46m), 3737 S21/07 (between 9m to 18m) and 3737 S21/08 (between 12m and 18m). Prolonged ingress of water (leaking wet services) did lead to subsurface erosion and the formation of a 20m diameter size sinkhole extending to a depth of 4m and a second sinkhole of 6m diameter size extending to a depth of 6m. Groundwater drawdown will occur at a great depth (145m below ground surface or 1424m AMSL) within dolomite bedrock (25m to >46m depth). Dolomite Hazard Zone 2: The blanketing layer consists typically of chert residuum underlain by dolomite residuum (ferroan soils) and highly weathered soft rock dolomite. It should be noted that colluvium was encountered above the chert residuum in Borehole BH7A to the east of the sinkhole and residual syenite was encountered within the dolomite residuum layer in Borehole BH7A. The chert residuum and dolomite residuum horizons exhibit moderate to rapid penetration times during drilling with an absence of high manganese content (wad) within the dolomite residuum horizon. Air loss was only encountered in Borehole 3737 S21/09 within the chert residuum (0m to 2m). No cavities or disseminated receptacles are recorded in any of the boreholes drilled in this zone. Prolonged ingress of water (leaking wet services) may however lead to

23

subsurface erosion and the formation of sinkholes and subsidences. Groundwater drawdown will occur at a great depth (145m below ground surface or 1424m AMSL) within dolomite bedrock (9m to >30m depth). ii.

Potential Development Space Dolomite Hazard Zone 1: The depth to the groundwater level (145m) and potential receptacles characterises the Potential Development Space (PDS) as mainly large to very large. Dolomite Hazard Zone 2: The depth to the groundwater level (145m) and potential receptacles characterises the Potential Development Space (PDS) as mainly medium to large.

iii.

Hazard Classification (with Ingress of Water as a Triggering Mechanism) Dolomite Hazard Zone 1: The typical subsurface profile consists of a thin to thick (1m to 30m) horizon of chert residuum (fines subordinate), typically characterised by moderate to good internal drainage characteristics. The thin (2m to 5m) dolomite residuum (ferroan soils) horizon intercepted from near surface to intermediate depth and the underlying thin to very thick (9m to >27m) dolomite residuum (manganiferous soils) horizon intercepted from shallow to great depths is anticipated to have a high mobilization potential. Cavernous conditions including disseminated voids are intercepted within the blanketing layer dolomite residuum (manganiferous soils) in four of the five boreholes drilled, sample and air losses were recorded. The thin colluvium horizon intercepted from near surface, only in Borehole BH7, is typically characterized by moderate internal drainage characteristics. The subsurface conditions are characterised as largely reflecting a high susceptibility of large to very large-size sinkhole and subsidence formation with respect to ingress water, i.e. Inherent Hazard Class 7/8. Dolomite Hazard Zone 2: The typical subsurface profile consists of a thin (1m to 4m) horizon of chert residuum (fines subordinate), typically characterised by moderate to good internal drainage characteristics. The thin to intermediate thick (2m to 14m) dolomite residuum (ferroan soils) horizon intercepted from near surface to intermediate depth is anticipated to have a moderate to high mobilization potential. Dolomite residuum (manganiferous soils), cavernous conditions including disseminated voids or sample losses were not recorded in any of the boreholes drilled. The thin colluvium horizon intercepted from near surface, only in Borehole BH7A, is typically characterized by moderate internal drainage characteristics. The thin (1m) residual syenite horizon with a low mobilization potential and poor internal drainage characteristics, encountered from an intermediate depth in Borehole BH7A, does not act as a barrier layer due to its general absence in other boreholes and the thickness of the horizon. The subsurface conditions are characterised as largely reflecting a medium susceptibility of medium to large-size sinkhole and subsidence formation with respect to ingress water, i.e. Inherent Hazard Class 3/4.

iv.

Hazard Classification (with Groundwater Drawdown as a Triggering Mechanism) Groundwater is anticipated at a depth of 145m or an elevation of 1424m AMSL. The groundwater level is therefore confirmed within dolomite bedrock within all the boreholes drilled.

24

The susceptibility to sinkhole and subsidence formation with respect to groundwater level drawdown is therefore considered to be low as the groundwater level is within dolomite bedrock. Consequently the study area is designated as an Inherent Hazard Class 1 with respect groundwater level drawdown. v.

Impact of Lowering of the Groundwater Level/Base of Erosion on the Action of Water Ingress Dolomite Hazard Zone 1: In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged i.e. IHC 7/8. Dolomite Hazard Class 2: In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged i.e. IHC 3/4.

vi.

Composite Hazard Classification Dolomite Hazard Zone 1: The composite hazard classification is Inherent Hazard Class 7/8//1, defined as an area characterised as largely reflecting a high susceptibility of large to very large-size sinkhole and subsidence formation with respect to ingress water and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. In the event that the groundwater level is drawdown significantly (6m or more), the hazard classification with respect to ingress of water remains the same i.e. Inherent Hazard Class 7/8. Dolomite Hazard Zone 2: The composite hazard classification is Inherent Hazard Class 3/4//1, defined as an area characterised as largely reflecting a medium susceptibility of medium to large-size sinkhole and subsidence formation with respect to ingress water and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. In the event that the groundwater level is drawdown significantly (6m or more), the hazard classification with respect to ingress of water remains the same i.e. Inherent Hazard Class 3/4.

d) Design Hazard Class for rehabilitation works Dolomite Hazard Zone 1: The provisional Design Hazard Class is 7. Dolomite Hazard Zone 2: The provisional Design Hazard Class is 3.

8.

CONCLUSIONS 8.1.

Results of the borehole drilling programme The boreholes drilled on site intercept a thin (2m to 3m) horizon of colluvium in Boreholes BH7 and BH7A, whilst this horizon is absent in all other boreholes. A thin (1m to 8m) horizon of chert residuum (fines subordinate) with the exception of Borehole 3737 S21/02 located to the south of the sinkhole area where a thick (30m) horizon of chert residuum is encountered. A thin (1m) horizon of residual syenite is encountered in Borehole BH7A to the east of the sinkhole. A thin to intermediate thick horizon of dolomite residuum-ferroan soils (2m to 14m) occurs from near surface to intermediate depth. This is underlain by thin to very thick horizons of dolomite residuum-manganiferous soils/wad (9m to >27m) from near surface to great depth in boreholes located to the south of the sinkhole area.

25

Disseminated voids are intercepted within the blanketing layer dolomite residuum (manganiferous soils) in four of the eleven boreholes drilled, all located to the south of the sinkhole. Sample and air losses are recorded in the chert residuum, dolomite residuum, highly weathered dolomite rock and in dolomite bedrock. Disseminated receptacles and cavernous conditions are recorded above the OWL within dolomite residuum (wad) in Boreholes 3737 S21/01 (between 6m to 12m), 3737S21/02 (>46m), 3737 S21/07 (between 9m to 18m) and 3737 S21/08 (between 12m and 18m). All these boreholes are located to the south of the sinkhole area. Dolomite bedrock is intercepted at 24m (3737 S21/03 and 04) to the west of the sinkholes; at 9m to 14m (3737 S21/05 and 06) to the north of the sinkholes; at 14m to 25m (3737 S21/01 and 09) to the east of the sinkholes; at 29m to >46m (3737 S21/02, /07 and /08) to the south of the sinkholes. The dolomite groundwater level at 145m (or 1424m AMSL) is within dolomite bedrock. A 20m diameter size sinkhole extending to a depth of approximately 4m and a second sinkhole (6m in diameter extending to a depth of approximately 6m) occurred within a radius of 30m on the 90 degree bend of a 1m diameter water supply steel pipe, located above ground and approximately 170m west of the Ada Reservoirs, on 31 July 2016. It is anticipated that subsurface erosion of highly erodible dolomite residuum (wad) material was initially triggered by a leak on the water supply line over an extended period of time. As a result of the initial leak, the water supply lines settled and failure of the wet services occurred, leading to the formation of a very large and large sinkhole. Water expelled under high pressure from a leaking water line will typically cause the erosion of highly erodible and compressible dolomite residuum (wad) and the formation of a cavity or erosion tunnel. Disseminated voids were intercepted in four of the nine boreholes during the drilling programme. An additional contributing factor is run-off surface water originating during heavy rains entering the sinkhole area further eroding subsurface materials. It should be noted that continuous and prolonged leaking wet services and surface water run-off water into the highly susceptible sinkhole areas will aggravate the situation, including the enlargement of the sinkhole laterally and vertically potentially affecting a larger portion of the water supply line. 8.2.

Dolomite Hazard Characterisation and Suitability of the site for the current land use The geotechnical data gathered during this investigation permits the dolomite hazard characterisation of the site in accordance with SANS 1936 (2012). Accordingly, the following Inherent Hazard Class areas had been identified on the site: Dolomite Hazard Zone 1: Area characterised as largely reflecting a high susceptibility of large to very large-size sinkhole and subsidence formation with respect to ingress water, and a low susceptibility of all-large size sinkhole and subsidence formation with respect to groundwater drawdown. Composite Inherent Hazard Class 7/8//1. In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged. Dolomite Hazard Zone 2: Area characterised as largely reflecting a medium susceptibility of medium to large-size sinkhole and subsidence formation with respect to ingress water, and a low susceptibility of all-size sinkhole and subsidence formation with respect to groundwater drawdown. Composite Inherent Hazard Class 3/4//1. In the event of groundwater level drawdown the Inherent Hazard Class remains unchanged.

26

9.

RECOMMENDATIONS The measures and recommendations outlined below are aimed at reducing the likelihood of a reoccurrence of a sinkhole or a subsidence in the affected area. Recommendations are based on experience gained during the investigation and rehabilitation of more than 100 instability features (sinkholes and subsidences) in the Ekurhuleni Metropolitan Municipality (EMM), City of Tshwane and West Rand District Area. 9.1.

Rehabilitation of Sinkhole Areas and related wet services Based on the problematic profile intercepted during these investigations, improvement of the subsurface conditions will require the use of the Inverted Filter Method and a Compaction Grouting (i.e. backfilling) programme to rehabilitate the affected area, including two sinkholes, a larger subsidence area and erosion voids at depth. The proposed rehabilitation area is displayed on Drawing VGI3737 S21/2. All services, including wet services (water) and electrical will need to be located prior to commencement of works. These services will need to be relocated beyond the area of works. 9.1.1. Safety precautions and site establishment The following procedures are recommended: 

Construct a temporary 1,8m high diamond fence around the proposed rehabilitation site and construction camp. Provide lockable access gates for each. The site shall be safeguarded by a 24-hour security service.



Establish base camp and security services.



Provide temporary chemical toilets for workers.



Determine the locality, depth, diameter and material type of all affected services (i.e. water and possibly electrical). Relocate affected services to outside the area of works if practically feasible.



Obtain wet services layout plans from Merafong Local Municipality for the proposed rehabilitation areas. Obtain way leaves from the various service providers for the affected area, before any work commence.



Ensure that the safety file is in order before any work commences. The Safety Officer will be permanently on site during the period of rehabilitation. Personnel and construction crews are to be informed of the hazardous conditions pertaining to working in and around the sinkhole areas.



The personnel and construction crews are to be made aware of the need to report new surface cracks, voids, any ground movement or sudden changes in soil conditions. If such features are reported, the Resident Engineer should immediately clear the site and inspect the conditions. If uncertain of the implications of the reported conditions the Engineer should request inspection by a dolomite specialist.



Personnel and construction workers executing work in and around the sinkhole are to be strapped in harnesses and safety ropes secured away from the sinkhole and excavation area or suspended from a crane or excavator parked in a safe position away from the feature.

27 

All mandatory safety procedures and requirements pertaining to working in excavations are to be applied. Use of independently attached harnesses is mandatory when workers are in the excavation. The Engineer is to ascertain the need for shoring and identify those areas of the Works where shoring is to be provided to ensure the safety of workers and equipment. An inspection of the bulk excavated area shall be conducted by the Engineer each day before workers enters the excavated area as part of the safety precautionary measures.



Place a soil berm around proposed area of rehabilitation to the north and east to prevent any surface runoff water entering the sinkhole area.

9.1.2. Soil improvements Improvement of subsoil conditions by means of the Inverted Filter Method and replacement of sub-surface services (water and possibly electrical) Improvement of the affected area (sinkholes and subsidence area) will require the use of the Inverted Filter Method. The recommended rehabilitation procedure will be to remove all material to a maximum depth of 6m below ground surface, covering a surface area of 40m by 40m. Over-excavate a 20m by 20m and 5m by 5m area, with the two sinkholes central to the over-excavated areas down to a depth of 12m. Benching may be required at a depth of 6m creating a 2m wide terrace. Create a lip or bench 1m wide and 1m deep around the margin of the final excavation to create a protective umbrella over the contact zone between made earth and the virgin soil during backfilling. Excavation should start from the north-east of the sinkhole area extending towards the south. Excavation slopes from ground surface to 8m depth should be 1:1 (V:H) and from 8m to 12m vertical. An access ramp should be provided from the north-east. All mandatory safety procedures and requirements pertaining to working in excavations are to be applied as outlined in Section 9.1.1. The Contractor’s Engineer must be satisfied with safety at all times. Backfilling of the excavation should involve blocking of the throats (if intercepted) with large boulders and stones filled with soilcrete compacted with an impact roller in 1m lifts up to 6m from ground surface. This should be followed by backfilling with low plasticity material: Cobbles/sandy gravels in 300mm thick layers up to a depth of 1,5m below ground surface (the first three layers should be stabilized with 3% cement), followed with silty/gravely sand (G5-quality material) in 150mm thick layers up to ground level. Compaction should at least be 95% of Modified AASHTO maximum dry density at optimum moisture. The upper two 150mm layers should be compacted at least to 98% of Modified AASHTO maximum dry density at optimum moisture content and extending 1m beyond the excavated area. Provision should be made in the bill of quantities for testing of layer works to determine if the required compaction has been reached. Stabilisation of side walls closest to structures must be provided to prevent damage. All subsurface services (HDPE pipes, butt welded) can be placed during the backfilling of the excavation. Reconnect all new wet services. Landscape the area to promote surface water run-off.

28

Improvement of subsoil conditions by means of a backfill grouting programme Based on the subsurface conditions encountered during the drilling programme around the sinkholes, the dolomite bedrock is dipping at a steep gradient towards the south. Dolomite bedrock is encountered at a depth of 25m to >46m to the south of the sinkholes. Disseminated receptacles and cavernous conditions are recorded above the OWL within dolomite residuum (wad) in Boreholes 3737 S21/01 (between 6m to 12m), 3737S21/02 (>46m), 3737 S21/07 (between 9m to 18m) and 3737 S21/08 (between 12m and 18m). All these boreholes are located to the south of the sinkhole area. Based on the above findings, improvement of the subsurface conditions will require a backfill grouting programme to fill erosion voids or zones of wet, very soft dolomite residuum (wad), intercepted in boreholes to the south of the sinkhole area. Multi stage grouting should be planned at a series of primary and secondary points, with the possibility of a tertiary stage if found necessary. All the primary points will be drilled first on a 3m grid spacing, followed by the secondary points some days later. The secondary points will be positioned midway between the primary points. The grouting of each point will be carried out from the bottom up, which is referred to as upstage grouting, or a combination of methods may be required also including downstage grouting. The pumping rates and pressure induced to inject the grout should be selected carefully and monitored throughout the grouting process as excessive pressure will cause fracturing of the overburden resulting in ground heave and potentially more damage. The grouting mixture generally used with a slump of between 25mm and 75mm, does not need to meet any strength requirements as the objective is not to form a structural element in the ground but to backfill voids and compact problematic zones. As a point of departure provision should be made for the following grouting points in the bill of quantities: 26 primary and 20 secondary grouting points extending to a depth of 29m to 50m below ground surface. It should be noted that the above recommended grouting depths are an estimate based on existing borehole information. All the grouting boreholes should be drilled into at least 4m of solid dolomite bedrock. The injection of grout should not exceed 0,1 MPa (or 10 Bar). Provision should be made in the bill of quantities for 1m3 per meter drilling 2MPa strength grout. Clear and landscape (contouring) of the site will be required after completion of the grouting programme. Provision should be made in the bill of quantities for 1 concrete cube test per day. The field report on the grouting programme, should include the applied pressure per meter (bar) and volume of grout (litre/metre) pumped at each grouting point. Also record any voids and their height. Care should be taken during the grouting programme to ensure that no damage is caused to surrounding structures. A crack survey is recommended before grouting work commences (both of the structures and the ground). 9.1.3. Monitoring of rehabilitated area Monitoring of the rehabilitated area is required on a daily basis the first three months after completion of works and thereafter on a monthly basis for the period three to six months after rehabilitation; and after six months on a three month interval to record

29

any stability problems. These visual inspections and observations should be recorded in a log book and signed by the inspecting official. Any deterioration must immediately be reported to the City Engineer for appropriate action. 9.2.

Precautionary measures Water is a triggering mechanism, in the majority of cases, of distress in dolomitic/limestone areas. It is therefore imperative that the concentrated ingress of water into the ground be avoided at all times. SANS 1936, Part 3 (2012): “Design and construction of buildings, structures and infrastructure” is in the public domain. The contents of SANS 1936 should be applied except where more stringent requirements are specifically required by the local authority. In accordance with SANS 1936, Part 3 (2012) all subsurface wet services should comprise of HDPE butt-welded pipes. It is recommended that all subsurface water lines (including manholes) around the affected sinkhole area be replaced with HDPE butt-welded pipes as specified by SANS 1936 Part 3 for areas underlain by dolomite. All subsurface stormwater pipes should also be HDPE buttwelded pipes.

9.3.

Stormwater Management Absolutely no ponding of water should be permitted on the site. An essential component of risk management is ensuring that stormwater is efficiently and effectively removed from the proximity of infrastructure and safely distributed or deposited into either the municipal stormwater system or natural river courses. Where stormwater canals are proposed on the site and carry large quantities of water, the canal should be lined.

9.4.

Monitoring Actions Immediate Monitoring Actions before rehabilitation takes place The procedure in the designation of monitoring activities and frequencies on site is described in Section 5. During the period before rehabilitation the designation for the site is as follows: Inherent HazardClass Area

Monitoring Designation

Dolomite Stability Zone 1: 7/8//1

(ABC)Daily DDaily E0

Dolomite Stability Zone 2: 3/4//1

(ABC)Daily DDaily E0 Explanation

A B

Visual inspections of ground, structures and above ground infrastructure (e.g. roads, storm water canals, ditches) daily. Visual inspection of stormwater systems crossing the site for blockages daily.

C

Testing of wet-services for leaks daily.

D

Monitoring of structures and ground levels required daily.

30

* Although not applicable to the site, any substantial lowering of the groundwater level may induce ground subsidence events in the general area. The groundwater level should be maintained within natural seasonal fluctuation limits i.e. dewatering resulting from the artificial drawdown of the groundwater level cannot be permitted. E The Department of Water Affairs should be notified if monitoring detects groundwater level drawdown beyond that which represents natural seasonal fluctuations. In such an instance the Department of Water Affairs should immediately be requested to investigate the cause and every effort made to cease further lowering of the groundwater level. Long Term Monitoring Actions The long term designation for the site is presented as follows: Inherent HazardClass Area

Monitoring Designation

Dolomite Stability Zone 1: 7/8//1

(ABC)3 D3 E0

Dolomite Stability Zone 2: 3/4//1

(ABC)3 D3 E0 Explanation

A B

Visual inspections of ground, structures and above ground infrastructure (e.g. roads, water supply lines, storm water canals, ditches) monthly. Visual inspection of stormwater systems crossing the site for blockages monthly.

C

Testing of wet-services for leaks quarterly.

D

Monitoring of structures and ground levels required quarterly. * Although not applicable to the site, any substantial lowering of the groundwater level may induce ground subsidence events in the general area. The groundwater level should be maintained within natural seasonal fluctuation limits i.e. dewatering resulting from the artificial drawdown of the groundwater level cannot be permitted.

E The Department of Water Affairs should be notified if monitoring detects groundwater level drawdown beyond that which represents natural seasonal fluctuations. In such an instance the Department of Water Affairs should immediately be requested to investigate the cause and every effort made to cease further lowering of the groundwater level. The rehabilitation works proposed are only an element of the mitigation and risk management measures required on the site. Ongoing risk management (SANS 1936 (2012) Part 4) is essential. 9.5.

Database of ground movement and stability conditions It is recommended that the sinkholes that occurred on the Adatta Pipeline at the Ada Reservoirs and all repair, upgrade and soil improvement work conducted is added to the Municipal’s database of ground movement events. Detailed historical records of this nature are most useful in developing a clearer perspective on the stability situation on site and management of a pro-active maintenance strategy.

31

10.

GENERAL These findings are based upon our interpretation of the data recovered during these investigations. While every effort has been made, within the limits of the project budget, time and present-day insight, to determine overall ground conditions on this site, poorer sub-areas may have been missed.

VGIconsult Projects (Pty.) Ltd. P.O. BOX 604 FOURWAYS 2055 TEL : (011) 469 0854 FAX : (011) 469 0961 FAX : 0866892847 e-mail: [email protected]

TABLES SUMMARISED BOREHOLE INFORMATION AND INHERENT HAZARD CHARACTERISATION

TABLE 1

TABLE 1: BOREHOLE DATA AND DOLOMITE INHERENT HAZARD CHARACTERISATION

Chert Residuu m

Hazard Characterisation Residual Syenite Highly weathered dolomite

Manganiferrous Soils

Fines subordinate

Dolomite Residuum

Ferroan Soils

BH No. (Collar Elev. m AMSL)

Colluvium

Blanketing Layer (m) – (m)

(m) – (m)

Dolomite Bedrock (m) – (m) (m AMSL)

3737 S21/1 (1570)

-

0-1

1-6

6-15

-

15-25

25-30 (1545)

3737 S21/2 (1569)

-

0-30

-

30-46

-

-

>46 (<1523)

3737 S21/3 (1568)

-

0-4

13-15

-

-

4-13 15-24

24-30 (1544)

3737 S21/4 (1568)

-

0-1

-

-

-

1-24

24-30 (1544)

3737 S21/5 (1568)

-

-

-

-

-

0-14

14-20 (1554)

3737 S21/6 (1570)

-

0-1

1-3

-

-

3-9

9-15 (1561)

3737 S21/7 (1568)

-

0-8

-

8-21

21-29

29-35 (1539)

3737 S21/8 (1568)

-

0-5

5-7 18-23

7-18

23-34

34-40 (1534)

3737 S21/9 (1570)

-

0-2

-

-

-

2-14

14-20 (1556)

BH 7 (1565)

0-3

-

-

3->30

-

-

>30 (<1535)

BH 7A (1568)

0-2

-

2-16 17->30

-

16-17

-

>30 (<1538)

-

-

Groundwater Rest Level (m) (m AMSL) OWL (m AMSL) Dry 146 1424 Dry 145 1424 Dry 144 1424 Dry 144 1424 Dry 144 1424 Dry 146 1424 Dry 144 1424 Dry 144 1424 Dry 146 1424 Dry 141 1424 Dry 144 1424

Ingress Water

Groundwater Drawdown

Air & Sample Losses

Cavity

(m)–(m)

(m)–(m)

Subsidence Formation

Sinkhole Formation

6-30

6-12

High

High

Low

Low

6-9 18-20 30-46

>46

High

High

Low

Low

-

-

Medium

Medium

Low

Low

-

-

Medium

Medium

Low

Low

-

-

Medium

Medium

Low

Low

-

-

Medium

Medium

Low

Low

2-4 5-22

9-18

High

High

Low

Low

3-19

12-18

High

High

Low

Low

0-2

-

Medium

Medium

Low

Low

15-30

-

High

High

Low

Low

-

-

Medium

Medium

Low

Low

Subsidence Formation

Sinkhole Formation

FIGURES LOCALITY PLAN TOPOGRAPHICAL SHEET LEGEND SITE LAYOUT WITH GROUND ELEVATION CONTOURS REGIONAL GEOLOGY GEOLOGY LEGEND REGIONAL GEOHYDROLOGY

FIGURE 1 FIGURE 1A FIGURE 2 FIGURE 3 FIGURE 3A FIGURE 4

DRAWINGS RESIDUAL GRAVITY, BOREHOLE POSITIONS AND DOLOMITE HAZARD ZONATION

VGI3737 S21/1

PROPOSED REHABILITATION AREA

VGI3737 S21/2

APPENDICES GRAVITY SURVEY REPORT APPENDIX 1 BOREHOLE PROFILES APPENDIX 2

APPENDIX 1 GRAVITY SURVEY REPORT

APPENDIX 2 BOREHOLE PROFILES