Note On Gravel

Use of natural gravel in sub base, base and patchwork Its adverse effects and some suggestive measures During rainy season, if the rainfall is good everybody expects good yield of crops and inevitably bad roads. If there are no or few rains in a particular rainy season, better condition of roads is assured. Formation of pot holes, sinking of roads, bad condition of berms and gravel roads in slushy is a common sight during rains. Good rains and bad roads go hand in hand in our state. Use of highly plastic gravel in sub base and WBM layers is the main cause of this statement because gravel attracts, retains water and becomes soft when water content gets higher. Such condition initiates and accelerates the deterioration of road condition. Gravels available in almost all the approved (and unotherised) quarries in our state do not satisfy the stipulated requirements for gravel or moorum specified in APSS or MORT & H (formerly MOST) standard specifications. Most of the so called gravels are highly plastic in nature and not suitable for use in a) In sub base construction, b) WBM patch work, c) as blindage in WBM and d) in gravel berms. The test results on thousands of gravel samples done in Soil Testing Cell Vijayawada and Road Research Station Hyderabad clearly confirm this. The so called gravel from approved quarries is nothing but red earth and it is suitable, at best, for sub grade or shoulder construction. Due to ignorance or lack of willingness to learn from failures, AP Highway Engineers are accustomed to using the so called gravel extensively in road construction. Digging pits in the damaged road stretches revealed the presence of wet gravel in WBM layers and in gravel base in almost all the cases. Also, in many cases bottom of BT surface is wet. The wetness below BT surface weakens BT layer.

Reasons for damage due to use of gravel Gravel due to presence of negative charged particles in gravelly soils, they attract, absorb and retain water.. Water molecules and negative charged particles in gravel are attracted to each other developing plasticity. Gravel present in WBM layers (or WBM patchwork) or in gravel base attracts water through a) ingress of water through berms and slopes, b) increased capillary rise due to negative charged particles (in sub grade soils, sub base and blindage gravel), c) high water table condition, d) seepage and e) Water retained during construction and curing. Once the water content in gravel exceeds plastic limit value, the moist gavel turns to plastic state. In sub base: Gravel in plastic state in sub base leads to plastic deformation failure because a material in plastic state is not capable of withstanding additional load other than its self weight. Development of plastic spots in gravel base results in sinking of BT roads at many places. Most Highway Engineers are of the view that such sinking is due to poor sub grade or embankment soils. In fact the sinking is due to the presence of gravel base as sub base and development of soft plastic spots in gravel after absorption of water. Similar sinking is observed where gravel base was laid over existing WBM or BT surfaces and filling deep pot holes with gravel prior to renewals. It is very difficult and costly to repair such roads when some soft spots are developed in gravel base. In base and patchwork: Water reaches where WBM is loose or gravel is more and Gravel turns into plastic state. It reduces internal friction among coarse aggregates and acts as a lubricant for them. It allows coarse aggregates to rotate or slide laterally under the impact of heavy loads. Such phenomena results in formation of pot holes and damage to freshly laid BT layers. Extensive pothole forming and development of undulations is due to movement of coarse aggregates enabled by wet and soft gravel in between them. It has to be noted here that some roads done with gravel base and WBM with gravel blindage are in reasonably good condition. As long as water is prevented from entering into sub base and traffic intensity is not heavy, the road condition will be satisfactory even if some unsuitable materials are used. It is very essential for every road Engineer to understand the requirements of sub base, base and berms as per standard specifications. He shall be equally thorough and knowledgeable about the quality and suitability of locally available materials.

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Sub base Requirements IRC: 37-2001 specifies a minimum CBR of 30% for roads with design traffic intensity more than 2msa and a minimum CBR of 20% for roads with design traffic intensity less than 2msa. For all R&B roads it is preferable to adopt a minimum CBR of 30% for sub base. MORT & H standard specification for road and bridge works (401) specifies that the mix of materials provided in sub base shall satisfy the following requirements: 1) It shall produce a minimum 4 days soaked CBR value specified 2) It shall satisfy the specified gradation or gradings given in table 400-1and 400-2 of MORT & H specifications for road and bridge works. 3) The material in the mix shall have 10 percent fines value of 50 KN or more. 4) The material passing 425 microns in the mix shall have liquid limit and plasticity index not more than 25% and 6 % respectively Requirement 4 is specified to prevent the use of water absorbing materials like the gravels being used. Higher plasticity index indicates higher water absorbing capacity which effects durability. Sub base needs the strength to withstand heavy compressive stresses induced under heavy traffic under adverse climatic conditions. The sub base shall neither deform nor get crushed under the impact of heavy loads. The sub base layer provided shall not be treated as sub base (and part of the road crust) if all the above requirements are not satisfied. In such situation, the net effective crust thickness provided is the thickness of metal and BT layers only.

Table 400-1. Grading for Close-Graded Granular Sub-Base Materials IS Sieve Designation 75.0 mm 53.0 mm 26.5 mm 9.50 mm 4.75 mm 2.36mm 0.425 mm 0.075 mm CBR Value(minimum)

Per cent by weight passing the IS sieve Grading I Grading II Grading III 100 --80-100 100 -55-90 70-100 100 35-65 50-80 65-95 25-55 40-65 50-80 20-40 30-50 40-65 10-25 15-25 20-35 3-10 3-10 3-10 30 25 20

Table 400-2. Grading for Coarse Graded Granular Sub-Base Materials IS Sieve Designation 75.0 mm 53.0 mm 26.5 mm 9.50 mm 4.75 mm 2.36 mm 0.425 mm 0.075 mm CBR Value (Minimum)

Per cent by weight passing the IS sieve Grading I Grading II Grading III 100 --100 55-75 50-80 100 10-30

15-35

25-45

< 10 30

< 10 25

< 10 20

In the MORT & H standard data 2003 Granular Sub Base courses furnished here, type of materials is not mentioned. Selection of materials is to be done based on the sub base requirements mentioned in clause 401, availability of materials and economy of construction. Locally available gravel

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from approved quarries may be one of the cheapest road construction materials but it is not satisfying any of the sub base requirements. In the above data, gravel is not mentioned.

Close Graded Granular Sub Base Materials 384 cum loose compacted to 300cum ( 128mm to 100mm )

Grading

I

53mm to 9.5mm =50%

9.5mm to 2.36mm =20%

II

26.5mm to 9.5mm =35%

9.5mm to 2.6mm =25%

III

9.5mm to 4.75mm =35%

4.75mm to 2.36mm =12.5%

CBR 2.36mm below =30% 2.36mm below =40% 2.36mm below =52.5mm%

30% 25% 20%

Coarse Graded Granular Sub Base table Materials 384 cum loose compacted to 300cum ( 128mm to 100mm )

Grading

CBR

I

53mm to 26.5mm =35%

26.5mm to 4.75mm =45%

2.36mm below =20%

30%

II

26.5mm to 4.75mm =75%

__

2.36mm Below =25%

25%

III

9.5mm to 4.75mm =66%

__

2.36mm below =34mm%

20%

Considering all the sub base requirements and including gravel fraction also, some combinations were tested for close graded GSB mixes with available materials. These combinations are based on practicability in execution and economy in estimation.

Tested Mixes of gravel associated GSB for Sub base Type of metal to be blended

CBR Required

Natural gravel

Metal

Stone dust or sand

Grade III

30%

45%

20%

35%

HBGranite

20%

50%

10%

40%

The above mixes are expected to satisfy the specified requirements of close graded GSB. The proportions of gravel, metal and stone dust are finalised considering the likely variations in mixing. They are tried on some roads and found to be satisfactory when there are no rains during execution. However, during rainy season, some problems arose while executing the above combinations. If it rains after collection of materials or after spreading of materials but before the completion of compaction, gravel becomes slushy and the materials are to be replaced. Hence extra care is to be taken while executing the above mixes also. Compaction of materials collected shall be completed before any rain. One greater disadvantage observed with these mixes is the poor drainage due to the presence of gravelly soil. Alternate internal drainage arrangements are to be provided if these mixes are opted.

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GSB mixes without any gravel Surface Drainage Measures: IRC; 37 states, “The performance of a pavement can be seriously affected if adequate drainage measures to prevent accumulation of moisture in the pavement structure are not taken. Some of the measures to guard against poor drainage conditions are maintenance of transverse section to good shape to reasonable cross fall to facilitate quick run-off of surface water and provision of appropriate surface and sub-surface drains where necessary. When the traditional granular construction is provided on a relatively low permeability sub grade, the granular sub base should be extended over the entire formation width in order to drain the pavement structural section. Care should be exercised to ensure that its exposed ends do not get covered by the embankment soil.” Such provision of drainage measures is not seen in any estimate sanctioned in our Department. Only in works taken up by NHAI drainage measures are observed. Now in new CRF road works the drainage aspect is considered. In view of the disadvantages with gravel base or gravel associated GSB, it is preferable to go for GSB mixes without any gravel for all roads to obtain the best results. However, combinations of locally available and comparatively cheaper materials may be adopted as far as possible. GSB layers without any gravel act as excellent drainage layers and they are to be extended to the outer edge of formation as stipulated in IRC: 37. Some GSB combinations are noted below: 1) Mix of hard blasted granite fractions such as various sizrs of metal and dust to satisfy the specified grading. 2) Crushed chatru (rock overburden) and about 30% of granite stones to satisfy GSB requirements. 3) Mix of different sizes of crushed HOG metal. 4) WBM with cheaper materials such as HOG in GSB Combinations 1 and 2 are being adopted in 4-laneing works taken up by National Highway Authority of India. They give excellent strength, act as drainage layer and executed under any condition. Where the sub base thickness required is very high, combination of lower sub base(satisfying grading III) and upper sub base (satisfying Grading I) provided the cost of lower sub base is considerably cheaper. A combination adopted for CRF work is shown here. Here, no gravel or such plastic materials which absorb moisture is provided. Also, the layers extended up to the edge of formation to act as drainage layer.

LSB

Upper sub base with grade III metal, 12.5mmand 6mm chips with stone dust to satisfy grading III of table 400-2

LSB

Lower sub base with 6mm chips and stone dust to satisfy grading III of table 400-2 (LSB)

Sub Bases for low traffic roads The following combinations may be adopted low traffic roads and by-lanes. 1) Sub base with crushed stone dust 128mm loose compacted to 100mm thick layers. 2) Gravel sand mix in 1:1 proportion 132mm loose compacted to 100mm thick layers.

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For shoulders or berms, it is preferable to adopt gravel and sand mix in 2:1 proportion 132mm loose compacted to 100mm thick layers.

Use of natural gravel in WBM layers and in patchwork In WBM construction, using 25% quantity of only gravel in its natural form is causing many early failures.

Disadvantages of using gravel in its natural form in WBM a) Under heavy vehicular traffic, extensive damage of the road will occur when the WBM is fully soaked due to ingress of water. b) Even under low traffic conditions, undulations will develop when the WBM becomes soaked during rainy season as presence of moisture in WBM reduces the bond with BT surfacing. Moisture below BT surfacing cause lstripping of bitumen. c) Sealing pot holes become very difficult once damage is started. New pot holes sprout up frequently adjacent to the recently sealed ones. d) Treatments done even after a few years are likely to be damaged due to presence of gravel in WBM. Ex: Failure of BM + SDBC done over previously (5 to 6 years back) widened carriageway

Stipulations of MORT & H standard specifications for road and bridge works for stone screenings and binder 404.2.6. Screenings: Screenings to fill voids in the coarse aggregate shall generally consist of the same material as the coarse aggregate. However, where permitted, predominantly non-plastic material such as moorum or gravel (other than rounded river borne material) may be used for this purpose provided liquid limit and plasticity index of such material are below 20 and 6 respectively and fraction passing 75 micron sieve does not exceed 10 percent. Screenings shall conform to the grading set forth in Table 400-8. The consolidated details of quantity of screenings required for various grades of stone aggregates are given in Table 400-9. The table also gives the quantities of materials (loose) required for 10 sq. m for sub-base/base compacted thickness of 100/75 mm. The use of screenings shall be omitted in the case of soft aggregates such as brick metal, kankar, laterites, etc. as they are likely to get crushed to a certain extent under rollers.

Table 400-8, Grading for Screenings Grading classification

Type A

Type B

Size of screenings

13.2 mm

11.2 mm

IS Sieve Designation

Percent by weight passing IS Sieve.

13.2 mm

100

11.2 mm

95-100

5.6 mm

15-35

180 micron

0-10

11.2 mm

100

5.6 mm

90-100

180 micron

15-35

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404.2.7. Binding material: Binding material to be used for water bound macadam as a filler material meant for preventing raveling, shall comprise of a suitable material approved by the Engineer having a Plasticity Index (PI) value of less than 6 as determined in accordance with IS: 2720 (Part-5). The quantity of binding material where it is to be used will depend on the type of screenings. Generally, the quantity required for 75 mm compacted thickness of water bound macadam will be 0.060.09 Cum/10 Sq. m and 0.08-0.10 m3/10 m2 for 100 mm compacted thickness. The above-mentioned quantities should be taken as a guide only, for estimation of quantities for construction etc. Table: 400-9. Approximate Quantities of Coarse Aggregates and Screenings required for 100/75 mm Compacted Thickness of Water Bound Macadam (WBM)

Sub Base/Base Course for 10 Sqm Area. Screenings. Crushable Type such as Moorum or Gravel.

Stone Screening. Classification

Size Range

Compacted Thickness

Loose Qty.

Grading 1

90mm to 45mm

100 mm

1.21 to 1.43 Cum

Type A 13.2 mm

For WBM Subbase / base course (loose quantity) 0.27 to 0.3 Cum

Grading 2

63mm to 45mm

75 mm

0.91 to 1.07Cum

Type A 13.2 mm

0.12 to 0.15Cum

-do-

-do-

-do-

-do-

-do-

Type B 11.2 mm

0.20 to 0.22Cum

-do-

-do-

Grading 3

53mm to 22.4mm

75 mm

-do-

-do-

-do-

-do-

Grading Classification & Size.

Grading Classification & Size. Not uniform

0.18 to 0.21Cum

Loose Qty. 0.3 to0.32 Cum 0.22 to0.24 Cum

Materials that satisfy screenings and binder: 6mm chips confirm to the requirements of type A screenings. Stone dust satisfies the type B screenings and binder

Quantities of aggregates, stone screenings and binder for WBM (MORT&H data 2003 Item

Coarse Aggregates

Type A screenings(6mm)

Type B screenings(dust)

Binder(dust)

Grad I WBM

1.21 cum

0.27 cum

nil

0.08 cum

Grade II WBM with type A screenings

0.91 cum

0.12 cum

nil

0.06 cum

Grade II WBM with type B screenings

0.91 cum

nil

0.20 cum

0.06 cum

Grade III WBM

0.91 cum

nil

0.18 cum

0.06 cum

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From the above data, material quantities for different WBM layers are: Description

Type of screenings

metal

6mm chips

Stone dust

WBM with Grade I

Type A (6mm)

1.21 cum

0.27 cum

0.08 cum

WBM with Grade II

Type A (6mm)

0.91 cum

0.12 cum

0.06 cum

WBM with Grade II

Type B (stone dust)

0.91 cum

0.20 cum

0.06 cum

WBM with Grade III

Type B (stone dust)

0.91 cum

0.18 cum

0.06 cum

WBM works done using stone dust instead of gravel are giving excellent performance in Vijayawada Municipal Corporation. Similarly, works done in Bhimavaram Division are quite satisfactory and it is the best way to improve the durability of road works. Even pot hole filling with metal and dust followed by BT patchwork is really good. Many other Departments are also shifting from using gravel to stone screenings. For works where there is no immediate BT proposal, WBM with gravel and sand or stone dust mixed in 1:1 or 1:2 proportion may be used. Here the mixing of sand or stone dust controls the harmful effects of plasticity but the capillary transmission of water is not prevented. Blending of stone dust is comparatively better than blending with sand. It gives best results if WBM is done without any gravel for all the layers. In some cases it is not possible and at least the top layer shall be done with stone screenings to keep the bottom of BT surface dry all the time. For controlling plasticity of gravel, a quantity of stone dust or sand 30 to 40% is necessary in laboratory tests. In field it is not possible to mix in the same proportion as in laboratory. Hence, blending of gravel with stone dust or sand is preferable in 1:2 proportions out of the 25 % blindage quantity. Pothole filling: Pot holes are to be filled with metal and dust and the top is to be covered with BT patchwork. This type of patchwork was proved to be a great success for a few roads in Bhimavaram Division. By this method no new pot holes will form adjacent to patches done as the dust present in pot hole filled absorbs moisture around the patchwork done. It is very economical when compared with semi grout patchwork for bigger pot holes. Berms: Good and durable berms are very essential for the durability of the carriageway and for the safety to road users. It is preferable to go for granular berms with gravel and sand proportioned in 2:1 ratio. Such mixes are likely to produce a CBR of 15% and resist deformation. It is always preferable to provide these granular berms up to the edge of road formation. Present practice of doing berms with unsuitable soils, though cheaper, do not serve any purpose. MORT&H Standard Specifications for road and bridge works clearly specifies a minimum maximum dry density of 1.75 g/cc for the material to be used in shoulders. It also specifies that no borrow pits are dug within 10m from toe of embankment as such digging of pits will effect the stability of formation slopes.

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A comparative study of advantages and disadvantages of WBM with gravel as blindage and WBM with stone screenings is presented hereunder: Item

WBM with gravel

WBM with screenings

Execution

Very fast and easy

Needs more time

cost

2 to 4% cheaper

2 to 4% costlier

Affinity with water

Attracts water from all directions, absorbs and assists capillary rise

No attraction to water. Temporarily absorbs but no retention

Drainage

Poor drainage

Excellent drainage

Internal friction

Reduces internal friction in coarse aggregates

Increases internal friction in coarse aggregates

Lubricant for aggregates

When gravel in WBM turns to plastic state, it acts as a good lubricant for coarse aggregates. It allows the aggregates to rotate, move in lateral direction resulting in formation of potholes and development of undulations. This is the reason for damage of renewals done after the WBM and BT. Even pot hole filling done using gravel creates similar problems for works done subsequently

All these problems can be avoided if we do WBM using screenings.

cohesion

Cohesion of gravel is good enough to hold fine aggregates but it is not sufficient to hold coarse aggregates.

There is no cohesion in screenings. Binding of coarse aggregates is by filling of voids

Durability

poor

Many times better

Not being used in many cases

It is necessary for top layer. The additional cost can be compensated by providing MSS or SDBC instead of WBM+BTSD+SC

Raking up problem is less when thick layer of gravel is laid on top.

Needs regular watering and early sealing of layers. In multy- layered construction careful planning is necessary.

Primer sealing

Maintainance

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Table 1: Test results on Gravel samples collected from approved quarries in West Godavari District, Krishna District, Guntur District and Prakasam District in Andhra Pradesh. Liqui Plasticity Sl % passing d CBR Location of Quarry index No. 75µ sieve limit in % in % in % Kunkudu quarry Km 67/500 of Narasapur – 1 31 36 14 11 Aswaraopet Road Road side quarry Km 91/440 of Narasapur – 2 32 39 17 12 Aswaraopet Road 3 Road side quarry Km 36/500 of E.G.K Road 35 37 17 9 Quarry at Km 26/000 of Eluru – Jangareddy Gudem 4 32 39 16 13 Road Janampeta quarry at Km 11/600 of E.M Road + 5 33 38 16 10 0.200 Km CT 6 Quarry at Km 16/900 of P.K Road + 0.800 Km CT 34 38 16 8 7

34

38

12

12

33

38

17

12

9

Gajjaram quarry at Km 2/508 of P.K Road Arugolanu quarry at Km 2/800 of P.N Road + 3.000Km CT Quarry at Km 135.180 of M-N-K Road

34

41

12

11

10

Quarry at Km 8.30 of SN Road + 0.40 Km CT

34

39

11

12

11

Quarry at Km 8.65 of Y.S.Road

32

37

13

10

12

33

36

16

9

36

37

13

10

34

37

12

11

33

40

15

12

35

41

13

10

36

37

13

13

28 24

47 38

11 16

14 10

30

36

17

9

21 22 23 24

Quarry at Km 11.60 of V.V.Road +0.2 CT Jakkampudi Quarry at Km 10.80 + 0.40 CT 1.0 of VV Road Purushotapatnam Quarry at Km 4.830 V.V.Road Gollanapalli Quarry at Km 23.80 with 2.20 CT to TVV Road Quarry at Veerapanenigudem at Km 35.4 of V.V.Road Ammanabrolu quarry Km 39/700 on Chirala – Ongole Road Dronadula quarry Km 0/0 on DTC road Ongole – Nanadyala Road Km 4/830+1.000 Km CT Podili – Nanadavaram Road Km 0/600+0.800 Km CT Medarimetla quarry Km 0/600 on MN Road Ollapalem – Vemulapadu Road Km 9/800 Quarry Km 12/800 of Kandukuru – Pamarru Road Macherla – Sri Sailam Road at Km 2.500

32 23 36 29

36 36 37 38

17 11 13 18

10 13 14 8

25

Macherla – VP South Road at Km 22.310

33

32

16

8

26

N – R Road at Km 4.90

29

37

14

11

8

13 14 15 16 17 18 19 20

Sd-D.V.Bhavanna Rao SE(R&B)

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