Design Of Earthfill Dams.pdf
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The Design of Earthfill Dams
Danie Badenhorst January 2005 10/13/2005
SANCOLD / University of Stellenbosch 2005
1
Contents of Presentation •
Reasons for earthtfill dam failures
•
Criteria for defensive design
•
Describe sectional layouts of fill dams
•
Soil material characteristics
•
Focus on 4 design aspects: – Compaction of earthfill – Seepage control – Stability of embankment slopes – Slope protection
10/13/2005
SANCOLD / University of Stellenbosch 2005
2
Reasons for earthfill dam failures
•
Overtopping of earthfill embankment due to floods and/ or settlement
•
Piping through embankment, along the bottom outlet most frequent
•
Piping through foundation
•
Slope failure
•
Erosion of slopes due to water waves or stormwater
10/13/2005
SANCOLD / University of Stellenbosch 2005
3
Criteria for design
•
Spillway size and freeboard adequate to accommodate Design Flood in accordance with acceptable risk
•
Provide camber to ensure that long-term crest line is on or above design crest line
•
Seepage control measures to be provided (belts and braces)
•
Stable slopes to be designed
•
Slopes to be protected
10/13/2005
SANCOLD / University of Stellenbosch 2005
4
Design Failure?
10/13/2005
SANCOLD / University of Stellenbosch 2005
5
Cross Section Layouts of Fill Dams
Homogeneous Section
Diaphragm Section 10/13/2005
SANCOLD / University of Stellenbosch 2005
6
Cross Section Layouts of fill dams
Zoned Section
10/13/2005
SANCOLD / University of Stellenbosch 2005
7
Material Properties of Zoned Embankment Dam (PI vers LL)
Core
LL
Shell
10/13/2005
LL
SANCOLD / University of Stellenbosch 2005
PI
PI
8
Material Properties of Zoned Embankment Dam ((rho rhomaks maks/w)
Core
Rho max
W opt
Shell W opt
Rho max
10/13/2005
SANCOLD / University of Stellenbosch 2005
9
Material Properties of Zoned Embankment Dam (PI vers Phi)
Core
PI
PHI triaxle
Shell
PI
10/13/2005
PHI triaxle
SANCOLD / University of Stellenbosch 2005
10
Casacrande Classification
10/13/2005
SANCOLD / University of Stellenbosch 2005
11
Material Properties of Zoned Embankment Dam LL
PI
10/13/2005
LS
K
W opt
MDD
SANCOLD / University of Stellenbosch 2005
PHI shearbox PHI triaxle
Cshearbox C triaxle
12
Zones of an Embankment Dam
10/13/2005
SANCOLD / University of Stellenbosch 2005
13
Compaction of Soil
•
Energy is applied to soil to increase density.
•
Air is expelled and water and soil are retained.
•
Compaction improves strength, decreases permeability and compressibility
•
Compaction of soil to a certain standard (i.t.o. density and water content) not only prevents excessive leakage and failure but also provides the basis for the determination of other characteristics e.g. strength, permeability, settlement and elasticity.
•
By applying compaction to a specific standard a norm is set against the behaviour can be measured.
10/13/2005
SANCOLD / University of Stellenbosch 2005
14
Compaction construction methods
•
Placement in thin 150mm to 600mm after compaction layers
•
Watering and mixing by disc plough are carried out to specified water content
•
Layer thicknesses are best confirmed by constructing test sections and testing throughout the layer.
•
Compaction of soil normally done with static roller
•
Rockfill and sand done by vibratory roller
10/13/2005
SANCOLD / University of Stellenbosch 2005
15
Compaction standards
10/13/2005
SANCOLD / University of Stellenbosch 2005
16
Compaction standards
•
For earth embankment dams the Standard Proctor is the recommended standard to use for the following reasons: – Modified AASHTO compacted earthfill embankments are too rigid, i.e. not elastic or plastic – The possibility of cracking increases rapidly for moisture contents lower than the Mod AASHTO optimum – A lower energy effort (less diesel) is required for the Standard Proctor method.
10/13/2005
SANCOLD / University of Stellenbosch 2005
17
Compaction standards
•
For sand, prevention of crack formation is not the first priority, but the decrease in permeability.
•
Modified AASHTO standard can be used as standard.
•
Well graded sands compact better than non well-graded sands.
10/13/2005
SANCOLD / University of Stellenbosch 2005
18
Compaction: water content variation and effect on geomechanical properties
•
• •
Laboratory compaction test study (volume change) on Driekloof Dam impervious and pervious materials revealed the following: – Impervious material compacted to 0 % to 2 % of optimum significant decrease in shear strength value, minor variation in cohesion and no change in elasticity – Semi-pervious material compacted to –1 % to +2 % of optimum, minor variation in shear strength value, small differences in cohesion but significant changes in elasticity occur. – If the water content is varied beyond the referred limits large changes in mentioned properties were obtained. Core - sealing, therefor elasticity and cohesion are dominant, strength second Shell zones: stability and strength first and elasticity not priority.
10/13/2005
SANCOLD / University of Stellenbosch 2005
19
Compaction: water content variation and density control
% of Maximum Standard Proctor dry density
Compaction Results: Core of Embankment Dam 104 102 100 98 96 94 -2
-1
0
1
2
3
4
Moisture content relative to optimum moisture content (%)
10/13/2005
SANCOLD / University of Stellenbosch 2005
20
Compaction: Quality control during construction
•
Frequency of testing
•
Test section
•
Calibrate Troxler machine
•
Changes in soil in borrow area
•
When placed and compacted layer dries out
•
Compaction in confined areas
10/13/2005
SANCOLD / University of Stellenbosch 2005
21
Compaction: Practices
•
Compaction of filters
•
Bottom outlet
10/13/2005
SANCOLD / University of Stellenbosch 2005
22
Seepage control
•
Seepage from the reservoir takes place through the embankment and foundation materials.
•
Phreatic surface
•
Hydraulic gradient
•
Permeability is the rate of water seepage
10/13/2005
SANCOLD / University of Stellenbosch 2005
23
Seepage control: failure modes •
Piping: Water concentrate and water washed out
•
Excessive hydraulic gradients at the downstream side of the embankment can cause erosion of soil particles under buoyant conditions. Water and solid particles are removed to form boils. In case of granular materials failure can occur due to shear resistance.
10/13/2005
SANCOLD / University of Stellenbosch 2005
24
Seepage control: failure modes
10/13/2005
SANCOLD / University of Stellenbosch 2005
25
Seepage control: Flow rate Q= kiA Q=kiA
10/13/2005
SANCOLD / University of Stellenbosch 2005
26
Seepage Control: Design •
Practice: Provide a number of seepage control measures
•
Defensive design is necessary to accommodate a series of unknown factors which can be as follows: – Unknown geology and foundation materials – Degradation or ageing of seepage control measures – Change in operation method of dam – Non conforming quality control during construction
10/13/2005
SANCOLD / University of Stellenbosch 2005
27
Seepage control measures
10/13/2005
SANCOLD / University of Stellenbosch 2005
28
Seepage control measures
•
Zoning – Dispersive materials or materials susceptible to piping in central zone – CFRD zones zoned to be stable under through flow conditions
10/13/2005
SANCOLD / University of Stellenbosch 2005
29
Seepage control measures
•
Drains – Chimney Drain • Unsaturated conditions in downstream zone • Downstream slope can be made steeper • Can prevent piping • can collapse in case of differential movement preventing failure • if taken high enough can prevent piping in dispersive materials • must be designed to meet permeability and filter criteria
10/13/2005
SANCOLD / University of Stellenbosch 2005
30
Seepage control measures
•
Drains – Collector Drain • Bottom of chimney drain • Collects water from chimney drain • Normally gravel is provided to ensure capacity • At strip drain positions small berms are provided to enable drainage of zones of embankment
10/13/2005
SANCOLD / University of Stellenbosch 2005
31
Seepage control measures
•
Drains – Strip Drain • Connects to chimney drain with the toe drain • Convenient location +-30m apart and drains at 3% slope • Note: not sufficient to drain complete foundation • Drainage, filter criteria to be met
10/13/2005
SANCOLD / University of Stellenbosch 2005
32
Seepage control measures
•
Drains – Blanket Drain • Horizontal drains close to natural ground level • Connects to chimney drain and toe drain • Seepage through the foundation can be intercepted • When chimney drain is omitted, can also drain upstream zone • Drainage, filter criteria to be met
10/13/2005
SANCOLD / University of Stellenbosch 2005
33
Seepage control measures
•
Drains – Toe Drain • Blanket and strip drain waters are canalised to lowest point downstream • Manhole on strip drain junction with V-notch • Stormwater from downstream face to be channelled separately • Rockfill toe drain - 1/3 of height
10/13/2005
SANCOLD / University of Stellenbosch 2005
34
Seepage control measures
•
Drains – Practical considerations • Chimney drain extended into core trench a must for use of dispersive soils and differential movement between embankment zone materials • Collars of filtersand to be provided around bottom outlet and outlet drain to be provided • Thickness of drain -practical considerations • Safety factors regarding theoretical capacity 30 to 500 acceptable to accommodate unknowns
10/13/2005
SANCOLD / University of Stellenbosch 2005
35
Seepage control measures
•
Filters – Piping Criteria • D15 filter/D85 basis = 5 or less • D15 filter/D50 basis < 25 • D15 filter/D50 basis < 5 for sandy silt and clay (D85 of 0,1 to 0,5) • D15 < 0,5 for fine clay (D85 from 0,3 to 0,1) • D15 < 0,3 for fine silts with low cohesion and plasticity (LL<30) • D15 < 0,3 for fine soil (D85 of 0,02)
10/13/2005
SANCOLD / University of Stellenbosch 2005
36
Seepage control measures
•
Filters – Criteria for permeability • 5 < D15 filter / D15 base < 40 • The grain size associated with the 0,075 sieve size based on the washed grading of sand must be less than 5 %
10/13/2005
SANCOLD / University of Stellenbosch 2005
37
Seepage control measures
•
Filters – Uniform criteria – Holes and slots – Inherent stability of filter layer – Dispersive clays • Compaction 98% of Standard Proctor, 0% to 2% above optimum
– Organic material • Less than 2% in filters
10/13/2005
SANCOLD / University of Stellenbosch 2005
38
Seepage control measures: Dispersive clays
10/13/2005
SANCOLD / University of Stellenbosch 2005
39
Seepage control measures
•
Criteria and practices regarding synthetic materials – Stresses in embankments due to movements can cause material to be torn – Construction practices can damage – Therefore access to location must be allowed in design.
10/13/2005
SANCOLD / University of Stellenbosch 2005
40
Seepage control measures
•
Core cut-offs – Extension of core into foundation – Depth of core trench where in-situ soil has the same or better compaction values as the to be placed core. – Extend to rock important. If 80% of depth is covered only 50% of permeability is saved. – Slope of core trench flatter than 1V:1H.
10/13/2005
SANCOLD / University of Stellenbosch 2005
41
Seepage control measures
10/13/2005
SANCOLD / University of Stellenbosch 2005
42
Seepage control measures
•
Grout curtains – provided in rock to approximately water head depth – Spacing as close as 1,25m – Lugeon unit is used to determine the permeability – Lugeon > 3 is normally used as cut-off – GIN method of grouting – Pressure relief wells to be provided with grouting – Tuba Manshet grouting is grouting of alluvium/sand
10/13/2005
SANCOLD / University of Stellenbosch 2005
43
Seepage control measures
10/13/2005
SANCOLD / University of Stellenbosch 2005
44
Seepage control measures
•
Slurry trench cut-off – +600mm wide rectangular curtain below water table connected to rock and core of embankment (Normal excavation is expensive) – Excavation of alluvium is carried out in phases - each phase is filled with bentonite/water mixture called slurry (bentonite expands 20X when wetted) – Excavation of the next phase is carried out below the slurry - filled again with slurry – Purpose of the high density slurry is to keep open the trench and to seal – Slurry is circulated by pump and viscosity of slurry is controlled – After completion the slurry can be replaced from the bottom by concrete using a tremmie pipe – A filter cake developes in the sides of the trench. – Excavation through boulders can be a problem – Special care must be taken regarding the contact between cut-off and core of the embankment
10/13/2005
SANCOLD / University of Stellenbosch 2005
45
Seepage control measures •
Upstream blanket – Impervious layer located in the basin and connected to the core of the embankment – It seals a dam with a pervious foundation – It decreases the seepage through a pervious foundation by lengthening the leakage path. – Design issues: • high hydraulic gradients can enhance piping • clay materials of a blanket can dry out • Filter material can be placed below blanket to prevent piping • The lowest draw down level can be chosen that the blanket is always covered • Geosynthetic clay liner can be used as sealing layer
10/13/2005
SANCOLD / University of Stellenbosch 2005
46
Seepage control measures
•
Treatment of embankment/foundation contact – purpose is to prevent leakage/failure due to piping caused by differential settlement of soil, cracking or below standard compaction – Treatment include: • remove soil with high organic load, roots etc • remove or treat materials not meeting permeability or stability requirements e.g. highly pervious alluvial materials, collapsible soils,sand susceptible to liquefaction or weathered uncompacted rock materials • remove topsoil
10/13/2005
SANCOLD / University of Stellenbosch 2005
47
Seepage control measures •
Treatment of embankment/foundation contact – Treatment include:
• Preparation earthfill dam rock foundations as follows: – Cleaning of joints and seams with water and air jets, removal of loose boulders, sweeping and/or washing if the surface and the infill of openings with a cement mortar – Flattening of steep slopes especially at the river banks to slopes meeting differential settlement criteria of above slopes. Maximum slopes of 1V:1H is recommended. – All uneven surfaces to be filled with mortar or cement to enhance positive compaction. • Special attention to first earthfill layer • Contact below the core must be treated with special care.
10/13/2005
SANCOLD / University of Stellenbosch 2005
48
Seepage control measures
10/13/2005
SANCOLD / University of Stellenbosch 2005
49
Seepage control measures
•
Provision of galleries – provides the facility from where monitoring, grouting or drainage can be done
10/13/2005
SANCOLD / University of Stellenbosch 2005
50
Seepage control measures
•
Downstream berms – For improving stability – For keeping pervious material in place.
•
Monitoring structures for seepage
10/13/2005
SANCOLD / University of Stellenbosch 2005
51
Stability of Embankment Slopes
•
Embankment and foundation are to be analysed for slope stability to ensure the most economical and safe section.
•
Always remember that below standard zones can develop in the embankment. Furthermore progressive failure can occur. One never knows when movement is completed.
•
Equilibrium methods and finite element techniques can be used.
•
For smaller dams equilibrium methods are used only
10/13/2005
SANCOLD / University of Stellenbosch 2005
52
Stability of Embankment Slopes
10/13/2005
SANCOLD / University of Stellenbosch 2005
53
Stability of Embankment Slopes Important issues
•
Saturated, partly saturated and buoyant weights
•
Selection of design strength and cohesion parameters
•
Hydrostatic pore pressure during construction
•
Critical cases for analyses
10/13/2005
SANCOLD / University of Stellenbosch 2005
54
Stability of Embankment Slopes equilibrium methods
•
Two dimensional equilibrium method is based on the following: – An embankment cross section is evaluated – The earthfill above the slip failure is divided into blocks – Each block is analysed for weight and shear resistance and the final safety factor determined – For all methods except the wedge method vertical blocks are selected. – Wedge method is applicable to the following: • horizontal layer with low shear resistance is available in the foundation
10/13/2005
SANCOLD / University of Stellenbosch 2005
55
Stability of Embankment Slopes Equilibrium methods
• • • • • • • •
Simple Bishop (1965) Spencer (1967) Janbu (1957) Wedge (1970) Carter (1971) Morgenstern and Price (1965) Maksumovic’s (1986) Ghugh (1986)
10/13/2005
SANCOLD / University of Stellenbosch 2005
56
Stability of Embankment Slopes Equilibrium methods
•
Determine the most critical slope failure by – plotting of safety factors with various radii but same centre point and plot on scale – redo above with various centre points – note that more than one critical surface can be possible for example ECRD
10/13/2005
SANCOLD / University of Stellenbosch 2005
57
Stability of Embankment Slopes Triaxial test assumptions
•
Undrained Unconsilidated
•
Consolidated Undrained
•
Consolidated drained
10/13/2005
SANCOLD / University of Stellenbosch 2005
58
Stability of Embankment Slopes Minimum Safety Factors Summary of design case, minimum safety factors, and shear test Design case
Minimum safety
Shear
Applicable to
factor
test ***
slope of embankment
End of construction
1,3*
UU or CD Upstream **
&
downstream slopes
Sudden draw down
1,0
CU or CD Upstream slope
from maximum level Sudden draw down
of full section 1,2
CU or CD Upstream slope
from full supply level Partly
water
with
of full section 1,5
CD
Upstream slope
Seepage full dam
1,5
CD
Upstream slope
Seismic
1,0
seepage
through
embankment
forces,
Both slopes
cases 1, 4 and 5 above
10/13/2005
SANCOLD / University of Stellenbosch 2005
59
Stability of Embankment Slopes Minimum Safety Factors
*For dams higher than 15m on relative poor foundations or in case of lack of safety factors use 1,4 to 1,5. **
In zones where no significant hydrostatic pore pressures are
excepted, use strengths as determined in CD test. ***
Refer Table 5.2.5(d) for definitions.
Note: Effective stresses are to be used.
10/13/2005
SANCOLD / University of Stellenbosch 2005
60
Stability of Embankment Slopes Important aspects
•
Change in shear strength of granular materials under high stress
•
Effects of differential settlement in steep valleys on stability
•
High hydrostatic pore pressures decrease shear resistance and safety factor
•
Ageing of materials
•
Dynamic forces
10/13/2005
SANCOLD / University of Stellenbosch 2005
61
Erosion Control in respect of slope protection
•
Erosion by wind, stormwater and water waves.
10/13/2005
SANCOLD / University of Stellenbosch 2005
62
Erosion Control in respect of slope protection
10/13/2005
SANCOLD / University of Stellenbosch 2005
63
Erosion Control in respect of slope protection
•
Measures against water waves: – Riprap – Flat slopes – Concrete slabs – Soil cement – Armorflex
10/13/2005
SANCOLD / University of Stellenbosch 2005
64
Erosion Control in respect of slope protection
10/13/2005
SANCOLD / University of Stellenbosch 2005
65
Erosion Control in respect of slope protection
10/13/2005
SANCOLD / University of Stellenbosch 2005
66
Erosion Control in respect of slope protection
•
New development in riprap design (1997, Montreal, CANCOLD): – Fines (10% through sieve fraction) better if excluded – Widely graded riprap is more stable on steep slopes (1V:2H)
10/13/2005
SANCOLD / University of Stellenbosch 2005
67
Embankment Dam Engineering
•
Golden rules:
•
Dam designer must have the judgement to know the unknowns in a multi-disciplinary environment
•
Knowledge is necessary
•
Experience is golden key
10/13/2005
SANCOLD / University of Stellenbosch 2005
68
Design Failure?
10/13/2005
SANCOLD / University of Stellenbosch 2005
69
Danie Badenhorst January 2005 10/13/2005
SANCOLD / University of Stellenbosch 2005
1
Contents of Presentation •
Reasons for earthtfill dam failures
•
Criteria for defensive design
•
Describe sectional layouts of fill dams
•
Soil material characteristics
•
Focus on 4 design aspects: – Compaction of earthfill – Seepage control – Stability of embankment slopes – Slope protection
10/13/2005
SANCOLD / University of Stellenbosch 2005
2
Reasons for earthfill dam failures
•
Overtopping of earthfill embankment due to floods and/ or settlement
•
Piping through embankment, along the bottom outlet most frequent
•
Piping through foundation
•
Slope failure
•
Erosion of slopes due to water waves or stormwater
10/13/2005
SANCOLD / University of Stellenbosch 2005
3
Criteria for design
•
Spillway size and freeboard adequate to accommodate Design Flood in accordance with acceptable risk
•
Provide camber to ensure that long-term crest line is on or above design crest line
•
Seepage control measures to be provided (belts and braces)
•
Stable slopes to be designed
•
Slopes to be protected
10/13/2005
SANCOLD / University of Stellenbosch 2005
4
Design Failure?
10/13/2005
SANCOLD / University of Stellenbosch 2005
5
Cross Section Layouts of Fill Dams
Homogeneous Section
Diaphragm Section 10/13/2005
SANCOLD / University of Stellenbosch 2005
6
Cross Section Layouts of fill dams
Zoned Section
10/13/2005
SANCOLD / University of Stellenbosch 2005
7
Material Properties of Zoned Embankment Dam (PI vers LL)
Core
LL
Shell
10/13/2005
LL
SANCOLD / University of Stellenbosch 2005
PI
PI
8
Material Properties of Zoned Embankment Dam ((rho rhomaks maks/w)
Core
Rho max
W opt
Shell W opt
Rho max
10/13/2005
SANCOLD / University of Stellenbosch 2005
9
Material Properties of Zoned Embankment Dam (PI vers Phi)
Core
PI
PHI triaxle
Shell
PI
10/13/2005
PHI triaxle
SANCOLD / University of Stellenbosch 2005
10
Casacrande Classification
10/13/2005
SANCOLD / University of Stellenbosch 2005
11
Material Properties of Zoned Embankment Dam LL
PI
10/13/2005
LS
K
W opt
MDD
SANCOLD / University of Stellenbosch 2005
PHI shearbox PHI triaxle
Cshearbox C triaxle
12
Zones of an Embankment Dam
10/13/2005
SANCOLD / University of Stellenbosch 2005
13
Compaction of Soil
•
Energy is applied to soil to increase density.
•
Air is expelled and water and soil are retained.
•
Compaction improves strength, decreases permeability and compressibility
•
Compaction of soil to a certain standard (i.t.o. density and water content) not only prevents excessive leakage and failure but also provides the basis for the determination of other characteristics e.g. strength, permeability, settlement and elasticity.
•
By applying compaction to a specific standard a norm is set against the behaviour can be measured.
10/13/2005
SANCOLD / University of Stellenbosch 2005
14
Compaction construction methods
•
Placement in thin 150mm to 600mm after compaction layers
•
Watering and mixing by disc plough are carried out to specified water content
•
Layer thicknesses are best confirmed by constructing test sections and testing throughout the layer.
•
Compaction of soil normally done with static roller
•
Rockfill and sand done by vibratory roller
10/13/2005
SANCOLD / University of Stellenbosch 2005
15
Compaction standards
10/13/2005
SANCOLD / University of Stellenbosch 2005
16
Compaction standards
•
For earth embankment dams the Standard Proctor is the recommended standard to use for the following reasons: – Modified AASHTO compacted earthfill embankments are too rigid, i.e. not elastic or plastic – The possibility of cracking increases rapidly for moisture contents lower than the Mod AASHTO optimum – A lower energy effort (less diesel) is required for the Standard Proctor method.
10/13/2005
SANCOLD / University of Stellenbosch 2005
17
Compaction standards
•
For sand, prevention of crack formation is not the first priority, but the decrease in permeability.
•
Modified AASHTO standard can be used as standard.
•
Well graded sands compact better than non well-graded sands.
10/13/2005
SANCOLD / University of Stellenbosch 2005
18
Compaction: water content variation and effect on geomechanical properties
•
• •
Laboratory compaction test study (volume change) on Driekloof Dam impervious and pervious materials revealed the following: – Impervious material compacted to 0 % to 2 % of optimum significant decrease in shear strength value, minor variation in cohesion and no change in elasticity – Semi-pervious material compacted to –1 % to +2 % of optimum, minor variation in shear strength value, small differences in cohesion but significant changes in elasticity occur. – If the water content is varied beyond the referred limits large changes in mentioned properties were obtained. Core - sealing, therefor elasticity and cohesion are dominant, strength second Shell zones: stability and strength first and elasticity not priority.
10/13/2005
SANCOLD / University of Stellenbosch 2005
19
Compaction: water content variation and density control
% of Maximum Standard Proctor dry density
Compaction Results: Core of Embankment Dam 104 102 100 98 96 94 -2
-1
0
1
2
3
4
Moisture content relative to optimum moisture content (%)
10/13/2005
SANCOLD / University of Stellenbosch 2005
20
Compaction: Quality control during construction
•
Frequency of testing
•
Test section
•
Calibrate Troxler machine
•
Changes in soil in borrow area
•
When placed and compacted layer dries out
•
Compaction in confined areas
10/13/2005
SANCOLD / University of Stellenbosch 2005
21
Compaction: Practices
•
Compaction of filters
•
Bottom outlet
10/13/2005
SANCOLD / University of Stellenbosch 2005
22
Seepage control
•
Seepage from the reservoir takes place through the embankment and foundation materials.
•
Phreatic surface
•
Hydraulic gradient
•
Permeability is the rate of water seepage
10/13/2005
SANCOLD / University of Stellenbosch 2005
23
Seepage control: failure modes •
Piping: Water concentrate and water washed out
•
Excessive hydraulic gradients at the downstream side of the embankment can cause erosion of soil particles under buoyant conditions. Water and solid particles are removed to form boils. In case of granular materials failure can occur due to shear resistance.
10/13/2005
SANCOLD / University of Stellenbosch 2005
24
Seepage control: failure modes
10/13/2005
SANCOLD / University of Stellenbosch 2005
25
Seepage control: Flow rate Q= kiA Q=kiA
10/13/2005
SANCOLD / University of Stellenbosch 2005
26
Seepage Control: Design •
Practice: Provide a number of seepage control measures
•
Defensive design is necessary to accommodate a series of unknown factors which can be as follows: – Unknown geology and foundation materials – Degradation or ageing of seepage control measures – Change in operation method of dam – Non conforming quality control during construction
10/13/2005
SANCOLD / University of Stellenbosch 2005
27
Seepage control measures
10/13/2005
SANCOLD / University of Stellenbosch 2005
28
Seepage control measures
•
Zoning – Dispersive materials or materials susceptible to piping in central zone – CFRD zones zoned to be stable under through flow conditions
10/13/2005
SANCOLD / University of Stellenbosch 2005
29
Seepage control measures
•
Drains – Chimney Drain • Unsaturated conditions in downstream zone • Downstream slope can be made steeper • Can prevent piping • can collapse in case of differential movement preventing failure • if taken high enough can prevent piping in dispersive materials • must be designed to meet permeability and filter criteria
10/13/2005
SANCOLD / University of Stellenbosch 2005
30
Seepage control measures
•
Drains – Collector Drain • Bottom of chimney drain • Collects water from chimney drain • Normally gravel is provided to ensure capacity • At strip drain positions small berms are provided to enable drainage of zones of embankment
10/13/2005
SANCOLD / University of Stellenbosch 2005
31
Seepage control measures
•
Drains – Strip Drain • Connects to chimney drain with the toe drain • Convenient location +-30m apart and drains at 3% slope • Note: not sufficient to drain complete foundation • Drainage, filter criteria to be met
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Seepage control measures
•
Drains – Blanket Drain • Horizontal drains close to natural ground level • Connects to chimney drain and toe drain • Seepage through the foundation can be intercepted • When chimney drain is omitted, can also drain upstream zone • Drainage, filter criteria to be met
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Seepage control measures
•
Drains – Toe Drain • Blanket and strip drain waters are canalised to lowest point downstream • Manhole on strip drain junction with V-notch • Stormwater from downstream face to be channelled separately • Rockfill toe drain - 1/3 of height
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Seepage control measures
•
Drains – Practical considerations • Chimney drain extended into core trench a must for use of dispersive soils and differential movement between embankment zone materials • Collars of filtersand to be provided around bottom outlet and outlet drain to be provided • Thickness of drain -practical considerations • Safety factors regarding theoretical capacity 30 to 500 acceptable to accommodate unknowns
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Seepage control measures
•
Filters – Piping Criteria • D15 filter/D85 basis = 5 or less • D15 filter/D50 basis < 25 • D15 filter/D50 basis < 5 for sandy silt and clay (D85 of 0,1 to 0,5) • D15 < 0,5 for fine clay (D85 from 0,3 to 0,1) • D15 < 0,3 for fine silts with low cohesion and plasticity (LL<30) • D15 < 0,3 for fine soil (D85 of 0,02)
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Seepage control measures
•
Filters – Criteria for permeability • 5 < D15 filter / D15 base < 40 • The grain size associated with the 0,075 sieve size based on the washed grading of sand must be less than 5 %
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Seepage control measures
•
Filters – Uniform criteria – Holes and slots – Inherent stability of filter layer – Dispersive clays • Compaction 98% of Standard Proctor, 0% to 2% above optimum
– Organic material • Less than 2% in filters
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Seepage control measures: Dispersive clays
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Seepage control measures
•
Criteria and practices regarding synthetic materials – Stresses in embankments due to movements can cause material to be torn – Construction practices can damage – Therefore access to location must be allowed in design.
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Seepage control measures
•
Core cut-offs – Extension of core into foundation – Depth of core trench where in-situ soil has the same or better compaction values as the to be placed core. – Extend to rock important. If 80% of depth is covered only 50% of permeability is saved. – Slope of core trench flatter than 1V:1H.
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Seepage control measures
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Seepage control measures
•
Grout curtains – provided in rock to approximately water head depth – Spacing as close as 1,25m – Lugeon unit is used to determine the permeability – Lugeon > 3 is normally used as cut-off – GIN method of grouting – Pressure relief wells to be provided with grouting – Tuba Manshet grouting is grouting of alluvium/sand
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Seepage control measures
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Seepage control measures
•
Slurry trench cut-off – +600mm wide rectangular curtain below water table connected to rock and core of embankment (Normal excavation is expensive) – Excavation of alluvium is carried out in phases - each phase is filled with bentonite/water mixture called slurry (bentonite expands 20X when wetted) – Excavation of the next phase is carried out below the slurry - filled again with slurry – Purpose of the high density slurry is to keep open the trench and to seal – Slurry is circulated by pump and viscosity of slurry is controlled – After completion the slurry can be replaced from the bottom by concrete using a tremmie pipe – A filter cake developes in the sides of the trench. – Excavation through boulders can be a problem – Special care must be taken regarding the contact between cut-off and core of the embankment
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Seepage control measures •
Upstream blanket – Impervious layer located in the basin and connected to the core of the embankment – It seals a dam with a pervious foundation – It decreases the seepage through a pervious foundation by lengthening the leakage path. – Design issues: • high hydraulic gradients can enhance piping • clay materials of a blanket can dry out • Filter material can be placed below blanket to prevent piping • The lowest draw down level can be chosen that the blanket is always covered • Geosynthetic clay liner can be used as sealing layer
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Seepage control measures
•
Treatment of embankment/foundation contact – purpose is to prevent leakage/failure due to piping caused by differential settlement of soil, cracking or below standard compaction – Treatment include: • remove soil with high organic load, roots etc • remove or treat materials not meeting permeability or stability requirements e.g. highly pervious alluvial materials, collapsible soils,sand susceptible to liquefaction or weathered uncompacted rock materials • remove topsoil
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Seepage control measures •
Treatment of embankment/foundation contact – Treatment include:
• Preparation earthfill dam rock foundations as follows: – Cleaning of joints and seams with water and air jets, removal of loose boulders, sweeping and/or washing if the surface and the infill of openings with a cement mortar – Flattening of steep slopes especially at the river banks to slopes meeting differential settlement criteria of above slopes. Maximum slopes of 1V:1H is recommended. – All uneven surfaces to be filled with mortar or cement to enhance positive compaction. • Special attention to first earthfill layer • Contact below the core must be treated with special care.
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Seepage control measures
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Seepage control measures
•
Provision of galleries – provides the facility from where monitoring, grouting or drainage can be done
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Seepage control measures
•
Downstream berms – For improving stability – For keeping pervious material in place.
•
Monitoring structures for seepage
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Stability of Embankment Slopes
•
Embankment and foundation are to be analysed for slope stability to ensure the most economical and safe section.
•
Always remember that below standard zones can develop in the embankment. Furthermore progressive failure can occur. One never knows when movement is completed.
•
Equilibrium methods and finite element techniques can be used.
•
For smaller dams equilibrium methods are used only
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Stability of Embankment Slopes
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Stability of Embankment Slopes Important issues
•
Saturated, partly saturated and buoyant weights
•
Selection of design strength and cohesion parameters
•
Hydrostatic pore pressure during construction
•
Critical cases for analyses
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Stability of Embankment Slopes equilibrium methods
•
Two dimensional equilibrium method is based on the following: – An embankment cross section is evaluated – The earthfill above the slip failure is divided into blocks – Each block is analysed for weight and shear resistance and the final safety factor determined – For all methods except the wedge method vertical blocks are selected. – Wedge method is applicable to the following: • horizontal layer with low shear resistance is available in the foundation
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Stability of Embankment Slopes Equilibrium methods
• • • • • • • •
Simple Bishop (1965) Spencer (1967) Janbu (1957) Wedge (1970) Carter (1971) Morgenstern and Price (1965) Maksumovic’s (1986) Ghugh (1986)
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Stability of Embankment Slopes Equilibrium methods
•
Determine the most critical slope failure by – plotting of safety factors with various radii but same centre point and plot on scale – redo above with various centre points – note that more than one critical surface can be possible for example ECRD
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Stability of Embankment Slopes Triaxial test assumptions
•
Undrained Unconsilidated
•
Consolidated Undrained
•
Consolidated drained
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Stability of Embankment Slopes Minimum Safety Factors Summary of design case, minimum safety factors, and shear test Design case
Minimum safety
Shear
Applicable to
factor
test ***
slope of embankment
End of construction
1,3*
UU or CD Upstream **
&
downstream slopes
Sudden draw down
1,0
CU or CD Upstream slope
from maximum level Sudden draw down
of full section 1,2
CU or CD Upstream slope
from full supply level Partly
water
with
of full section 1,5
CD
Upstream slope
Seepage full dam
1,5
CD
Upstream slope
Seismic
1,0
seepage
through
embankment
forces,
Both slopes
cases 1, 4 and 5 above
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Stability of Embankment Slopes Minimum Safety Factors
*For dams higher than 15m on relative poor foundations or in case of lack of safety factors use 1,4 to 1,5. **
In zones where no significant hydrostatic pore pressures are
excepted, use strengths as determined in CD test. ***
Refer Table 5.2.5(d) for definitions.
Note: Effective stresses are to be used.
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Stability of Embankment Slopes Important aspects
•
Change in shear strength of granular materials under high stress
•
Effects of differential settlement in steep valleys on stability
•
High hydrostatic pore pressures decrease shear resistance and safety factor
•
Ageing of materials
•
Dynamic forces
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Erosion Control in respect of slope protection
•
Erosion by wind, stormwater and water waves.
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Erosion Control in respect of slope protection
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Erosion Control in respect of slope protection
•
Measures against water waves: – Riprap – Flat slopes – Concrete slabs – Soil cement – Armorflex
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Erosion Control in respect of slope protection
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Erosion Control in respect of slope protection
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Erosion Control in respect of slope protection
•
New development in riprap design (1997, Montreal, CANCOLD): – Fines (10% through sieve fraction) better if excluded – Widely graded riprap is more stable on steep slopes (1V:2H)
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Embankment Dam Engineering
•
Golden rules:
•
Dam designer must have the judgement to know the unknowns in a multi-disciplinary environment
•
Knowledge is necessary
•
Experience is golden key
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Design Failure?
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