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Foundation Preparation and Foundation
Treatment in dams
Types of Dams Gravity dams.Gravity dams.
Concrete Gravity damsConcrete Gravity dams Masonry Dams Masonry Dams R.C.C damsR.C.C dams
Embankment dam.Embankment dam. Earthen damsEarthen dams Rock fill damsRock fill dams Rock fill dam with U/s membraneRock fill dam with U/s membrane
Arch dam.Arch dam. Buttress Dam.Buttress Dam. Other damsOther dams
Causes of Failure of Dams
OVERTOPPING 29% FOUNDATION 53% OTHERS 18%
Dam Foundation requirement
Foundation is the lower most Foundation is the lower most part of the Dam which transmit part of the Dam which transmit the load from the dam safely to the load from the dam safely to the rock stratum. the rock stratum.
Dam foundation requirements Dam foundation requirements are based on the type of dam are based on the type of dam proposed and is largely proposed and is largely dependent on the strength, dependent on the strength, deformation, and permeability deformation, and permeability characteristics of site materials. characteristics of site materials.
To determine the depth of To determine the depth of excavation needed to achieve excavation needed to achieve an adequate foundation, an adequate foundation, observation of site conditions in observation of site conditions in borings and test pits, field borings and test pits, field testing of soil and rock, testing of soil and rock, laboratory testing of laboratory testing of representative samples and, representative samples and, ultimately, design analysis is ultimately, design analysis is needed.needed.
Terminologies Related to Dam Foundations
Grouting:Grouting: A process A process of pouring or injecting grout in the joints, hollows, cracks, faults, shattered zones or fracture zones, etc.
Lugeon:Lugeon: The Lugeon is the unit of permeability Which is most The Lugeon is the unit of permeability Which is most popular and relevant unit for grouting purposes popular and relevant unit for grouting purposes 1 lugeon unit = 1 liter of water taken per meter of test 1 lugeon unit = 1 liter of water taken per meter of test
length, length, per minute, at 10 bars pressure per minute, at 10 bars pressure Pattern:Pattern: Arrangement of holes in plan and vertical section. Drainage:Drainage: It is the disposal of surface and seepage water in the abutments, foundation and the body of the Dam. Shear Key:Shear Key: A longitudinal trench in the foundation of concrete/ masonry dam and backfilled with concrete in order to increase
the resistance against sliding.
Foundation Investigations
The Shear Strength The Shear Strength Permeability of each MaterialPermeability of each Material The Deformation Modulus to define the deformation of the The Deformation Modulus to define the deformation of the
foundation.foundation. Depth of Over burdenDepth of Over burden Joint patternJoint pattern Fold Orientation, etc.Fold Orientation, etc.
These parameters can be Determined by Surface or Sub-These parameters can be Determined by Surface or Sub-Surface explorationsSurface explorations
Most of the dams have to be built on complex foundations requiring special treatments. Various types of geological features encountered are:
Foundation ProblemsFoundation Problems
1. Faults2. Shear Zones3. Shear Seams4. Shattered/Highly
jointed rock5. Foundations with
more than one type of rock with different properties
6. Folds7. Buried Channels8. Jointing pattern of
the rock mass 9. Caverns/Cavities10. Springs etc.
Faults and Shear zonesFaults and Shear zones
A A faultfault or or fault linefault line is a is a planar planar fracturefracture in rock in in rock in which the rock on one side which the rock on one side of the fracture has moved of the fracture has moved with respect to the rock on with respect to the rock on the other side. the other side.
A A shear zoneshear zone or or shearshear is a is a wide zone of distributed wide zone of distributed shearing in rock. Typically shearing in rock. Typically this is a type of this is a type of faultfault but it but it may be difficult to place a may be difficult to place a distinct fault plane into the distinct fault plane into the shear zone. Shear zones shear zone. Shear zones may form zones of much may form zones of much more intense more intense foliationfoliation, , deformationdeformation, and , and foldingfolding. . En echelon veinsEn echelon veins or or fractures may be observed fractures may be observed within shear zones. within shear zones.
Foundation ProblemsFoundation Problems
Those Geological features can cause1. Mechanical problems leading to excessive stress
concentrations, deformations, stability problems etc.2. Hydraulic problems like possibility of piping below the dam
along the weak features such as faults, shear zones etc.3. Sliding stability problems along shear seams/joints etc.
Foundation Problems (Faults)Foundation Problems (Faults)
Shih-Kang spillway weir damaged by 1999 Shih-Kang spillway weir damaged by 1999 Chi-Chi earthquake: Spillway openings 17 Chi-Chi earthquake: Spillway openings 17 and 18 destroyed by fault movementand 18 destroyed by fault movement
Effect of fault movement on Shih-Effect of fault movement on Shih-Kang dam: Detail of destroyed Kang dam: Detail of destroyed spillway openings 17 and 18spillway openings 17 and 18
Foundation ProblemsFoundation Problems
Concrete dams are more susceptible to damage from differential settlement than embankment dams,
Conditions are most severe where the foundation comprise materials with different moduli. For the condition shown in Fig-(A) , differential settlement can induce stresses in the concrete. And elastic and non- elastic behavior of rocks causes permanent deformation of the foundation caused by cycles of reservoir filling and emptying Fig (B).
Fig - (A)Fig - (A) FIG – (B) FIG – (B)
Foundation ProblemsFoundation Problems
Rock Foundation:
Most natural and induced cavities develop in soluble rocks, most notably limestone, dolomite, gypsum, and rock salt.
Typical karst conditions develop in lime stones and dolomites by solution-widening of joints and bedding planes caused by flowing ground water. Eventually, his process develops into a heterogeneous arrangement of cavities with irregular sinkholes occurring where cavity roofs have collapsed. The amount of solution that occurs in limestone and dolomite would be negligible in the lifetime of a typical project. Hence, existing cavities are the major concern.
Gypsum and anhydrite are less common than lime stones, but they have the additional concern of solution and collapse or settlement during the useful life of a typical structure.
Flow of ground water, particularly to water supply wells, has been known to dissolve gypsum and cause collapse of structures.
Foundation Preparation for Foundation Preparation for Gravity DamsGravity Dams
Generally a considerable length of area needs to be excavated to enable the various operations to go on continuously.
Excavation of foundation for concrete spillway and similar overflow structures should be carried out to the desired depth and concrete should be laid on sound rock.
Excavation within 500 mm of the foundation grade
should be done just before concrete is laid on sound rock and should not be allowed to remain exposed for long time. In case the geological exploration indicates existence of faults, shear or weak zones, necessary treatment of the foundation should be carried out before laying the concrete.
Section Along Dam AxisSection Along Dam Axis
500
490
480
470
460
450
440
430
420
410
400
390
380
370
360
350
340
330
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310
300
290
280
270
260DATUM 250
510
520
ELEVATION(m)
DISTANCE(m)
508
500
490
480
460
450
440
420
410
390
380
370
350
320
310
320
330
00.0
03.
828.
8915
.31
23.3
429
.33
33.5
5
43.9
948
.09
52.8
158
.90
64.5
670
.16
78.5
2
90.8
3
358.
50
370.
85
NSL
NSL
350
360
370
380
390
400
410
420
430
440
EL.419.00
EL.415.60
NSL
INSPECTION GALLERY
EL.382.00
EL.378.60
EL.385.40
EL.382.00
EL.388.80
EL.385.40
EL.393.40
EL.390.00
EL.396.80
EL.393.40
EL.400.20
EL.396.80
EL.404.80
EL.401.40
EL.406.00
EL.409.40
EL.406.00EL.406.00 EL.406.00 EL.406.00 EL.406.00
EL.412.80
EL.409.40
EL.420.80
EL.289.80
EL.286.40
EL.267.60
EL.265.40
EL.265.80
EL.266.80
EL.423.80
EL.300.00
EL.296.60
EL.334.40
EL.331.00
EL.330.00
EL.329.00
EL.325.60 EL.325.60
EL.318.80
EL.315.40 EL.315.40
EL.293.20
EL.289.80
EL.286.40
EL.283.00 EL.283.00
EL.279.60 EL.279.60
EL.276.20
EL.274.80
EL.273.00
EL.271.80
EL.267.40
EL.269.00
EL.270.60
EL.271.80
EL.272.60
EL.275.40
EL.278.80
EL.281.80
EL.283.20
EL.284.00
EL.285.20
EL.287.60
EL.293.40
EL.296.80
EL.300.00
EL.302.40
EL.305.80
EL.309.20
EL.305.80
EL.309.20
EL.315.60
EL.312.20
EL.319.00
EL.315.60
EL.322.40
EL.319.00
EL.325.80
EL.322.40
EL.329.20
EL.325.80
EL.330.00
EL.332.40 EL.332.40
EL.335.80
EL.339.20
EL.335.80
EL.342.60
EL.339.20
EL.346.00
EL.342.60
EL.349.40
EL.346.00
EL.352.80
EL.349.40
EL.356.20
EL.352.80
EL.359.60
EL.356.20
EL.363.40
EL.360.00
EL.368.40
EL.365.00EL.365.00
EL.371.80
EL.368.40
EL.375.20
EL.371.80
EL.378.60
EL.375.20
450
460
470
480
490
500
340
381.
47
391.
3739
6.97
402.
9440
9.54
417.
90
426.
94
441.
67
485.
52
498.
30
509.
83
521.
56
535.
78
552.
86
585.
2258
9.99
608.
56
EL. 362.89
TOP OF DAM
ELEVATORSHAFT
19
CJ
GUIDE WALL
CJ CJ
CJ
1 2 6 . 7527 2 1 9 . 7000 1 2 7 . 8992
4 7 4 . 3519
ROCK LINE
FOUNDATION GRADE
ROCK LINE
FOUNDATIONGRADE
NSL
NSL
ROCK LINE
FOUNDATION GRADE
24
CJ
CJ
CJ
GUIDE WALL
1
8
ELEVATOR SHAFT
2 5 . 0 000
BREAST WALL1
4
STAIR CASE
CJ23
2221
20
18171615141312111098765
4
3
21
ROCKLINE
ROCK LINE
EL.423.80
EL.420.80
EL.417.20
EL.414.20
EL.411.20
EL.409.40
EL.406.40
EL.404.60
EL.401.60
EL.399.80
EL.396.80
EL.394.00
EL.390.00
EL.388.20
EL.385.20
EL.382.20
EL.379.20
EL.376.20
EL.374.40
EL.372.60
EL.365.40
EL.369.00
EL.362.40
EL.360.00
EL.356.60EL.356.60
EL.354.80
EL.351.40
EL.354.80
EL.348.00
EL.351.40
EL.344.60
EL.341.20
EL.337.80
EL.334.40
EL.337.80
EL.341.20
EL.344.60
EL.348.00
EL.331.00
EL.329.00
EL.322.20 EL.322.20
EL.318.80
EL.312.00
EL.309.40
EL.306.40
EL.303.00
EL.300.00
EL.296.60
EL.293.20
EL.303.00
Foundation Preparation for Foundation Preparation for Gravity Dams Gravity Dams
Blasting operation should be carried out carefully without affecting the rock mass beyond the required area of excavation and shall be restricted to minimum 500 mm above the foundation levels and at least 30 m away from any existing structure.
However, in special cases the blasting may be carried out at a distance of less than 30 m from any existing structure using controlled blasting. If necessary, field tests may be carried out to decide various parameters of controlled blasting.
It is desirable to keep the foundation slope upwards in downstream direction
Foundation Preparation for Foundation Preparation for Gravity Dams Gravity Dams
Immediately before placing concrete/ masonry, all surface of foundations upon or against which the concrete/masonry is to be placed, should be free from standing water, mud, debris, organic deposits, and other foreign material which may prevent a tight bond between the rock and concrete/masonry.
All surfaces of rock upon or against which the concrete/masonry is to be placed should be clean, solid and free from all objectionable coatings, sand loose, semidetached or unsound fragments and should be sufficiently rough to ensure satisfactory bond with the concrete/ masonry.
Foundation Preparation for Foundation Preparation for Gravity Dams Gravity Dams
Foundation Preparation for Foundation Preparation for Gravity Dams Gravity Dams
Stepping in the foundation should be avoided and a continuous foundation profile provided . The slope in foundation grade should not exceed 45°.
If the foundation grade has a slope greater than 45° suitable berm has to be provided.
Whenever the foundation is sloping steeply and shear zones, faults and other weak layers exist in the foundation, necessary treatment of the foundation should be carried out before laying the concrete.
Foundation Preparation for Foundation Preparation for Gravity Dams Gravity Dams
Foundation Treatment Against Foundation Treatment Against
SlidingSliding The following measures are usually taken to improve the The following measures are usually taken to improve the
Sliding resistance of dam Foundation Sliding resistance of dam Foundation Roughening of Base.Roughening of Base. Provision of Shear Key. Provision of Shear Key. Upstream Sloping of Dam base.Upstream Sloping of Dam base. Use of Anchors.Use of Anchors. Use of Mat Foundation.Use of Mat Foundation. Providing Buttress Structure.Providing Buttress Structure. Struts bearing against sound rock ledge on the Down Struts bearing against sound rock ledge on the Down
stream side.stream side. Adopting a Slightly arched arrangement of the dam.Adopting a Slightly arched arrangement of the dam.
Shear KeyShear Key
Rock AnchorsRock Anchors
Treatment of Fault and Weak Treatment of Fault and Weak ZonesZones
Faults and weak zones exist in most rock formations, their size, Faults and weak zones exist in most rock formations, their size, Continuity and orientations are important factors in Continuity and orientations are important factors in determining the suitability of a foundation for any dam.determining the suitability of a foundation for any dam.
Type of FaultType of Fault Problem to Problem to FoundationFoundation
Treatment Treatment
Low- Angle Faults (Dip angle < 450)
Providing Providing Inadequate Sliding Inadequate Sliding ResistanceResistance
1.1. Excavating out the Excavating out the weak materialweak material
2.2. Providing Shear Providing Shear KeysKeys
3.3. Use Rock AnchorsUse Rock Anchors
High – Angle High – Angle FaultsFaults(Dip angle > 450)
The main problem The main problem is that of Bridging is that of Bridging over of the over of the Structure and the Structure and the resulting Stress resulting Stress ConcentrationsConcentrations
1.1. Dental treatmentDental treatment
2.2. Providing Seepage Providing Seepage Cut-off on U/sCut-off on U/s
3.3. Use Rock AnchorsUse Rock Anchors
Dental Treatment for Faults and Dental Treatment for Faults and Seams.Seams.
Very often the faults, shear seams or shattered zones met with after excavation extend to such depths that it is impracticable to clean them out entirely.
These conditions require special treatment. Stress concentrations may occur in the dam due to the presence of such low modulus zones.
To minimize the build up of stresses in the dam, a portion of the weak zone is replaced by concrete. The procedure for reinforcing and consolidating such weak geological features is frequently called Dental Treatment.
These seams are excavated to a depth sufficient to produce elastic qualities (stresses/ strains /deformations) and bearing strength nearly equal to the values (both in the dam and foundation) had such seams not been present in the foundations.
Dental Treatment for Faults and Dental Treatment for Faults and Seams.Seams.
Based on theoretical studies carried out for Shasta and then for Friant dams, USBR has developed the following approximate formulae for determining the depth of concrete plug:
d = 0.0066 b H + 1.5 for H > 46 m d = 0.3 b + 1.5 for H < 46 m
where,
H = Height of the dam above foundation level in m
b = width of the weak zone in m
d = Depth of excavation of weak zone below surface of adjoining
sound rock for providing concrete plug in m.
Dental Treatment for Faults and Dental Treatment for Faults and Seams.Seams.
Dental Treatment for Faults and Dental Treatment for Faults and Seams.Seams.
Other measures Other measures
• In swelling rocks hold –down piers and tensioned rock anchors are used to reduce heave due to rock swelling
Grouting and Drainage Provisions Grouting and Drainage Provisions
for Foundationsfor Foundations
The principal purpose of grouting is to fill openings in a foundation and render it impervious to percolating water.
It is also used to improve the strength and elastic properties of the foundation material into which it is injected.
The method of grouting, in large measure, depends upon the nature of treatment to be given to foundation materials which, in turn, depends upon the geological features.
For details of grouting for foundation treatment, reference may be made to IS 6066 : 1994.
Grouting and Drainage Provisions Grouting and Drainage Provisions for Foundations for Foundations
Consolidation Grouting - For filling up the joints, cracks, crevices etc. and there by making the foundation homogeneous.
Grouting and Drainage Provisions Grouting and Drainage Provisions for Foundations for Foundations
Curtain Grouting - For making a curtain to cutoff/
minimize seepage
Grouting and Drainage Provisions Grouting and Drainage Provisions for Foundations for Foundations
Drainage arrangements - To drain off seepage water and thus reduce uplift. Some rules in this respect are
Grouting and Drainage Provisions Grouting and Drainage Provisions for Foundations for Foundations
Consolidation Grouting :
Holes at 3-6 m c/c 6 m to 15m deep, Normal to dam - rock interface
Curtain Grouting : As per IS 11293 (Part 2): 1993 The depth of grout curtain
depends upon the type and conditions of the rock mass with respect to its permeability. The following empirical criteria may be used as a guide which is based on going practice.
D=2/3 H +8 D= Depth of the grout curtain in m, and H= Hydraulic head Holes at 3.0 m c/c
Grouting and Drainage Provisions Grouting and Drainage Provisions for Foundations for Foundations
Name: St Francis DamLocation: a water supply dam for Los Angeles, California, USA.
THIS DAM COLLAPSED! •It had unsuitable foundation rock, and it was NOT GROUTED!It was built in 1924-26, before the need for grouting was understood.Max height = 205ft [63m]
Foundation And Abutment Preparation of Dams
The Foundation and Abutment preparation of Dams Can The Foundation and Abutment preparation of Dams Can be Explained Under the Following Headings,be Explained Under the Following Headings,
1. Earth foundations. 1. Earth foundations.
2. Rock Foundations.2. Rock Foundations.
3. Abutment Treatments.3. Abutment Treatments.
Earth Foundations The design of dams on earth foundations is based on the in
situ shear strength of the foundation soils. Foundation preparation usually consists of clearing,
grubbing to remove stumps and large roots in approximately the top 3 ft, and stripping to remove sod, topsoil, boulders, organic materials, rubbish fills, and other undesirable materials.
It is not generally necessary to remove organic-stained soils. Highly compressible soils occurring in a thin surface layer or in isolated pockets should be removed.
After stripping, the foundation surface will be in a loose condition and should be compacted.
Note : If a silty or clayey foundation soil has a high water content and high degree of saturation, attempts to compact the surface with heavy sheep-foot or rubber-tired rollers will only remold the soil and disturb it
Earth Foundations (Cont.)
Stump holes should be filled and compacted by power-driven hand tampers.
For dams on impervious earth foundations not requiring a cutoff, an inspection trench having a minimum depth of 6 ft should be made. This will permit inspection for abandoned pipes, soft pockets, tile fields, pervious zones, or other undesirable features not discovered by earlier exploration.
Differential settlement of an embankment may lead to tension zones along the upper portion of the dam and to possible cracking along the longitudinal axis in the vicinity of steep abutment slopes at tie-ins or closure sections. To minimize this possibility, steep abutment slopes and foundation excavation slopes should be flattened, if feasible, particularly beneath the impervious zone of the embankment
Rock FoundationsRock Foundations
Rock foundations should be cleaned of all loose fragments, including semidetached surface blocks of rock spanning relatively open crevices.
Projecting knobs of rock should be removed to facilitate operation of compaction equipment and to avoid differential settlement.
Cracks, joints, and openings beneath the core and possibly elsewhere should be filled with mortar or lean concrete according to the width of opening.
The excavation of shallow exploration or core trenches by blasting may damage the rock. Where this may occur, exploration trenches are not recommended, unless they can be excavated without blasting. Where core trenches disclose cavities, large cracks, and joints, the core trench should be backfilled with concrete to prevent possible erosion of core materials by water seeping through joints or other openings in the rock.
Rock FoundationsRock Foundations (Cont.) (Cont.)
Where an earth dam is constructed on a jointed rock foundation, it is essential to prevent embankment fill from entering joints or other openings in the rock.
Shale foundations should not be permitted to dry out before placing embankment fill, nor should they be permitted to swell prior to fill placement.
Limestone rock foundation may contain solution cavities and require detailed investigations
Where faults or wide joints occur in the embankment foundation, they should be dug out, cleaned and backfilled with lean concrete
This type of treatment is obviously required beneath cores of earth and rock-fill dams and also beneath rock-fill shells.
Abutment TreatmentAbutment Treatment
The principal hazards that exist on rock abutments are due to irregularities in the cleaned surfaces and to cracks or fissures in the rock.
Cleaned areas of the abutments should include all surfaces beneath the dam with particular attention given to areas in contact with the core and filters. It is good practice to do both a preliminary and final cleanup of these areas.
The purpose of the preliminary cleanup is to facilitate inspection to identify areas that require additional preparation and treatment. Within these areas, all irregularities should be removed or trimmed back to form a reasonably uniform slope on the entire abutment.
Overhangs must be eliminated by use of concrete backfill beneath the overhang or by barring and wedging to remove the overhanging rock.
Abutment Treatment Abutment Treatment (Cont.)(Cont.)
Concrete backfill may have to be placed by shotcrete, gunite, or similar methods to fill corners beneath overhangs.
Vertical rock surfaces beneath the embankment should be avoided or, if permitted, should not be higher than 5 ft, and benches between vertical surfaces should be of such width as to provide a stepped slope comparable to the uniform slope on adjacent areas.
Relatively flat abutments are desirable to avoid possible tension zones and resultant cracking in the embankment, but this may not be economically possible where abutment slopes are steep.
Flattening of the abutment slope may reduce the effects of rebound cracking (i.e., stress relief cracking) that may have accompanied the development of steep valley walls.
The cost of abutment flattening may be offset by reductions in abutment grouting. The cost of foundation and abutment treatment may be large and should be considered when selecting dam sites and type of dam.
