FOUNDATION
TREATMENT FOR DAMS
1. INTRODUCTION
Dams
are very important structures and have very stringent foundation requirements,
viz., excellent bearing capacity, no differential settlement, almost
impervious, perfect bond with dam material, no adverse effect due to
earthquakes etc. Such a perfect foundation condition is seldom available,
especially at locations where other conditions, i.e., hydrological, geological,
seismo-tectonical, etc., are favourable for construction of a dam. Thus, we
need to treat the existing foundation, so as to achieve geologic or
geotechnical conditions that meet foundation performance requirements.
The
preparation of the foundation and abutments for a dam is a most difficult and
important phase of construction; the thoroughness with which it is done is
reflected in the performance of the completed structure. It is often difficult
or sometimes impossible to correct foundation and abutment deficiencies that
show up after construction is well underway or completed. Hence, one need to
take preventive actions by way of thorough geotechnical investigations and
treat the foundation appropriately well in time, before it is too late and the
safety & performance of the structure is compromised for years to
come.
About
59% of dam failures is due to foundation problems.
ALSO READ: DESIGN PRINCIPLES OF EARTH DAM
2. OBJECTIVES
The
primary objectives of foundation and abutment treatment are;
1. To provide
adequate stability,
2. To obtain
positive control of under seepage,
3. To prepare
surfaces to achieve satisfactory contact with overlying compacted fill,
and,
4. To
minimize differential settlements and thereby prevent cracking in the
fill.
These
objectives can be achieved by defining foundation objectives, which serve as a
descriptive tool to convey design intent information to construction
engineering staff and the contractor. The general approaches available for
defining foundation objectives are;
Attain a specific
geologic unit
An
example of this approach is a narrative goal such as “extend
the cutoff trench excavation completely through the alluvium and three feet
into the underlying granitic rock.” This approach requires that
sufficient exploration has been performed to identify a continuous geologic
unit judged to possess adequate properties for the foundation.
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Excavate to a grade based on field testing
results
Two
examples of this approach are the quantitative statements “excavate the shell
foundation to an elevation that encounters dense silty sand with an SPT N1(60)
value of 30 blows per foot,” and “excavate the cutoff trench to an elevation
that encounters crystalline rock with a low permeability of less than 10
lugeons.” This approach is typically used at well-explored sites, where a
sufficient data set exists to identify the materials with stated properties
across the site.
Attain a specific rock
quality
An
example of this approach would be the narrative objective to “excavate the dam
foundation to slightly weathered granitic rock.” Qualities of rock that are
potentially significant to dam construction include degree of rock weathering
and the density, orientation, aperture, and infilling of the discontinuities.
This approach requires that sufficient exploration be performed to identify the
consistent presence of the rock quality specified and reason to believe that
rock with similar properties underlies the chosen surface.
Achieve a surface that meets a construction
control test
Two
examples of this approach are “excavate to a surface with a relative compaction
of 95% ASTM D-1557,” and “excavate to a surface with an in-place dry density of
120 pounds per cubic foot.” This approach is often used for poorly explored
sites, where prejudgments cannot be made with confidence. Estimates of the
excavation needed to achieve adequate foundation material can be poorly
constrained, potentially increasing excavation costs. It requires an ability to
physically test the foundation materials during construction, and a belief that
materials with adequate properties will underlie the chosen surface.
Excavate to a surface
based on the ability of excavation equipment
An
example of this approach is “excavate to blade refusal of a Caterpillar D-10N
tractor dozer.” This approach usually requires a calibration test between the
capability of the equipment, the character of the material on which it refuses,
and adequacy of that material for foundation. This approach is not appropriate
for stratified rock, where weaker materials may underlie a stronger layer.
Excavate to a depth indicated by design
analysis
This
approach is often used when exploration indicates there is no expectation of
material improvement within conventional excavation depth. The adequacy of the
foundation is based on engineering analysis in conjunction with design
solutions that mitigate the impact of the undesirable foundation
materials. Achieve a material judged
adequate based on visual observation
An
example of this approach is to “excavate to a depth directed by the engineer.”
This approach requires the ability to make observations and judgments of
strength and permeability during construction, and an expectation that adequate
materials underlie the surface chosen. This approach is generally not used as
the primary method of identifying adequate dam foundation materials, but should
always be specified to confirm the adequacy of any surface indicated by other
approaches, and to deal with unexpected foundation materials.
ALSO READ: DESIGN PRINCIPLES OF EARTH DAM
3. METHODS
Foundation
treatment methods depend on the actual site conditions, and as such, are as
varying as the site conditions. The generalization brought out in this lecture
is only intended to generalize approaches for treatment of foundation of dams
for sake of easy comprehension.
Practicising
engineers need to identify the potential problems and to suggest and adopt
treatments, as appropriate on case to case basis.
Clearing, Grubbing, Stripping, and Cleaning
Clearing
consists of removal of all aboveground obstructions, including trees,
vegetation, felled timber, brush, abandoned structures, local dams, bridges,
and debris. Grubbing includes removal of all objectionable below ground
obstructions or material including stumps, roots, logs, drain tiles, and buried
structures or debris. Stripping consists of the removal of sod, topsoil,
boulders, and organic or foreign materials. Shaping and cleaning consists of
removing large loose rocks, overhangs, and projecting knobs by scaling,
handpicking and wedging, and light blasting pressure washing followed by some
form of “dental treatment” to fill all holes, cracks, joints, crevices, and
depressions. These treatments are required to remove those materials having
undesirable engineering qualities, such as, low shear strength, high
compressibility, undesirable permeability, or other characteristics, which
would interfere with compaction operations; and provide a surface favourable
for a good bond with the overlying fill. Blasting should be avoided if
possible; if unavoidable, explosive charges should be kept small as possible.
The final preparation of almost all foundations should be by hand labor with
adequate time given for inspections. The use of heavy or tracked vehicles on
the final foundation should be avoided, especially if the rock is thinly bedded
or badly jointed.
Seepage Control
Seepage
control is by and large the most important foundation treatment measure. This
is usually done by providing following;
Cut-offs
Foundation
cutoffs or core trenches serve as barriers to underseepage. The design of
foundation cutoffs is based largely on borings made during field investigations
for Detailed Project Report. Some common types of cut-offs are being described
below;
(a) Compacted backfill trenches - Backfill
compacted into a seepage cutoff trench is one of the most effective
construction devices for blocking foundation seepage. Material and compaction
requirements are the same as for the impervious section of the embankment.
(b) Slurry trenches - The
slurry trench method of constructing a seepage cutoff involves excavating a
relatively narrow trench with near-vertical walls, keeping the trench filled
with a bentonite slurry to support the walls and prevent inflow of water, and
then backfilling with a plastic impervious mixture of well graded clayey gravel
to protect against piping, to reduce seepage, and to minimize consolidation of
the backfill material.
(c) Grout curtains - Grouting
is the injection by pressure of grout (a mixture of water, cement, and other
chemical compounds) into openings (voids, cracks, or joints) in a rock mass.
The grout is designed to be injected as a fluid and to stiffen or solidify
after injection. The rock foundation and abutments of most large dams require
grouting to reduce seepage and to reduce hydrostatic uplift pressures in dam
foundations. Grout curtains are frequently tied into the bottom of cutoff
trenches which extend through soil overburden to the rock foundation. Grouting
procedures must be tailored to the formation characteristics of the foundation
being grouted, and close supervision and inspection are required to obtain
satisfactory and economical results.
(d) Upstream impervious blankets
A
horizontal upstream impervious blanket controls underseepage by lengthening the
path of underseepage. The effectiveness of the blanket depends on its length,
thickness, continuity, and the permeability of the material/soil from which it
is constructed.
Pressure relief wells, drainage galleries,
toe-drains
Pressure
relief wells, drainage galleries & tunnels, toe drains etc. are constructed
to intercept seepage water, which might have passed through cutoffs, described
above. Interception of underseepage water and relieving of excess uplift
pressures prevents the transport of soil, which might occur in the formation of
sand boils and also prevents heaving at the toe.
(a) Pressure relief wells
Relief
wells are installed along the downstream toe of an embankment to intercept
underseepage water and relieve excess uplift pressures that would otherwise
develop at the toe of an embankment.
(b) Drainage galleries and tunnels
To
facilitate foundation and abutment grouting and interception of seepage water,
drainage galleries and tunnels are provided in high dams.
(c) Toe drains
Toe
drains collect and facilitate removal of seepage water at the downstream toe of
the dam to prevent formation of soft boggy areas and/or boils. Toe drains are
generally connected to the horizontal drainage blanket and sometimes to the
relief well system to collect and remove seepage water in thin pervious strata
in the upper foundation that the deeper relief wells cannot drain. Treatment of Unfavourable Conditions
Unexpected
unfavorable conditions are frequently discovered during early construction, and
may range from undesirable deposits of material not detected in exploratory
drilling to adverse seepage conditions that were impossible to predict. Some
common undesirable conditions are discussed below;
ALSO READ: DESIGN PRINCIPLES OF EARTH DAM
3.3.1. Unfavourable soil conditions
(a) Highly compressible and low strength
soils -
Organic soils exhibit high compressibility and low shear strength and are
generally recognized by their dark color, the presence of organic particles,
and often a distinctive “organic” odor. Inorganic clays with high water content
also exhibit high compressibility and low shear strength. If an embankment is
constructed on a deposit of either highly organic soil or highly compressible
inorganic soil, excessive differential settlement could cause cracking of the
embankment, or shear failure might occur; if significant deposits of either of
these materials are discovered during early construction, their extent should
be established and, if it is feasible, they should be removed and replaced with
acceptable compacted backfill.
(b) Clay shales – Clay
shales are among the most troublesome and unpredictable soils. They are often
termed “compaction” or “soil-like” shales, if they have been highly over
consolidated by great thicknesses of overlying sediment and have no appreciable
cementation. Clay shales tend to slake rapidly, when subjected to cycles of
wetting and drying; some exhibit very high dry strength, but upon wetting swell
and slake profusely, losing strength rapidly. They swell or expand considerably
when unloaded by excavating overlying material due to release of stored strain
energy. Therefore, excavating in clay shales should be completed and backfilled
without delay. The last foot or so of excavation into slaking clay shale should
be deferred, until just prior to backfill operations in order to minimize the
time of exposure of the final clay shale surface.
(c) Collapsible soils -
“Collapsible” soils are generally soils of low density and plasticity, which
are susceptible to large decreases in bulk volume when they are exposed to
water. Collapsible soils are characterized by bulky grains (in the silt-to-fine
sand grain size) along with some clay. Collapse results from softening of clay
binder between larger particles or the loss of particle-to-particle
cementation
due to wetting. Volume change from collapse occurs rapidly (relative to
consolidation) and can be very significant, especially if the soil is under
high stress.
(d) Loose granular soils - Loose,
water-saturated sands and silts of low plasticity may have adequate shear
strength under static loading conditions; however, if such materials are
subjected to vibratory loading, they may lose strength to the point, where they
flow like a fluid. The process in which susceptible soils become unstable and
flow when shocked by vibratory loading is called liquefaction, and it can be
produced by vibration from blasting operations, earthquakes, or reciprocating
machinery. In very loose and unstable deposits, liquefaction can occur as the
result of disturbances so small that they are unidentifiable. Such loose silt
and sand deposits may be compacted by blasting (generally not effective in
densifying loose granular deposits, because the vibratory energy produced is of
such high frequency), vibroflotation, and driving compaction piles; however,
the effectiveness of these procedures for deposit densification is not
predictable. (e) Steep abutment slopes -
Steep abutment slopes of earth tend to increase the possibility of transverse
cracks developing in the embankment after construction. During construction,
they may become unstable and endanger construction personnel. Slides can occur
in clays, sands, and gravel, particularly in slopes subjected to seepage.
Slides may damage completed works and require costly repairs. In many cases, it
may be necessary to bench the slopes to provide safety against sloughing
material and sliding.
(f) Old river channels - Old
abandoned river channels filled with pervious or impervious materials are often
encountered unexpectedly during construction. As mentioned earlier, the extent
of these deposits is often difficult to establish accurately during the
exploratory stages, and in some cases an entire deposit may be missed. Where
the existence of such deposits has been revealed, additional exploration by
borings, test pits, etc., to establish their extent may be necessary. Old river
channels beneath a dam foundation, filled with course-grained pervious material,
would constitute a dangerous open path of seepage. Channel fillings of soft
fine-grained materials can cause differential settlements and cracking of the
embankment, if not removed and replaced with properly compacted material.
3.3.2. Unfavourable rock conditions
(a) Weathered rock - Weathered
rock may have undesirable characteristics, such as, high compressibility, low
strength, and high permeability. Removal of weathered rock is generally
required for embankments founded on rock to obtain impervious contact beneath
the core and to eliminate the possibility of differential settlements and low
shear strengths beneath the core and other zones. The weathering of rock is a
transitional process; a sharp line of demarcation does not exist between weathered
and unweathered zones.
In
fact there are nine categories of weathering, as under;
(i) Fresh (Intact rock) – W1;
(ii) Slightly weathered to Fresh – W2;
(iii) Slightly weathered (shallow oxidation or
discolouration) – W3; (iv) Moderately to Slightly weathered – W4;
(v) Moderately weathered (significant oxidation
or discolouration, body of rock
slightly weakened, open joints) – W5;
(vi) Intensely to Moderately weathered – W6; (vii) Intensely weathered (thoroughly
oxidised or discoloured, body of
rock significantly weakened) –
W7;
(viii)Very
Intensely weathered (joints fully separated)
– W8;
(ix)
Decomposed (more like soil) –
W9., The degree of weathering usually
decreases with depth, thus, it may be necessary to excavate deeper in some
areas than in others to remove weathered rock beyond category W2.
(b) Open joints and fractures - All open joints, cracks, fissures, and
fractures in the foundation rock surface must be filled to prevent erosion or
scour of embankment material at the rock contact. A sand-cement mortar is
generally used to fill these openings.
(c) Cavities and solution features - Cavities,
potholes, and other voids caused by solution of the rock are dangerous, and
field personnel should always be on the lookout for such conditions during
foundation preparation. More care should be taken, where a dam is being built
on rock susceptible to solution, such as limestone or gypsum. Potholes and
cavities exposed or “day lighted” on the foundation surface are usually
remedied by dental treatment. Concrete should be thoroughly vibrated &
rodded into the voids and its upper surface brought up to the general level of
the surrounding rock. Dental treatment serves to smooth up the foundation to
reduce compaction difficulties as well as provide a non-erodable impervious
seal, a measure of protection against scour of the embankment fill along the
rock contact.
(d) Overhangs and surface depressions - Overhangs
and other irregularities in the rock surface of an abutment or foundation must
be corrected. Overhangs should be removed by drilling and blasting, preferably
with pre-splitting, so as not to disturb the adjacent sound rock. Concrete
dental treatment can be used to fill depressions created by blasting and to
remedy some types of overhangs. Tamping of soil under overhangs instead of
removal or dental treatment must not be permitted. If the rock is very
irregular, it may be more economical to cover the entire area with a concrete
slab. It should be noted that a gently undulating rock surface is desirable,
and only when surface depressions interfere with compaction of the first lift,
should concrete backfilling be required.
(e) Springs - Springs, often
encountered in rock foundations and abutments, are simply groundwater sources
seeping to the ground surface driven by artesian pressure. Attempts to place
impervious fill over springs issuing from joints or rock fractures will result
in extremely wet soil in the vicinity of the spring, which is impossible to
properly compact. Depending on the flow rate and pressure driving the spring,
seepage will continue through the wet soil, creating an uncompacted weak zone
of increasing size, if fill placement is continued without properly removing
this source of water. The zone created around an improperly controlled spring
is a very dangerous situation, which will cause problems both during
construction and over the life of the embankment. Where the water is under a
low head and has a single point of issue, a standpipe can usually be installed.
A corrugated metal pipe of a diameter depending upon the size of the spring is
placed over the spring area, and a damp mixture of quicksetting cement, sand,
and gravel is packed around the standpipe base. Earth is then compacted around
the outside of the pipe at the base. The water is kept pumped down within the
standpipe until an impervious seal is obtained and enough pipe sections have
been added to retain the head of water in the pipe. The pipe is then filled
with vibrated concrete or grout, and construction of the fill continued upwards
and across the top of the plugged pipe by conventional methods. The area is
then examined for evidence of new springs, which often appear after an old
spring is plugged. This procedure can also be used for springs on the abutment
when the fill reaches the same elevation as the spring. While filling
operations are progressing below the spring, a small pipe can be grouted into
the source of seepage and discharged away from the fill as a temporary
measure.
Dewatering and Drainage of Excavated Areas
Inadequate
control of groundwater seepage and surface drainage during construction can
cause major problems in maintaining excavated slopes and foundation surfaces
and in compacting fill on the foundation and adjacent to abutment slopes.
Dewatering systems should be adequate enough to control seepage and hydrostatic
uplift in excavations, and for collection and disposal of surface drainage and
seepage into excavations.
Dewatering - Potential troubles
can often be detected in early stages by visual observation of increased
seepage flow, piping of material from
the foundation of slopes, development of soft wet areas, uplift of excavated
surfaces, lateral movement of slopes, or failure of piezometer water levels to
drop sufficiently as pumping is continued. Water pumped from dewatering systems
must be observed daily at the discharge outlet; if the discharge water is muddy
or contains fine sand, fines are being pumped from the foundation. This is
important as the
pumping
of fines from the foundation can cause internal erosion channels or piping to
develop in the embankment structure; if this happens, it is crucial that
corrective measures be taken.
Failure
of the dewatering system can result in extremely serious problems, often
requiring extensive and expensive remedial work. In excavations bottoming in
impervious material, unchecked artesian pressure in underlying pervious strata
can cause heaving of the excavation bottom. If the impervious stratum ruptures
under these pressures, boils (violent emission of soil and water) will develop,
causing the loss of the underlying foundation material and thereby endangering
the entire structure. Similar boils could develop on the bottom of an
excavation from excessive artesian pressures in the underlying strata. Failure
of excavation slopes may also occur because of excessive artesian pressures. In
order to prevent failure of the dewatering system, all power sources should
have standby gas or diesel-powered pumping or generating equipment, and standby
pumps should be available.
Sumps and ditches - When an
excavation, such as a cutoff trench, is extended to rock or to an impervious
stratum, there will usually be some water seeping into the excavation and/or
“wet spots” in the bottom of the excavation, even with deep wells or well point
systems in operation. Water seeping into the excavation from the upstream and
downstream slopes of a long cutoff trench can usually be captured by excavating
narrow longitudinal ditches or drainage trenches at the intersection of the
slopes and the bottom of the excavation, or by forming such trenches with
sandbags, with sumps located as necessary for pumping the water out. If the
bottom of the excavation will still not dry out, smaller ditches can be cut
through the problem areas and sloped to drain to the side trenches.
Surface erosion - Surface erosion may
present problems on slopes cut in silts, fine sands, and lean clays. Eroded
material will wash down and fill in the excavation below the slope. The slope
itself will be left deeply scoured and rutted, making it necessary for costly
smoothing operations to be performed before the fill can be placed against it.
The best way to combat surface erosion of temporary excavation slopes is to
backfill as soon as possible, thus cutting down on exposure time. This often
cannot be done, however, and it becomes necessary to take other measures. Grass
cover on the slopes is a good means of preventing surface erosion, if it can be
readily established and if the slopes are to remain open for a season or two.
Other slope protection measures such as rip-rap or asphaltic treatment are
rarely justified for construction slopes. Thus, it is necessary to keep as much
water off the slope as possible. Most slopes can withstand rain falling
directly on them with only minor sloughing. Perimeter ditches and/or dikes at
the top of the slope are needed to carry other surface waters away from the
excavation, if surface waters outside the excavation would otherwise run into
it. Ditches may be needed at several elevations along the excavation slopes to
catch surface waters.
Other seepage control measures - Other
means of stabilizing excavation slopes and preventing seepage from entering an
excavation (such as electro-osmosis, freezing, sheet-piling, and grouting) have
been used for structure excavations. These methods are not economically
feasible for extensive foundation excavations for dams, but might be used in
structures, where conventional dewatering methods are not suitable for various
reasons.
ALSO READ: DESIGN PRINCIPLES OF EARTH DAM
