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OCR for page 213
7
Embankment Dams
TYPES OF DAMS AND FOUNDATIONS
Embankment-type dams have been classified in a number of different
ways, but various authorities have not always been in agreement on termi-
nology. Classification generally recognizes (l) the predominant material
comprising the embankment, either earth or rock; (2) the method by which
the materials were placed in the embankment; and (3) the geometric con-
figuration or internal zoning of the cross-section. A classification modifier
is often included to denote the purpose or use of the dam, such as diversion
dam, storage dam, coffer dam, tailings dam, afterbay dam, etc.
A formal, rigid classification is less important than an understanding of
the performance characteristics and purposes of the zones and components
forming the total dam.
Embankment dams are constructed of natural materials obtained from
borrows and quarries and from waste materials obtained from mining and
milling operations. The two primary types are the earthfill dam, an em-
bankment dam in which more than one-half of the total volume is formed
by compacted or sluiced fine-grained material, and the rockfill dam in
which more than one-half of the total volume is formed by compacted or
dumped pervious natural or quarried stone.
Earthfill Dams
Homogeneous Earthfill Dams
Homogenous earthfill dams are composed of materials having essentially
the same physical properties throughout the cross-section. Modern homo-
213
OCR for page 214
214
SAFETY OF EXISTING DAMS
geneous dams usually incorporate some form of drainage zones or other
elements for controlling internal saturation and seepage forces; however,
many small older dams do not have these provisions. Rock toes, horizontal
blanket drains, vertical and inclined chimney drains, line drains, and fin-
ger drains or a combination of these various forms have been used for these
purposes. The drainage facilities are composed of pervious sand, gravel, or
rock fragments separated from direct contact with the main body of the
dam by properly graded filter zones to prevent migration of fine-grained
soils into the drain elements and to reduce rapidly the hydraulic gradient of
the seepage flow.
Hydraulic Fill Dams
Hydraulic fill dams are constructed of materials that are conveyed into
their final position in the dam by suspension in flowing water. Originally
this sluicing was assumed to sort out and deposit the coarser materials near
the faces of the dam and the finer materials near the center of the cross-
section. With few exceptions, dams of this type have not been constructed
in the United States since about 1940 mainly because the development of
large, efficient earth-moving machines has made other types of embank-
ment dams more economical and because the seepage and structural per-
formance of these other types are more predictable (Jensen et al. 1976~.
The experience record during the period 1920-1940 demonstrates the unre-
liability of the theory of idealized grading and sorting into pervious shells
and impervious cores and the propensity for failure during construction.
The vulnerability of hydraulic fill dams to accidents and failures from
long-duration seismic ground motions was vividly demonstrated during the
1971 San Fernando, California, earthquake. Consequently, many old hy-
draulic fill dams in California have either been replaced or extremely mod-
ified and strengthened. Others, after site-specific investigations, have been
declared safe and are in service. Hydraulic fill dams and earthquakes are
not confined to California. Although they are no longer favored in the
United States, a substantial number of hydraulic fill dams are in service in
the United States and require surveillance and safety evaluation (tops et al.
1978~.
Zoned Earthfill Dams
Zoned earthfill dams are composed of an impervious zone or core of fine-
grained soils located within the interior of the cross-section and supported
by outer zones or shells of more pervious sand, gravel, cobbles, or rock frag-
ments. Transition zones of intermediate permeability are frequently in-
OCR for page 215
Embankment Dams
215
ctuded between the core and the shells for economic utilization of all mate-
rials that must be excavated for the project and to prevent intermingling or
transport of materials at the zone interfaces.
Various configurations and positions of the core zone have been used.
The zone may be centered on the dam axis with positive slopes or it may
have a vertical or overhanging downstream slope. The selection of the vari-
ous shapes is controlled by the properties and quantities of the available
construction materials and the stability and seepage control objectives of
the design.
Diaphragm Earth Dams
Diaphragm earth dams consist of a pervious or semipervious embankment
together with an impermeable barrier formed by a thin membrane or wall.
The diaphragm may be positioned in the embankment along the axis or on
the upstream face of the embankment. The stability of the dam is supplied
by the mass of the embankment, and the water retention capability is sup-
plied by the diaphragm. Cement concrete, asphaltic concrete, and steel
plate have been used for diaphragms.
Stone~vall-Earth Dams
Stonewall-earth dams are composed of rubble-masonry walls and an earth
filling. This type of dam is generally quite old 100 or more years and of
modest height. Some have only a downstream wall, in which case the up-
stream face of the earth filling is sloped. Others have both an upstream and
a downstream wall that retains the interposed filling. The exposed surfaces
of the walls are usually vertical or near vertical. The filled surfaces are
sometimes battered or sloped. The walls are usually dry rubble but may
occasionally be mortared.
Rockfill Dams
Faced Rockfill Dams
Faced rockfill dams consist of a pervious rock embankment with an imper-
meable membrane on the upstream face. The rock mass provides stability
and the membrane, or facing, retains the water. Older faced rockfill dams
were constructed by dumping the rock in relatively high lifts or tips. Some-
times the rock was sluiced in an attempt to reduce settlement by washing
the rock fines and spells into the interstices of the mass and creating direct
contact between the larger blocks of rock. An upstream narrow zone of
OCR for page 216
216
SAFETY OF EXISTING DAMS
derrick-placed stone was commonly used to create a uniform surface to
support the facing and to reduce the amount of movement and distress in
the facing.
Since about 1960 the construction procedures for faced rockfill dams
have been improved considerably, through the efforts of I. B. Cooke and
others, resulting in less embankment settlement and less damage to the fac-
ing. A large percentage of the rock is placed and compacted in horizontal
lifts. A special zone of selected small-size rock is used to support the face.
The main body of the embankment is zoned with the rock sizes in the zones
increasing toward the downstream face. All but the zones of larger-sized
rock are compacted by vibratory rollers or rubber-tired compactors. The
largest-size rock is usually dumped in lifts of moderate height.
The facings consist of reinforced Portland cement concrete, asphaltic
concrete, reinforced or unreinforced "unite or shotcrete, and timber. Dif-
ferent thicknesses, joint details, and spacing for concrete facings have been
developed over the years.
Newer dams of this type have been constructed with thinner slabs, re-
duced amount of reinforcement, minimum joint spacing, and closed verti-
cal construction joints. Horizontal joints have been limited to those re-
quired for construction purposes. A zone of compacted fine-grained soil has
been placed over the lower elevations of the facing when the dam site is V-
shaped or where there is an inner gorge (Davis and Sorenson 1969~.
Impervious Core Rockfill Dams
Impervious core rockfill dams consist of an interior impervious zone or ele-
ment supported by zones of dumped or compacted rock. The interior ele-
ment controls the retention of the water and is usually a compacted imper-
vious soil protected by filter or thin transition zones. A few old dams have
thin vertical concrete core walls located on the central dam axis. Depend-
ing on the position and configuration of the core, these dams are usually
classified as central core, inclined core, or sloping core rockfilis, and each
has its own stability, seepage control, construction advantages, and site
compatibility characteristics.
The composition and construction of the filter and transition zones are
especially critical in this type of dam because of the relative thinness of the
core and the magnitude of the hydraulic gradient.
Rockfilled Crib Dams
Rockfilled crib dams consist of a framework of interlocked timbers or con-
crete prismatic bars that confine rock blocks and fragments. The water fac-
ing and overpour surfaces are usually timber fastened to the crib members.
OCR for page 217
Embankment Dams
217
Construction of this type of dam was common around the turn of the cen-
tury, and some were later modified by the construction of concrete super-
structures. The stability characteristics of a crib dam resemble those of a
concrete gravity section; however, it is listed here because of its rock com-
position. Timber crib dams are sometimes constructed for diversion pur-
poses.
Foundations
Embankment dams can be constructed on foundations that would be un-
suitable for concrete dams. The foundation requirements for earthfill dams
are less stringent than those for rockfill dams (Engineering Foundation
1974~. Foundations for embankment dams must provide stable support un-
der all conditions of saturation and loading without undergoing excessive
deformation or settlement. The foundation must also provide sufficient re-
sistance to leakage where excessive loss of water would be uneconomic.
Foundations are extremely variable in their geologic, topographical,
strength, and water retention characteristics. Each is unique and is an inte-
gral part of a dam. During design and construction the foundation charac-
teristics can be modified and improved by such treatments as excavation,
shaping, curtain and consolidation grouting, blanketing, densification, in-
stallation of sheet piling, prewetting, etc. These various forms of treatment
are primarily for the purposes of (1) strengthening, (2) safely controlling
seepage and leakage, and (3) limiting the influence of the foundation on
embankment deformations. However, for an existing dam one can only
evaluate the effectiveness of the treatment from the construction record
and observable performance.
Based on strength and resistance to seepage and leakage, foundations can
be~typified as (1) rock, (2) sand and gravel, and (3) silt and clay or a combi-
nation thereof. Earthfill dams have been adapted to all three of these types
of foundations. Types 2 and 3 have generally been determined unsuitable
for faced rockfill dams. Type 3 has been determined unsuitable for imper-
vious core rockfill dams without extensive foundation excavation and treat-
ment.
The foundation types have been treated in a variety of ways depending
on the designers' versatility and objectives and the type and configuration
of the dam. The foundations of many existing dams will not have received
any special treatment and present safety concerns. Where treatment was
afforded, it varies under the different zones of the dam, depending on the
intended functions of the zones and the foundation type.
Foundations have been treated for seepage control by (1) earth back-
filled cutoffs, (2) concrete or sheet piling cutoff walls, (3) slurry walls, (4)
grout curtains, (5) vertical drains, (6) relief wells, and (7) impervious earth
OCR for page 218
218
SAFETY OF EXISTING DAMS
blankets or a combination of these methods (Wilson and Marsal 1979~.
Foundations have been treated for strengthening by (1) excavating weak
materials and formations; (2) consolidation grouting; (3) prewetting col-
lapsible soils; (4) installing vertical drains to accelerate consolidation and
accompanying strength gain during embankment placement; and (5) to a
limited extent, vibratory densification. Foundations containing saturated,
fine, cohesionless sand of low density are suspect, especially in regions of
higher seismicity, because of the tendency of the sand to collapse and liq-
uefy Luring long-duration ground shaking from earthquakes. Many foun-
dations of this type have probably received no treatment for such a condi-
tion.
DEFECTS AND REMEDIES
It has been emphasized that dam failures are usually caused by a complex
chain of events that involves one or more defects and that failure can be
averted by properly identifying and remedying the defects. For embank-
ment dams the major nonhydraulic defects causing failure ultimately in-
volve slope or foundation structural instability and/or slope or foundation
seepage instability. Closely associated defects are excessive settlement,
slope erosion, malfunctioning drains, problems at the abutment or founda-
tion/embankment interface, and/or excessive vegetation and rodent activ-
ity.
Equally important threats to the overall structural or seepage stability of
the dam are defects in appurtenant structures, such as spillways and con-
duits, and associated outlet works, such as gates, hoists, and valves.
The following sections include discussions of common defects that can
cause partial or total failure of the dam, indicators of these defects, possible
causes of each defect, effects on the dam, methods of investigating the de-
fects, and potential remedial measures, with brief examples of actual appli-
cations on existing dams. Table 7-1 is an evaluation matrix for embank-
ment dams that briefly summarizes these discussions.
In applying these and other remedies the complex interrelationships be-
tween the clam and its foundation, appurtenant structures, and reservoir
margin must be considered. Furthermore, extreme caution must be exer-
cised to avoid creating a new defect in the process of remedying an existing
one.
Slope and Foundation Instability
Instability of embankment dams or their foundations may occur as a result
of (1) extended periods of high reservoir level that result in high pore pres-
OCR for page 219
Embankment Dams
219
sures within the embankment, (2) rapid drawdown of the reservoir from a
high level, (3) earthquake shaking, or (4) deterioration of effectiveness of
drains and other factors. Each of these conditions deserves careful atten-
tion when dam safety is evaluated.
Unless an embankment shows signs of instability or high pore pressures
during normal operations, there is no way to determine by inspection
whether it will be stable under the above described loading conditions.
Evaluating the stability of an embankment for such conditions requires de-
termination of the strengths of the embankment and foundation materials
and comparison of these strengths with the stresses that result from the
loading.
Failures of dams due to extended periods of seepage at high pool have
occurred in at least one large dam and in a number of smaller dams. If a
dam is found to be unstable for steady seepage at high pool level, the most
common remedy is to install drains, relief wells, or other seepage control
measures to reduce the magnitudes of the pore pressures within the em-
bankment and/or its foundation.
Rapid drawdown has caused instability in the upstream slopes of many
dams, including Pilarcitos Dam and San Luis Dam in California and many
others. Rapid drawdown slides in embankments ordinarily do not have any
significant potential for loss of impoundment, because they usually involve
sliding at a depth of only a few feet in the upstream slope and do not extend
through the top of the dam. In the case of San Luis Dam, however, the
sliding was considerably deeper, extending into a layer of highly plastic
clay in the foundation. Although the slide at San Luis Dam did not extend
through the top, deep-seated failures of this type do have a potential for
doing so, and this possibility needs to be considered when stability during
drawdown is evaluated. Stability during drawdown can be improved by
flattening the upstream slope or by adding a layer of free-draining material
to blanket the upstream slope.
Earthquakes have caused instability and complete failures in dams built
of loose, cohesionless (sandy or silty) soils and dams built on foundations
containing such soils, which can "liquefy" or lose all strength under cyclic
loading. Examples include Sheffield Dam and Lower San Fernando Dam
in California. Sheffield Dam failed completely as a result of liquefaction of
loose sands in the foundation, and the entire reservoir was released. Lower
San Fernando Dam suffered a deep-seated upstream slide that extended
through the top of the dam and lowered the top to within about 3 feet of
reservoir level. The reservoir was lowered as quickly as possible, and com-
plete failure was avoided by a narrow margin.
Usually when a dam is found to be unsafe during an earthquake because
of a liquefiable foundation, the remedy is to build another, more stable
OCR for page 220
220
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OCR for page 248
248
SAFETY OF EXISTING DAMS
the early life of a dam may become plugged or broken, or otherwise mal-
function, thereby creating excessive uplift (internal hydrostatic pressures)
within the dam and foundation. It is also possible that an outward appear-
ance of drain malfunction can be created not by changes in the drains
themselves but by deterioration of the dam embankment or foundation,
permitting increased seepage that the drainage system was not designed to
handle. On the other hand, silt deposits in the reservoir may reduce the
seepage appearing at foundation drain outlets. It is extremely important,
therefore, that any situation that appears to involve a deterioration of
drainage capacity be thoroughly evaluated by an experienced engineer be-
fore corrective measures are defined and implemented.
One excellent means of inexpensively providing information that can be
highly useful for diagnosing a potentially deteriorating drainage system is
to channel to one or two locations and regularly measure visible seepage
flows emerging from the dam toe area and any installed drains. This should
be done over as wide a range in reservoir levels as possible, so that a rela-
tionship can be established between reservoir level and anticipated drain-
age flow rate. Any significant changes in the flows defined by this relation-
ship may be cause for further investigation. Drain water samples can also
be taken for chemical testing if it is suspected that chemical or bacterial
reactions are involved in changing drain flows. If flow reductions are
found to be occurring, backflushing of the drains can sometimes alleviate
the problem. If flow increases are occurring, the water chemistry testing
might indicate foundation solution activity.
Other corrective measures for malfunctioning drains or inadequate
drainage can assume a number of forms. Excessive uplift resulting from
inadequate control of seepage can be reduced to acceptable levels. If there
are foundation drains and formed drains in the dam that have become
plugged with chemical deposits, they can sometimes be reamed and their
effectiveness restored if they are accessible from drain galleries or from the
top of the dam. New foundation drains, both vertical and horizontal, can
be drilled. If water losses are excessive, the foundation can be regrouted
from galleries, if they exist, or from the top of the dam, but usually the
more effective way to reduce uplift is by the addition of drainage.
Founciation-Embankment Interface
The foundation-embankment interface is a critical area from the principal
standpoints of both overall stability and seepage prevention and control. A
poor bond between embankment and foundation can lead to piping by cre-
ating a favored seepage path along the contact. Improper or incomplete
OCR for page 249
Embankment Dams
249
treatment of foundation joints and fractures, alone or coupled with an in-
adequate filter relationship between the embankment and the joints, can
also lead to embankment piping and eventual collapse from internal ero-
sion. Inadequate stripping of loose or otherwise undesirable foundation
materials can create a weak plane along which major embankment insta-
bility could occur.
Where one or more grout curtains are constructed beneath a dam, it is
imperative that continuity of the impervious dam zone and the grout cur-
tain be maintained. This cannot be achieved without proper treatment of
the foundation-embankment interface.
Once a dam embankment is constructed, it is obviously rather difficult,
and sometimes impossible, to correct construction deficiencies along the
foundation-embankment interface without first removing the embank-
ment material from the problem area. As discussed earlier, problems such
as incomplete treatment of foundation fractures and joints can sometimes
be remedied by grouting through the dam embankment if the deficient
areas are discovered and their extent defined, and they can be corrected
before a major failure occurs.
The physical features of a defect at the foundation-embankment inter-
face usually are not directly observable because they are hidden by the
dam. The presence of the defect characteristics must, therefore, be de-
duced from indirect as well as direct evidence, obtained instrumentally or
from drilling cores and logs and a study of visual manifestations, such as
dissolved solids in seepage water or movements in the dam itself. For this
type of problem, evaluation by an experienced engineer is essential, but
even if the problem is properly defined, the cost of its solution may be very
high.
Trees and Brush
Trees and brush are frequently allowed to grow on the slopes and tops of
embankment dams. These forms of vegetation should be removed, espe-
cially for small dams, for the following reasons:
· Potential for loss of freeboard and breaching if trees on the top are
blown over during high-water conditions.
· Potential dangerous loss of dam cross section if trees on or near the
slopes are blown over.
· Potential initiation of leakage by piping if trees die and root systems
rot to become channels for flow.
· Obstruction of visibility and access to hamper observation and main-
tenance of embankments.
OCR for page 250
250
SAFETY OF EXISTING DAMS
Each of these potential problems is considered in the following paragraphs
and recommended general criteria for removal are included.
As a general rule, trees and brush should be removed from, and their
growth prevented on, dam and dike embankments. Vegetation on slopes
should consist of grass and should be cut at least annually so that there can
be effective monitoring for animal burrows and seepage. Trees and brush
adjacent to embankment slopes should be cut back at least far enough to
permit such observation and to allow access to the toes of the slopes by
maintenance equipment.
Tree root systems will vary with soil type and groundwater conditions.
The following general comments, subject to further consideration at each
site, are offered for general guidance:
· Root systems of usual tree types do not grow into the zone of satura-
tion, i.e., below the steady-state phreatic surface. One result is that trees in
swamp areas are shallow rooted and easily blown over.
· The spread of root systems is generally comparable to the spread of the
branches but will vary with tree type and soil conditions.
· Root system penetration tends to be as follows:
Pine: typical mat depth of 1 to 2 feet, maximum of 2.5 to 3 feet.
Softwoods: generally shallow rooted.
Oak: both deep and shallow rooted, typically 2 to 5 feet maximum
mat depth (in glacial till likely to be 1 to 3 feet, more typical in loose-to-
medium-compact, fine sand).
Maple: 10 to 20 % shallower than oak, typically 1 to 2.5 feet for major
part of mat.
Ash: relatively deep rooted but less dense mat than oak.
Birch: relatively shallow rooted, typically 1 to 2 feet maximum mat.
Criteria for removal of individual trees and stumps should also consider
the potential for damage due to the root systems. Living or dead trees
whose uprooted root systems could endanger a dam or dike should be cut.
This would include trees that could damage upstream slope protection
and trees on a crest where uprooting could leave less than a 10- or 12-foot
width of undamaged embankment. Trees on or near a downstream slope
should be removed if their root systems can penetrate significantly into
the minimum necessary embankment cross section.
Stump and major root removal should also be based on potential for
damage. Major root systems in the top of the dam or within a minimum
embankment section offer some potential for embankment damage by de-
cay and should probably be removed. However, the decay will generally be
to humus rather than to a void. There is some possibility of inside root de-
OCR for page 251
Embankment Dams
251
cay with attendant potential for water flow, more so with hardwoods, and
it would be prudent to remove major roots that traverse the top of a dam.
Stumps and root systems that are not in critical locations can be left to rot in
place unless removal is necessary for slope grading. The soil will gradually
close in.
In connection with tree work on dam and dike embankments, the fol-
lowing additional comments are offered:
· The integrity of trees that remain in place should be considered—their
root systems may be damaged by adjacent work, or they may be more ex-
posed to wind.
· Cutting shallow roots to limit growth will not work without a barrier.
Cutting will stimulate additional growth.
· Vegetation must be reestablished in work areas to protect embank-
ments from erosion.
· Backfill material after stump or root removal should have characteris-
tics similar to the embankment material at that location.
Rodents and Other Burrowing Animals
The burrowing of holes in earthfill dams by rodents is a widespread main-
tenance problem. This problem is known or suspected to have caused sev-
eral failures of small dams. The animals that have caused the most prob-
lems are beavers, muskrats, groundhogs, foxes, and moles. Beavers and
muskrats cause the largest problem because they operate below the water
level. They sometimes burrow holes below the water from the lakeside all
the way through the dam.
Frequent visual inspections of the earthfill embankment should be made
to detect the presence of animals or the holes they have made. If the pres-
ence of these animals is detected in the vicinity of the dam area, the animals
should be eradicated by either trapping, shooting, or with poison. If they
have made holes that are carrying or could carry water through the dam,
these holes should be immediately repaired by excavating and recompac-
tion or by filling with a thick slurry grout.
STABILITY ANALYSES
The stability of an embankment dam, in conjunction with its foundation,
must be evaluated from a number of different standpoints, as can be appre-
ciated from the preceding discussions concerning the many potential de-
fects that can create unstable conditions within the structure-foundation
system. Among the various methods of stability analyses available to the
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252
SAFETY OF EXISTING DAMS
engineer is the conventional analysis of slope stability, which, although it is
a valuable too] in the assessment of embankment adequacy, must be per-
formed and used with experienced judgment or it can produce completely
misleading results that could lead to erroneous, even disastrous, conclu-
sions regarding the safety of a dam. This is true because, as with most other
types of numerical analysis, the final results are only as valid as the data
used as input to the computations.
In the case of earth dams and their foundations, the input data them-
selves are often subject to fairly wide ranges of interpretation simply be-
cause the engineer is working with native materials that have been altered
in varying degrees by the forces of nature.
Especially in the case of the analysis of stability of existing embankment
dams, the exploration, sampling, laboratory testing, and materials proper-
ties evaluation program always has physical and economic constraints that
limit the extent of knowledge that can be gained about the important phys-
ical properties of any given structure. For this reason the physical proper-
ties data that must be input to a numerical stability analysis are always
subject to varying uncertainties that must be put in their proper perspective
for each individual case. It is in this critical area that experience, as well as
engineering judgment, are critical to the performance of the numerical
analysis and evaluation of results. Even when the results of an analysis ap-
pear favorable, they cannot be viewed in a vacuum but must be integrated
with all the other information available on the safety of the particular
structure, thus becoming an important part, but still only a part, of the
overall stability evaluation.
Methods of Slope Stability Analysis
Various methods of slope and foundation stability analyses are available.
The more common ones are two-dimensional and are based on limiting
equilibrium. These analyses are known by a variety of titles, including slip
circle, Swedish circle, Fellenius method, method of slices, and sliding
block. There are differences in assumptions and force resolutions in the dif-
ferent methods. When forces representing earthquake effects are included,
the analysis is often termed pseudostatic.
An analysis is made by assuming some form and location of failure sur-
face, such as a circular arc, compound curved surface, or a series of con-
nected plane surfaces. The configuration and positioning of the surface de-
pend on the kind of embankment dam, the internal zoning, and the
foundation's geologic structure. For example, connected plane surfaces are
often used for an inclined or sloping core rockfill dam. Also, the trial failure
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Embankment Dams
253
surfaces are positioned judgmentally to pass through weaker or more
highly stressed regions. Thus, a plane surface may be positioned in a con-
fined fluvial foundation susceptible to high pore pressure. The most critical
surface is defined as the one having the least computed factor of safety. The
factor of safety is considered to be the ratio of forces or moments resisting
the movement of the mass above the surface being considered to the forces
or moments tending to cause movements. Both embankment slopes are an-
alyzed for the specific service conditions expected.
Allied analyses are used during stability studies to determine seepage
patterns and amounts, pore pressures, uplift forces, hydraulic gradients,
and escape gradients in the embankment zones and the foundation by the
application of the principles of flow through porous media and the graphi-
cal or mathematical modeling of flow nets (Cedergren 1967~.
It is beyond the scope of this report to present the details of the many
numerical methods available to analyze the stability of an embankment
dam foundation system. These methods are discussed in great detail, with
examples, in university textbooks for fundamentals; professional engineer-
ing society publications, such as the journals of the American Society of
Civil Engineers, which consider practical, specific applications, and design
manuals, monographs, handbooks, and standards of federal and state
agencies engaged in the design of earth dams, such as the U.S. Army Corps
of Engineers and the U. S. Bureau of Reclamation. The reader is referred to
the references in this chapter for details of the subjects discussed in the fol-
lowing sections.
Loading Conditions
Dams that have been stable for a period of time may become unstable
when subjected to more severe loading conditions. Conditions that should
be considered in the stability analyses of dams are listed in Table 7-2. These
include (1) steady seepage with the highest pool level that may persist for a
significant period, (2) rapid drawdown from normal pool to lower pool
elevations, and (3) earthquake loading conditions. If a dam has not been
subjected to the most severe loading conditions expected, its safety can be
evaluated by measuring the strengths of the materials of which it is built
and by performing analyses to compare these strengths to the stresses in the
dam.
For modern dams, where factors of safety as shown in Table 7-2 were
evaluated during design, sufficient information may already be available
such that only a review is needed to establish the adequacy of the embank-
ment and its foundation. As-built drawings, construction records, tests and
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SAFETY OF EXISTING DAMS
TABLE 7-2 Loading Conditions, Required Factors of Safety, and Shear
Strength for Evaluations for Embankment Dams
Case
Loading Condition
Required Factor
of Safetya
Shear Strength for
Evaluationb
1
2
Steady seepage at high pool level
Rapid drawdown from pool level
Earthquake reservoir at high pool
for downstream slope; reservoir
at intermediate pool for
upstream slope
1.5
1.2
1.0
S strength
Minimum composite of
R and S
R tests with cyclic
loading during shear
aRatio of available shear strength to shear stress, required for stable equilibrium.
bTerminology from U.S. Army Corps of Engineers. R = total stress shear strength from con-
solidated-undra~ned shear tests; S = effective stress shear strength from drained or consoli-
dated undrained shear tests.
SOURCE: U.S. Army Corps of Engineers.
record samples, and performance records of piezometric levels and move-
ments can be used to confirm or refute the suitability of design assumptions
and evaluations.
For older dams, where factors of safety as shown in Table 7-2 have not
been calculated, evaluating safety will require collecting data to estimate
the strength properties of the embankment and the pore pressures within it,
and performing stability analyses.
Steady Seepage Conditions
The highest reservoir level that may persist over a significant period of time
constitutes the most severe conditions of steady seepage, resulting in the
lowest factor of safety for the downstream slope. A knowledge of water
pressures within the various zones in a dam and its foundation is essential
for a stability analysis. Field data may be obtained from observation wells
and piezometers, as described in Chapter 10. Thereafter, pore water pres-
sure throughout the embankment can be predicted from seepage analyses,
providing the information needed for an effective stress analysis of slope
stability.
Rapid Drawdown Condition
Rapid drawdown subjects the upstream slopes of dams to severe loading by
quickly reducing the stabilizing effect of the water acting against the slope
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Embankment Dams
255
without significant reduction of the pore water pressures within the soils
forming the upstream embankment. Rapid drawdown slides occur in soils
of low permeability, which do not drain freely. Generally, they are shal-
low slides within the upstream slope that pose no significant threat of loss of
impoundment. In some cases (notably the slide at San Luis Dam) rapid
drawdown slides extend into the foundation, and such deeper slides may
pose a hazard for loss of impoundment if the slide cuts through the top of
the dam.
Earthquake Condition
Earthquake accelerations impose forces on embankments and their foun-
dations; these forces are superimposed on the static forces. As a result, em-
bankment dams may suffer a number of kinds of damage during earth-
quakes. According to Seed et al. (1977), these include disruption by fault
movement, loss of freeboard owing to fault movement, slope failures in-
duced by ground shaking, liquefaction of the foundation or embankment
material, loss of freeboard due to slope failures, loss of freeboard owing to
compaction of embankment materials, sliding of the dam on weak founda-
tion soils or rock, piping through cracks induced by shaking, overtopping
by earthquake-generated waves in the reservoir, and overtopping by waves
caused by earthquake-induced landslides or rockfalls into the reservoir.
The type of damage is highly dependent on the type of soil acted on by the
earthquake forces. For example, embankment fill material that contains
significant amounts of clay is able to withstand short-lived increases in load
without a catastrophic failure; however, such embankments may suffer
some slumping and permanent deformation. Cohesionless soils that are sat-
urated may suffer dramatic loss of shearing resistance when subjected to
cyclic loading. In the extreme case, saturated cohesionless materials may
assume the properties of a dense viscous liquid. This liquefied state may
persist for several minutes under the earthquake motion and cause the em-
bankment fill and/or the foundation to flow as a liquid. The most severe
failures of embankment dams during earthquakes have occurred as a result
of this liquefaction of loose sandy soils.
To evaluate the possible effects of earthquakes on embankment dams,
two possibilities must be considered: (1) the fault motion in the foundation
can disrupt the embankment or cause loss of freeboard and (2) there may be
some form of damage caused by the ground shaking. In the first case the
dam must be able to absorb cracks and shears without suffering damaging
piping or erosion; it must have an adequate amount of freeboard prior to
the earthquake; and the dam designer must accurately estimate the poten-
tial magnitude, location, and direction of the fault movement during the
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256
SAFETY OF EXISTING DAMS
earthquake. The second effect can be sliding within the embankment or
the foundation material, settlement due to compaction of the soil materi-
als, and cracking and/or subsequent erosion of the embankment materials.
To survive this type of earthquake motion the dam must be designed with
adequate density of soils, the construction process must have had adequate
quality control, and finally the intensity of the earthquake shaking must
have been properly estimated by the designer. Experience with embank-
ment dams during earthquakes has shown a marked difference in perfor-
mance dependent on the type of material of which the dam is built and the
quality of construction. The performance of nearly 150 dams during earth-
quakes has shown that hydraulic-fill dams and dams built on loose materi-
als frequently suffered severe damage. On the other hand, dams built of
clay soils on stable foundations performed very well, although many were
subjected to very strong shaking (Seed et al. 1977~.
Shear Strength Evaluation
Soil strengths for stability analyses are most often evaluated through labo-
ratory biaxial or direct shear tests. To provide useful information, the tests
must be performed under conditions corresponding to those in the field
(drained or undrained, static or cyclic loading), and the samples must be
representative of the soils in the field with respect to density and water
content.
For most cohesive soils it is possible to obtain "undisturbed" samples for
testing that retain essentially the same properties as in the field. It is possi-
ble to sample cohesionless soils only by very expensive and elaborate proce-
dures requiring highly sophisticated equipment and procedures, and it is
common to estimate the in situ relative densities of such soils based on the
results of static or dynamic penetration tests. The shearing resistance of co-
hesionless soils may be evaluated by performing laboratory tests on samples
compacted to the in situ relative density, by correlations between shearing
resistance and relative density for similar soils, or by large-scale field direct
shear tests.
Large biaxial shear testing equipment developed in the past 30 years has
enabled more accurate determination of strengths of rockfills. Friction an-
gles vary widely, depending on characteristics of the rock in the fill. Con-
fining pressure is an important parameter (tops 1970~.
Seismic Analyses
Pseudostatic methods of analysis, in which dynamic earthquake loads are
represented by static loads, can be used to assess the stability of dams built
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Embankment Dams
257
of cohesive soils on stable foundations (Makdisi and Seed 1977~. Pseudo-
static analyses do not provide a suitable means for evaluating stability of
dams built of or built on loose cohesionless materials, because they do not
provide a means for including the potential these materials have for
strength loss under cyclic loading. To evaluate the stability of loose cohe-
sionless materials, more realistic dynamic analyses should be used, in con-
junction with special laboratory tests to evaluate soil strength under cy-
clic loading.
Although they generally perform well during earthquakes, dams of co-
hesive soils on stable foundations may suffer some permanent deforma-
tion and loss of freeboard due to earthquake shaking. These deformations
may be estimated using a simplified procedure suggested by Makdisi and
Seed (1977), or they may be analyzed in greater detail through dynamic
finite element analyses of embankment and foundation response to seis-
mic loading.
Factors of Safety
Typical factors of safety for the loading conditions discussed previously are
shown in Table 7-2. These are the minimum values required for dams un-
der the jurisdiction of the U. S. Army Corps of Engineers and thus represent
standards of practice that find wide application, even though they may not
be universally accepted by all agencies and for all circumstances.
REFERENCES
ASCE/USCOLD (1975) Lessons from Dam Incidents, USA, American Society of Civil Engi-
neers, New York.
Cedergren, H. R. (1967) Seepage, Drainage, and Flow Nets, John Wiley & Sons, New York.
Civil Engineering-ASCE (1981) Dike Safety Upgraded with Millions of Square Feet of Fabric,
January.
Davis, C. V., and Sorenson, K. E. (1969) Handbook of Applied Hydraulics. Section 18 by
John Lowe III, Embankment Dams, and Section 19 by I. C. Steele and J. B. Cooke, Con-
crete-Face Rock-Fill Dams.
Fetzer, C. A. (1979) Wolf Creek Dam, Remedial Work Engineering Concepts, Actions and
Results, Transactions of ICOLD Congress, New Delhi.
Gordon, B. B., Dayton, D. J., and Sadigh, K. (1973) Seismic Stability of Upper San Leandro
Dam, ASCE, San Francisco.
Jansen, R. B., lDukleth, G. W., and Barrett, K. G. (1976) Problems of Hydraulic Fill Dams,
Transactions of ICOLD Congress, Mexico.
Japan Dam Foundation, Tokyo (1977) "Use of Slurry Trench Cut-Off Walls in Construction
and Repair of Earth Dams," in World Dam Today '77, 576 pp.
Koerner, R. M., and Welsh, J. P. (1980) Construction and Geotechnical Engineering Using
Synthetic Fabrics, John Wiley & Sons, New York.
OCR for page 258
258
SAFETY OF EXISTING DAMS
Lamberton, B. (1980) "Fabric Forms for Erosion Control and Pile Jacketing," Concrete Con-
struction Magazine, May.
Leps, T. M., Strassburger, A. G., and Meehan, R. L. (1978) "Seismic Stability of Hydraulic
Fill Dams," Water Power and Dam Construction, October/November.
Leps, T. M. (1970) "Review of Shearing Strength of Rockfill," Journal' ASCE Soil Mechanics
and Foundations Division, July.
Makdisi, F. I., and Seed, H. B. (1977) A Simplified Procedure for Estimating Earthquake-
Induced Deformations in Dams and Embankments, EERC, University of California.
Pravidets, Y. P., and Slissky, S. M. (1981) "Passing Floodwaters Over Embankment Dams,"
Water Power and Dam Construction, July.
Proceedings of Engineering Foundation Conference (1974) "Foundation for Dams," Asilo-
mar, Calif.
Seed, H. B., Makdisi, F. I., and DeAlba, P. (1977) The Performance of Earth Dams During
Earthquakes, Report No. UCB/EERC-77/20, Earthquake Engineering Research Center,
University of California, Berkeley.
Sowers, G. F. (1962) Earth and Rockfill Dam Engineering, Asia Publishing House.
Timblin, L. O., Jr., and Frobel, R. K. (1982) Geotextiles—A State-of-the-Art Review, pa-
per presented to USCOLD annual meeting.
U.S. Army Corps of Engineers (1982) National Program for Inspection of Nonfederal Dams.
Final Report to Congress, May 1982 (contains ER 1110-2-106, September 26, 1979~.
U.S. Bureau of Reclamation (1974) Design of Small Dams, Water Resources, Technical Publi-
cation, Government Printing Office, Washington, D.C.
United States Committee on Large Dams (1981) "Mt. Elbert Forebay Reservoir," USCOLD
Newsletter, March.
Wilson, S. D,. and Marsal, R. J. (1979) Current Trends in Design and Construction of Em-
bankment Dams, ICOLD/ASCE, New York.
RECOMMENDED READING
ASCE, Soil Mechanics and Foundation Division (1969) Stability and Performance of Slopes
and Embankments, New York.
Cortright, C. J. (1970) "Reevaluation and Reconstruction of California Dams," Journal of the
Power Division, ASCE, January.
D'Appolonia, D. J. (1980) "Soil-Bentonite Slurry Trench Cutoffs," Journal of the Geotechni-
cal Engineering Division, ASCE, April.
Golze, A. R., et al. (1977) Handbook of Dam Engineering, Van Nostrand Reinhold Company,
New York.
Hollingworth, F., and Druyts, F. H. W. M. (1982) Filter Cloth Partially Replaces and Supple-
ments Filter Materials for Protection of Poor Quality Core Material in Rockfill Dam, Trans-
actions, ICOLD.
USCOLD Newsletter (1977) "Diaphram Wall for Wolf Creek Dam," July.
Representative terms from entire chapter:
existing dams
