seismic remediation of dams in california, an engineering geology perspective
TRANSCRIPT
Seismic Remediation of Dams in California, An Engineering Geology Perspective
William A. Fraser, P.G., C.E.G.
Chief, Geology Branch
CA Division of Safety of Dams
PO Box 942836
Sacramento, CA 94236-0001
Abstract
Over the past 25 years a number of dams in California have been analyzed for seismic
stability and have required remediation to improve their seismic stability. Embankment
dams are subject to slope failure under earthquake loading which causes deformation
and settlement of the dam crest, potentially resulting in uncontrolled release of the
reservoir. Detailed review of a dams design and construction history is an essential
component of predicting its future seismic performance. Embankment dams with
liquefiable alluvial foundations are perhaps the most common cause of seismic
instability. Older embankment dams constructed of poorly compacted soils and dams
constructed using hydraulic fill methods are also problematic. These dams are usually
remediated by improving the foundation strength and constructing a buttress against the
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embankment slope and improved foundation. Concrete arch dams become
overstressed when the seismic loads exceed the tensile strength of the concrete
resulting in cracking of the concrete. Remediation of a concrete arch dam usually
involves thickening of the arch with additional concrete section to improve its seismic
performance. Multiple arch dams rely on buttresses that are susceptible to cross-
channel toppling causing loss of support for the arch. The remediation of a multiple
arch dam often involves filling the area between the buttresses with mass concrete to
resist toppling. This paper presents case histories of several of remediation projects
that the author has worked on during his over 25 years with the California Division of
Safety of Dams, emphasizing the role of the engineering geologist in seismic
remediation.
Seismic Remediation of Dams in California, An Engineering Geology Perspective
Introduction
Since 1929, the California Division of Safety of Dams (DSOD) has supervised the
design, construction, maintenance and operation of non-Federal dams in California.
California is one of the more seismically active areas in the United States and
earthquakes are one of the more severe loading conditions dams need to withstand. It
is not surprising that DSOD dedicates significant resources to insuring jurisdictional
dams perform well in major earthquake events.
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As part of the project team, engineering geologists play an important role in
characterizing the seismic hazard of a given site and characterizing the site conditions
needed to evaluate a dam’s ability to resist seismic loading. Engineering geologists also
play an important role in the design and construction of the remedial measures that are
taken to improve a dam’s seismic performance. This paper discusses the author’s
vision of the role of engineering geologists in these civil engineering activities.
Identification of Dams with Seismic Deficiencies
The design and construction history of a dam is one way to recognize a potentially
deficient dam. In the first half of the 20th century, neither the ability to predict
earthquake loads nor the tools to evaluate the performance of dams under seismic
loading were available. Seismic design was only a minor consideration as compared to
static stability and seepage considerations. Unfortunately, some designs and
construction practices of that day resulted in dams which often proved to be poor
performers under earthquake loading. Two such designs are multiple arch dams and
hydraulic fill dams.
Hydraulic fill dams consist of sand, silt, and clay soils transported and placed by
hydraulically sluicing though an often elaborate conveyance system of flumes
constructed for the project (Photo 1). The sluiced soils were usually ponded between
two dumped rockfill dikes placed at the upstream and downstream toes of the dam
under construction. This technology was an outgrowth of the hydraulic mining industry,
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once a common mining technique in California outlawed in 1888. This approach was
used to build many dams in California through the 1930’s before mechanized
construction equipment was readily available. However, slope failures of hydraulic fill
dams during construction were not uncommon due to the lack of compaction and high
consolidation pore pressure. Their seismic deficiency was fully appreciated during the
1971 San Fernando Earthquake when the upstream slope and the crest of the Lower
San Fernando Dam slumped into the reservoir. Although the dam was badly damaged,
the reservoir was not released fortunately averting a catastrophic flood in urban Los
Angeles.
A multiple arch dam consists of a series of concrete buttresses which support a series
of concrete arch barrels between the buttresses (Photo 2). The design was popular in
the early 20th century because it required a smaller volume of concrete compared to a
massive concrete gravity dam. However, the buttresses of multiple arch dams can
topple when subjected to earthquake loading in the cross channel direction resulting in
the loss of support for the arches and release of the reservoir.
A second way that a potentially unstable dam can be recognized is by a history of poor
static performance or evidence of deterioration. Dams that perform marginally under
static conditions are likely to also perform poorly under earthquake loading. Excessive
movements of the embankment in response to changes in reservoir storage or a history
of instability are all symptomatic of a dam that will likely prove to be deficient under
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earthquake loading. A poorly maintained dam, such as a concrete dam with evidence
of deterioration of the concrete, could also prove to be seismically deficient.
A third approach to identifying seismically deficient dams is by periodic reassessment of
seismic stability. In California, dams which pose high consequences of failure have
historically been reassessed for their seismic stability every 25 to 30 years. This level of
scrutiny is warranted due to the periodic advancements in geology, engineering
seismology, and engineering practice. New seismic sources such as blind thrusts faults
have been recognized in California as recently as the 1980’s, increasing design loading
in some areas of California. The ability to characterize earthquake loading continues to
improve. The dense strong motion accelerometer network has provided many more
earthquake recordings resulting in improved ground motion prediction equations. While
peak ground acceleration remains a common intensity measure, the use of multiple
intensity measures can better describe the characteristics of the expected earthquake
motions. Studies of the performance of soils during historic earthquakes have led to
improved ability to predict future performance of those soils under future earthquake
loading. Finally, the analytical tools used to evaluate seismic performance of dams
have improved. Greater computational processing power has allowed the use of
software which contains complex constitutive models that simulate the changes in
engineering properties that occur as the earthquake proceeds.
Since 1999, DSOD has had an ongoing reevaluation program to identify dams that are
candidates for seismic reassessment. Considering there are more than 1250 dams of
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jurisdictional size in California, dams with the greatest seismic exposure combined with
the highest consequence of failure are evaluated first.
The approach used by DSOD to evaluate seismic exposure involves determining the
peak ground acceleration (PGA) probabilistically, which increases as the seismic
exposure increases. In a probabilistic analysis, sites essentially accumulate PGA (or
any intensity measure) as earthquakes from various seismic sources near the site are
sequenced through time. The timing of earthquakes is a function of the slip rates for
nearby faults. Therefore, a listing of dams by increasing PGA (at a constant return
period and site condition) essentially ranks dam sites in order of increasing seismic
exposure. The 500-year return period was chosen so sites which accumulate seismic
hazard quickly are emphasized. An improvement to this approach would be to perform
the probabilistic seismic hazard analysis using a time-dependent fault model that
considers the earthquake recurrence cycle on a fault and time since the last event. This
was the motivation for beginning the reanalysis program with the Hayward fault, in light
of the well-publicized analysis by the USGS that placed a very high probability of a
future event on that fault in the next 30 years.
Dams in high hazard-high consequence categories are then initially screened by a team
consisting of a design engineer and engineering geologist for attributes that suggest
seismic fragility, that is, the potential for poor performance during a seismic event. The
dams identified by this initial screening are given a detailed file review of site geology,
design, construction, and maintenance history. The file review is followed by an
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independent characterization of the seismic hazard and site conditions and an
evaluation of the dam’s performance under the design earthquake load. If the dam is
found to be of marginal or inadequate stability, the owner is notified and asked to
undertake a reevaluation of the seismic stability. This effort usually involves additional
field exploration using current standards to better characterize the dam and its
foundation so more confident seismic stability analyses can be performed.
Role of the Engineering Geologist in the Reevaluation of Dams with Potential
Seismic Instability
Poor compaction of earthfill is a common cause of seismic instability in older
embankment dams. The earliest dams in California were compacted by livestock.
Even the early rolled-fill dams sometimes lacked the compaction effort to sufficiently
improve shear strength to resist earthquake loading. Many early embankment dams
were designed without internal drainage to reduce the level of saturation within the dam.
The combination of poor embankment compaction and development of a high saturation
level within the embankment has resulted in many embankment dams proving to be
deficient when analyzed for seismic stability. Engineering geologists are typically
involved in the field investigations, which include field description, field testing, and
undisturbed sampling of the embankment and foundation soils, as well as the
installation of piezometers to determine the level of saturation in the embankment.
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A very common reason for seismic deficiency is the presence of potentially liquefiable
alluvium in the foundation of embankment dams. Dams built before the 1960’s often
have alluvium in their foundations that was left in place purposely by the designers of
that era to promote internal drainage. The alluvium is strong under static conditions, but
under earthquake shaking the saturated alluvium may lose strength resulting in seismic
instability.
The investigation and characterization of the alluvium involves engineering geologists
working in conjunction with geotechnical engineers. Although, the extent of the alluvium
is sometimes shown on the as-built plans for the dam, the existence and extent is
sometimes completely unknown and its limits and thickness must be learned through
exploration. Sonic drilling or other methods that provide continuous sampling of the soil
mass is critical at this stage. The potential for triggering liquefaction of cohesionless
alluvium is evaluated through an understanding the gradation and penetration
resistance of the alluvium. Depending on the gradation of the soil, one or more of the
penetration resistance field tests is used to determine if the soil will liquefy. Current
practices use the Standard Penetration Test (SPT) and Cone Penetration Test (CPT)
for sandy and silty soils and Becker Hammer Penetration Test (BPT) for gravelly soils.
Cohesive alluvium and colluvium is not prone to classic liquefaction, but these soils can
also lose strength and are evaluated by the laboratory testing of undisturbed samples.
If the alluvium proves to be liquefiable, its post-liquefaction residual strength is also
determined by empirical relations that use penetration resistance. Practical
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considerations often require that variable foundation soils be grouped and characterized
together within a generalized strength model. A key contribution made by the
engineering geologist is the use of geomorphologic and time-stratigraphic principals to
assist in defining packages of foundation soils with similar engineering properties. For
example, the different depositional environments produce soil deposits with unique
gradations (i.e., fluvial, colluvial or aolian) and logically can be the basis for grouping
penetration resistance test intervals in a foundation strength model. Also, older alluvium
should possess greater relative density than recent alluvium and these materials can be
distinguishing by elevated position above the modern drainage at sites experiencing
uplift through geologic time. In deep sedimentary basins where deposition has been
occurring over long periods of time, older alluvium can be recognized by the soil profiles
that have developed prior to burial by younger alluvium. Recognizing the depositional
environment is essential to characterizing the strength of unconsolidated materials and
will often provide the framework for confident strength modelling of a dam foundation.
In California, concrete dams rarely have alluvium in their foundations. The most
common cause of seismic deficiency in concrete dams is overstressing of the concrete
under earthquake shaking. Similar to rock, concrete has considerable compressive
strength but much less strength in tension. Strong earthquake shaking can induce
stresses that exceed the tensile strength of the concrete and cause the concrete to
crack. This is especially problematic for arch dams that are generally thin and therefore
more susceptible to failure from cracking. Although the analysis of concrete
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overstressing is beyond the scope of the paper, I do present the remediation of Big Bear
Dam where the multiple arch dam was remediated for seismic stability.
Concrete dams are susceptible to sliding on continuous planar discontinuities that are
adversely oriented with respect to the dam and topography. Bedding planes in folded
sedimentary rock are especially problematic because they are usually continuous over
the scale of a dam foundation and are often filled with weak clay gouge developed
during the folding of the rock mass. Joints are generally not as continuous as bedding,
but can be problematic because they usually occur with multiple orientations. The
orientation of jointing relative to the foundation excavation surface influences the shear
strength of the foundation surface, and wedges formed by adversely intersecting joints
can be unstable under the loading imposed by the dam and reservoir.
Geologists contribute to the characterization of the foundation rock strength by
identifying the presence, orientation, and condition of the discontinuities. Many larger
dams have detailed geologic maps of the foundation showing discontinuities and their
orientations as revealed by the extensive construction excavations. Additional
description of the discontinuities can be made by visual observation test pits and
logging of rock core. Orientation is determined by direct measurement with a Brunton
compass or borehole orientation techniques, such as acoustic or optical televiewers.
Structural geology principals are used to confidently project the position of the
discontinuities away from known points. A sometimes difficult characterization is
determining the shear strength of bedding plane gouge or weathering product on a joint
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surface. A confident assessment usually involves using multiple approaches such as
visual observation of discontinuity roughness and infilling, laboratory testing, and back-
calculating observed slope performance.
Finally, engineering geologists are involved in the seismic hazard assessment and
development of ground motions used in the engineering analysis. This activity involves
either a deterministic or probabilistic hazard assessment, or a combination approach to
develop target ground motion intensity parameters. Multiple intensity measures are
developed such as peak ground acceleration, response spectra, Arias intensity, peak
ground velocity, and significant duration. Generally, for dams in high seismic regions
with high consequence of failure, an 84th percentile deterministic loading from the
controlling seismic source is used for dam safety evaluations. A probabilistic seismic
hazard assessment can also be used to develop intensity measures or to estimate the
return period associated with the deterministic design load. Risk analysis methods are
becoming more common, which require a variety of seismic loading conditions up to or
near the point of failure. Most modern analyses use acceleration time histories as the
basis for the earthquake motion. Natural earthquake recordings are usually modified by
spectral matching or linear scaling to represent safety evaluation earthquake levels. For
scenarios such as Magnitude 8 events where natural records are not available,
synthetic time histories are being used increasingly.
Seismic Remediation Approaches
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If a dam is found to require seismic remediation, several stabilization approaches are
available and chosen on a case-by-case basis. To illustrate some of these remediation
techniques, this section will present several cases histories of dams that have been
remediated during the past 25 years.
An initial determination is needed to decide if the existing dam can be salvaged. In
some cases a deficient dam is so poorly built it is unwise to attempt to remediate it. If
the dam has very poor documentation, was built in multiple stages, has a history of
failure, or has had unsuccessful remediation attempts it may be best to build a
completely new dam.
If an existing dam can be confidently modeled and analyzed, approaches are available
to improve its stability. For embankment dams, these techniques include providing
buttresses, removing and replacing weak soils, and in-situ remediation of weak soils.
Often a combination of these techniques is used. For concrete dams, remediation
techniques typically involve various ways of thickening the concrete section to better
distribute earthquake induced stresses and change the dynamic response of the
structure. Other techniques involve post-tension anchoring, providing or improving
drainage, foundation grouting, upstream liners, and lowering the dam crest.
New Calaveras Dam
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The Calaveras dam, owned by the City and County of San Francisco, is located near
Sunol, California. The 220-foot high dam retains a 100,000 acre-foot reservoir, which is
a major component of the Hetch Hechy Aqueduct that provides water to San Francisco
and other Bay Area communities. The dam is essentially within the Calaveras fault
zone located about 1000 feet from the main trace.
The original Calaveras Dam was partially built by hydraulic fill methods and completed
in 1925. Two construction issues complicated confident characterization of the original
dam. The engineering properties of the hydraulic fill material changed when a new
source area was used mid-way through construction, and more significantly, the
hydraulic fill was placed too quickly and the embankment failed during construction.
When work was resumed several years later, the remaining hydraulic fill section was
reshaped and a compacted rolled-fill was used to construct the upper portion of the
dam. The dam is founded on Franciscan mélange and Tertiary-age Temblor sandstone
with alluvium beneath the channel section.
The dam was found to be seismically deficient in the 1970’s and remediated using an
approach no longer favored today. In 1999 the dam was again identified as a candidate
for seismic reanalysis because of continued embankment deformations in response to
reservoir loading cycles and its close proximity to the Calaveras fault. An extensive
geotechnical investigation was performed in 2001 to characterize the engineering
properties of the embankment and its alluvial foundation. Because of its complex
construction history, the material properties varied widely making confident
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characterization difficult. The new engineering analysis indicated the dam was unstable
under the relatively high design earthquake loads appropriate for the site. It was
decided to construct a replacement dam rather than attempt to remediate the existing
dam again. In the interim, the water storage against the existing dam is restricted to a
level which assures no uncontrolled release of the reservoir in the event of a major
earthquake.
A modern earthfill-rockfill dam, located immediately downstream of the existing dam,
was designed. The existing dam functions as a coffer dam during construction (Photo
3). Since the project is within the Calaveras fault zone, an extensive investigation of
the faulting involving field mapping, geomorphic terrain analysis, and paleoseismic
trenching was undertaken to identify the active fault traces as part of the design review.
An extensive exploration program was also performed to identify the available
construction materials and to investigate geologic hazards and dam foundation issues.
The replacement dam requires a completely new spillway and substantial modifications
to the existing low-level outlet. The replacement dam is currently under construction
with an expected completion date of 2018. The existing dam will then be partially
removed to a configuration which poses no impact to the safety or operation of the new
facility in the event of a major earthquake.
Crane Valley Dam
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Crane Valley Dam, owned by Pacific Gas and Electric Company, is a 145-foot high
hydraulic fill dam built in 1910. The dam impounds Bass Lake, a 45,410 acre-foot
popular recreation area near Mariposa, California.
The dam was suspected of seismic instability because of its hydraulic fill design and
evidence of continuing deformation under static conditions despite earlier efforts to
stabilize the dam. The investigation to evaluate the dam began in 2004, which included
exploration of the dam and its foundation using both conventional land-based and
barge-mounted drill rigs (Photo 4). The hydraulic fill was predicted to liquefy and the
dam was found to deform excessively under the relatively low design loads appropriate
for the Sierra Nevada. The reservoir was then restricted to a level at which the potential
for uncontrolled release during an earthquake was eliminated.
The remediation of Crane Valley Dam is an example of buttressing the slopes of an
embankment dam. Rockfill buttresses for both the upstream and the downstream slope
were designed. In general, the buttressing approach involves construction of a well-
compacted earthfill or rockfill section placed against the existing dam and founded on a
high shear strength foundation surface. The design of the buttress considers the
strength of the existing dam and its foundation, as well as the strength buttress to be
built. The size of the buttress is then optimized to reduce deformations to an acceptable
level. The design often considers the effect of removing a portion of the existing dam to
provide a more strategic location for the buttress and the addition of internal drainage to
insure the buttress is not saturated.
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To be effective, the buttress must be founded on high strength material to resist
excessive deformations resulting from sliding along a low strength surface beneath the
buttress. The Crane Valley site is weathered granitic rock overlain by residual soils and
colluvium. The likely presence of reservoir silt should always be expected in the vicinity
of the upstream toe. The design called for the removal of all reservoir silt, colluvium and
residual soils, providing a severely weathered rock foundation for both the upstream
and downstream buttresses.
At Crane Valley, engineering geologists played an important role in recognizing the
depth to severely weathered rock within the footprints of the buttresses. A detailed
predetermination of the elevation of the top of severely weathered rock was important to
this project because of the short construction season and the need to design an
effective dewatering system to allow excavation for the downstream buttress while
maintaining storage in the reservoir. To accomplish this, an additional exploration
program consisting of closely spaced test pits and borings was performed to identify the
top of severely weathered rock. The exploration also included an additional phase of
barge-based exploration on the upstream side (Photo 5). In this granitic rock,
determination of acceptable foundation was based primarily on visual observations, that
is, recognizing textural characteristics associated with severely weathered rock and the
unacceptable overlying materials in the test pits and boring samples.
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On the downstream side, foundation excavations of about 10 feet were needed to reach
severely weathered rock foundation. The characteristics of the foundation rock could be
confirmed by visual inspection of the exposures (Photo 6). The granitic site location in
the high Sierra allowed a high shear strength buttress comprised of clean coarse rockfill
to be specified. The rockfill was then placed and compacted by conventional methods.
On the upstream side, dredging was performed (Photo 7) using an instrumented
excavator down to the elevation of the top of severely weathered rock as predetermined
through closely spaced exploration. Excavations of up to 20 feet were required to
encounter severely weathered rock. The upstream buttress was placed by
systematically dumping the rockfill from a barge through a few tens of feet of water.
The final configuration of upstream buttress was confirmed using sonar and manual
depth probing.
Sunset North Dam
Sunset North Dam, owned by the City and County of San Francisco, is a lined and
covered 275 acre-foot reservoir built in 1938 within an urban setting in the Sunset
District of San Francisco. A 74-foot high embankment dam forms the northwestern
corner of the reservoir, with the remaining sides of the reservoir created largely by cut.
The dam is founded on up to 20 feet of dune sand and alluvium overlying Franciscan
Formation metamorphic rock. The site is approximately 5 kilometers from the San
Andreas fault.
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Because of its deposition by wind, dune sand is especially clean and poorly graded
which increases its susceptibility to liquefaction if saturated (Photo 8). The dune sand
was recognized during original construction and the records indicate that the sand was
either removed or recompacted to a dense condition, especially beneath the lined
portions of the reservoir. However in the late 1990’s, subsurface exploration revealed
that saturated medium dense dune sand and alluvium still existed beneath the
downstream portion of the Sunset North embankment. After an extensive piezometric
and geotechnical investigation, the dune sand and underlying alluvium were found to be
liquefiable and embankment deformations were judged to be unacceptably large under
the design loading.
Because the urban setting of this reservoir severely limited construction staging areas,
removal and replacement of the dune sand was considered impractical and a program
of insitu foundation remediation using Cement Deep Soil Mixing (CDSM) was
developed.
CDSM is a technique that involves a large specialty drill rig that bores several
overlapping auger holes approximately 3 to 5 feet in diameter (Photos 9). The holes are
advanced to target depth, and upon withdrawal cement slurry is injected and mixed with
the native materials creating soil-cement columns. The columns are configured in a
box-like cell arrangement with roughly 45% of the foundation material within each cell
made into CDSM columns. The approach does not prevent liquefaction of the untreated
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foundation material; however, the strength and arrangement of the columns provide
sufficient shear strength to limit embankment deformations. A berm is often placed over
the treated foundation area to further improve stability.
The advantage of deep soil mixing for foundation improvement is that columns with
predictable dimension and shear strength characteristics are built. Since there is no
attempt to improve the density of the alluvium itself, extensive field testing of the pre
and post treatment condition of the alluvium is not needed. The quality control
standards do include core drilling of a fraction of the CDSM columns to test their
strength and evaluating the percent recovery as a proxy for consistent field mixing of the
cement within the column. A potential disadvantage of the technique is that the cells
can decreases the permeability of the remediated foundation and care must be taken to
provide sufficient drainage to avoid increasing the phreatic line beneath the downstream
portion of the embankment.
At Sunset North, a total of 8000 linear feet of CDSM columns arranged in 27 cells were
constructed. The work was staged from three temporary benches excavated into the
downstream slope of the dam (Photo 10). The columns extend from the base of the
dam through the dune sand, alluvium and several feet into the Franciscan formation
rock. Because of the high costs of the column construction, the actual depth to rock
throughout the remediation area was important to avoid constructing the columns
unnecessarily deep. A map contouring the elevation of the top of rock based on the
extensive existing exploration was helpful for an initial assessment and the
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instrumented performance of the CDSM drill confirmed when rock was encountered on
a column-by-column basis. After the foundation improvement was completed the
embankment rebuilt to its original configuration. Similar CDSM methods were employed
at San Pablo Dam, and will be used at Perris Dam to limit seismic deformation due to
potentially liquefiable foundation alluvium.
Bear Valley Dam
Bear Valley Dam, owned by the Big Bear Municipal Water District, was originally
constructed as a multiple arch dam in 1911. The dam retains the 74,000 acre-foot Big
Bear Lake an important recreational area in the San Bernardino Mountains. The dam is
founded on granitic rock.
The dam was determined to be seismically deficient in the early 1980’s due to buttress
weakness in the cross channel direction. The approach selected to strengthen the dam
was to fill the downstream side of each arch barrel with mass concrete block against
both the arch and the two lateral buttresses that support each arch barrel. This
approach essentially transfers the earthquake induced stresses to the new mass
concrete blocks, essentially converting the multiple arches into a gravity structure. The
remediation was designed for both a M8.0 event on the San Andreas fault and a M6.0
local event on the Bear Creek fault. The highest loadings from these two scenarios
were applied in both the channel parallel and cross channel directions.
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The stabilization was constructed during the summer of 1988. The inclined arches
prevented the use of mechanical equipment to perform the excavation. Because of the
restricted access, hand excavation was required to remove the soil debris and severely
weathered rock to prepare the foundation for the mass concrete. The contractors used
picks, shovels and wheel barrows to excavate the material. Care was taken to not
undercut the foundation support of the existing arches or buttresses. The mass
concrete blocks were ultimately placed on a clean and unyielding surface of moderately
to slightly weathered rock (Photo 11). Finally, a formed narrow interface between the
existing arches and each mass concrete block was grouted to make the structure
monolithic.
A major remediation is a special opportunity to improve the understanding of the
foundation geology of a facility, and these observations are especially important for
older dams without geologic documentation during construction. During the
remediation, I performed detailed geologic mapping of the excavated foundation surface
at a scale of 1 inch equals 20 feet to improve the geologic documentation for this dam
(Figure 1). Geologic mapping of the buttress foundation excavation has become routine
practice DSOD staff during the construction of dam remediation measures.
The remediated dam was tested during the 1992 M6.6 Big Bear earthquake with an
epicenter just several kilometers from the dam. The dam performed well during this
major earthquake and the timely remediation of the dam just a few years earlier may
have averted a potential disaster.
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Acknowledgments
The author would like to thank the management of the California Division of Safety of
Dams for supporting this paper. I would like to thank the numerous geologists and
engineers I have had the pleasure to work with on the seismic remediation efforts in
California. I would especially like to thank Senior Engineering Geologist James
Lessman for his work on the Sunset North and Calaveras Dams, and Senior
Engineering Geologist Chris Tracy for his work on Crane Valley Dam. I also thank Mark
Schultz, Wallace Lam and Karyn Heim for their thoughtful reviews which improved this
manuscript.
Selected References
AMEC Geomatrix, Inc., 2012, Revised Final Design Report, Seismic Retrofit of Crane
Valley Dam: Consultant’s report prepared for Pacific Gas and Electric Company.
Bruce, Donald A., (editor), 2013, Specialty Construction Techniques for Dam and Levee
Remediation: CRC Press.
Fraser, William A., 2001, Engineering Geology Considerations for Specifying Dam
Foundation Objectives: in Ferriz, H., and Anderson, R., (editors), Engineering Geology
Practice in Northern California, AEG Special Publication 12.
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Olivia Chen Consultants, 2001, Sunset Reservoir North Basin Stability Assessment:
Consultant’s report prepared for San Francisco Public Utilities Commission.
R.W. Beck and Associates, 1987, Report on the Structural Analysis of Rehabilitation,
Bear Valley Dam Project: Consultant’s report prepared for Big Bear Municipal Water
District.
Schafer Dixon and Associates, Inc., 1987, Multiple Arch Dam Photographs, Big Bear
Lake, California: Consultant’s report prepared for Big Bear Municipal Water District.
URS Corporation, 2006, Design Basis Memorandum Calaveras Dam Replacement
Project, Final Design: Consultant’s report prepared for San Francisco Public Utilities
Commission.
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