HomeMy WebLinkAbout07/18/2000, 1 - SALINAS RESERVOIR EXPANSION PROJECT - DAM SAFETY EVALUATION council ;ui is 2000
!!j acenaa Report
CITY OF SAN LUI S O B I S P O
FROM: John Moss,Utilities Dire cto
Prepared By: Gary Hende n, Water Division Manager 4:9 1
SUBJECT: Salinas Reservoir Expansion Project- Dam Safety Evaluation
CAO RECOMMENDATION
Receive and file report
DISCUSSION
The Salinas Dam was constructed in 1941/42 by the federal government to provide water for
Camp San Luis Obispo and secondarily to provide water for the City of San Luis Obispo. The
dam is a 135-foot high concrete arch-shaped facility which has a current maximum storage
capacity of 23,843 acre feet. The facilities are under the control of the U.S. Army Corps of
Engineers (Corps) but are operated by the San Luis Obispo County Flood Control and Water
Conservation District. - Since the dam is federally owned, it does not currently fall under the
jurisdiction of the State Division of Safety of Dams (DSOD).
The City of San Luis Obispo has rights to all the water diverted from the reservoir under state
water rights Permit 5882. The Corps is also listed as a co-permittee on the water rights permit
but no water is utilized for federal purposes.
The City has proposed expanding the storage capacity of the reservoir for many years by
installing 19-foot high spillway gates in the existing dam. With the installation of the gates, the
storage capacity would be increased to 41,792 acre feet with a resulting increase in safe annual
yield of 1,650 acre feet.
Since the dam no longer serves any federal purpose, the Corps has stipulated that prior to
initiation of the Salinas Reservoir Expansion Project, the facility would have to be transferred to
a local agency. In California, dams owned by local agencies are under the jurisdiction of the
California Department of Water Resources, Division of Safety of Dams, and must meet current
dam safety standards. As such, a dam safety evaluation has been performed to address dam
safety issues, such as safety during earthquakes and flooding. Based on this requirement, the
City contracted with URS (formerly Woodward-Clyde) to evaluate the safety of the dam
considering the proposed reservoir expansion project and the ability of the dam structure to meet
current state standards with the increased water storage.
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Salinas Reservoir Expansion Project—Dam Safety Evaluation
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Previous Studies
Woodward-Clyde Consultants completed an analysis of the feasibility of expanding the reservoir
storage capacity in February 1989 (revised in December 1990). The study analyzed the seismic
stability and the hydrology associated with the design flood flow event (Probable Maximum .
Flood). The analysis was based on design standards at the time and on an understanding of local
faults at that time. The analysis indicated that the dam expansion project could meet seismic
standards with minor improvements to the facility. The analysis did indicate that the design
flood would overtop the dam for both the existing condition and with the spillway gates installed.
The flow over the dam could potentially erode the area at the base of the dam and therefore the
report indicated that armoring(concrete) at the dam base would be necessary.
Updated Studies
The recently completed analysis for seismic loads and the flood flows associated with the
Probable Maximum Flood (PMF) have been updated based on the current standards and DSOD
requirements. Both the PMF and seismic loads have increased as discussed below.
Probable Maximum Flood
The PMF has been updated based on the new requirements which were issued in 1998. The
current PMF rainfall event equates to 26.0" in 72 hours. The previous analysis was for 25.2" for
72 hours. With this increase in rainfall, the total flow into the reservoir will increase by about
2%. The new analysis predicts that 108,000 acre feet would flow into the reservoir. This is an
extremely large amount of water and would fill the entire capacity of the existing reservoir 4'/z
times. With these flows, the existing dam and the modified dam with spillway gates installed
would be overtopped. Therefore, armoring of the dam base is still recommended to prevent
erosion in the unlikely event of a storm of this magnitude.
Earthquake Event
A number of faults have been identified which could potentially impact the Salinas Dam. These
faults were evaluated to estimate which fault would generate the largest loads on the structure.
The Rinconada fault, as in the previous analysis, was judged to be the controlling fault for
seismic analysis. Seismic design criteria are updated as new earthquakes occur and our
understanding of resulting seismic forces increases. An earthquake on the Rinconada fault is the
controlling event (same as assumed in 1989), however, the earthquake forces that are predicted
from a fault rupture have increased by approximately 70% for the reasons discussed below.
In 1989, the activity (historic movement) of the Rinconada fault was reviewed with respect to
deciding if the fault was active and therefore capable of generating a damaging earthquake. The
evidence was inconclusive, but to be conservative for the purposes of dam safety, the fault was
assumed to be capable of generating a design earthquake. The ground motions from the
earthquake were selected in the middle range of motions (for known active faults ground motions
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Salinas Reservoir Expansion Project—Dam Safety Evaluation
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are selected towards the upper range, however, it was judged that a conservative assumption
regarding the activity of the fault had already been made and that higher ground motions were
judged to be overly conservative at that time).
The Rinconada fault has three separate segments and the previous study assumed that a fault
rupture would include up to 50% of the fault length. This assumption was consistent with design
standards at that time. In 1992, an earthquake occurred near Landers, California which has
resulted in revisions to seismic design for this project. The Landers earthquake occurred on a
fault with three separate sections and demonstrated that a fault rupture could jump to several
segments. The potential for multiple fault segment ruptures and additional evidence of fault
activity have resulted in an increase of the assumed seismic loads by approximately 70% (0.65g
to 1.10 g). This increase results in a significant increase in the potential seismic induced stresses
in the dam.
The structural computer analysis of the dam indicates that the existing dam (without the spillway
gates) would be capable of withstanding the design earthquake and satisfies modern seismic
performance evaluation criteria. The analysis also indicates that"the dam is not strong enough to
withstand strong ground shaking with the increased load from a raised reservoir." Remedial
measures will be necessary to strengthen the dam prior to installation of the spillway gates.
Strengthening Alternatives
There are two alternatives which could be utilized to strengthen the dam for the increased water
storage. The two alternatives are discussed in more detail in Attachment 2 to this report. The
potentially least costly alternative would involve placement of an additional layer(about equal to
the existing dam thickness) of concrete on the downstream face of the dam. While the costs may
be less, there are questions concerning the "bond" strength between the existing concrete and the
new concrete which may not be acceptable to DSOD. This method of strengthening has been
approved by DSOD for a dam of this type in California, however, Salinas Dam has potentially
greater seismic loading and therefore it may be difficult to receive approval. The estimated cost
range for this alternative is $5.6 to $8.5 million which is in addition to other project related costs
(i.e. spillway gate, recreation facilities relocation,environmental mitigations, etc.).
The second alternative involves the placement of a "buttress" against the downstream face of the
dam. The buttress would be constructed using "roller compacted concrete" (RCC) and would be
a stepped wedge structure which would transform the current arch dam into a curved gravity
structure. This method was successfully used on the Gilbraltar Dam near Santa Barbara to
strengthen the existing concrete arch dam. This type of remedial strengthening measure has been
used on a number of dams in California and has been accepted previously by DSOD, therefore
this alternative would likely gain relatively easy approval from DSOD. The estimated cost for
this alternative, excluding other project costs, would range between$7.2 and $11.0 million.
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Salinas Reservoir Expansion Project—Dam Safety Evaluation
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Next Steps
The current draft dam safety evaluation report has not been submitted to_the State Division of
Safety of Dams or the U.S. Arany Corps of Engineers for their review. The seismic design
criteria for the structural analysis was presented to DSOD during development of the computer
model. DSOD concurred with the criteria and assumptions to be used in the computer model
runs. Following the presentation of the information to the City Council, staff will transmit the
reports to DSOD and the Corps for their review and comment.
The previously approved concrete core sampling component of the study has been placed on hold
pending review of the analysis by the Corps and DSOD. The coring may not be necessary or
may be reduced in scope based on review of this recent analysis by the state. Funding for the
concrete coring and strength testing is currently included in the contract with URS.
Based on this new information, staff will be preparing a thorough review and update of other the
alternative water supply projects which could meet the city's projected future water supply
deficit based on growth identified in the General Plan. In addition, the City is still waiting for a
decision on our water rights permit hearing relative to increasing the storage capacity at Salinas
Reservoir. Staff anticipates that a decision on the water rights permit should be received within
the next several months. Staff will prepare an evaluation of alternative water supply projects and
strategies for Council consideration this fall.
Summary
The updated seismic evaluation of the Salinas Dam indicates that the existing facility meets
current seismic design criteria. The analysis concludes that if the reservoir storage capacity were
increased, the dam would not be strong enough to withstand the design earthquake on the
Rinconada fault. Remedial strengthening measures are available to safely allow the expansion of
the reservoir to move forward, and would likely cost an additional $5.6 to $11.0.
ATTACHMENTS
1. Executive Summary of"Salinas Reservoir Expansion Project—Dam Safety Evaluation"
2. Correspondence titled "Salinas Reservoir Expansion Project — Phase 1, Dam Engineering,
Dam Strengthening Alternatives"
On-File in Council Office:
1. Complete 2 Volume Report"Salinas Reservoir Expansion Project—Dam Safety Evaluation"
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Attachment 1
Salinas Reservoir Expansion Project
Dam Safety Evaluation
Executive Summary
INTRODUCTION
Salinas Dam is a 135-foot high concrete arch-shaped dam, located in San Luis Obispo
County on the Salinas River, about 9 miles southeast of the town of Santa Margarita,
California. The reservoir at the dam is referred to as Santa Margarita Lake. The dam is
owned by the US Army Corps of Engineers, and was constructed by the Corps between
late 1941 through early.1942. The original purpose of the dam was to provide wartime
water supply to Camp San Luis Obispo. The dam was constructed in accordance with the
engineering standards of practice at the time,and visually appears to be in good shape.
The County of San Luis Obispo currently operates the reservoir for the Corps of
Engineers,to provide municipal water supply to the City of San Luis Obispo (City) and
for recreation. The reservoir provides approximately 70 percent of the water supply to
the City, and along with the Whale Rock Reservoir, is one of two surface water reservoirs
providing water for the City's use.
REASONS FOR STUDY
The City is considering expanding the capacity of Santa Margarita Reservoir to meet the
growth proposed in the City's General Plan. The current capacity of the reservoir of
23,840 acre-feet(af) would be increased by 17,950 of following completion of the
expansion project. The"safe yield"(the amount of water which would be available for
use each year through a critical drought period) would increase by approximately 1650
acre-feet per year. A separate Environmental Impact Report has been prepared
describing the project, impacts from the project, and proposed mitigation measures.
The layout of the dam includes a concrete-lined spillway channel on the north (right) side
of the dam to convey flood flows past the dam. The crest elevation of the spillway (the
spillway's highest location) is approximately 20 feet below the top of the dam.
Therefore,to store additional water, spillway gates or other barriers would be installed at
the spillway crest location. With the gates in place the reservoir level would be
approximately 19 feet higher than currently exists when the reservoir is full. The dam
would not be increased in height.
As part of the proposed reservoir expansion, dam ownership would be transferred from
the Corps of Engineers to local control and the safety of the dam would therefore come
under state jurisdiction. The California, Department of Water Resources, Division of
Safety of Dams(DSOD) is responsible for dam safety for all jurisdictional non-federal
dams in California. The DSOD takes a careful and stringent review of dam safety,
because the consequences of the failure of a dam can be considerable. They have
developed general guidelines for use in assessing dam safety during an earthquake or
flood.
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Executive Summary Attachment 1
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Under an agreement(dated 1999) with the City of San Luis Obispo, URS Greiner
Woodward Clyde conducted a study to update the previously performed engineering
feasibility of increasing the storage capacity of Salinas Reservoir. The engineering
studies addressed both the seismic and hydrologic consequences of raising the reservoir.
The criteria for this current evaluation are based upon meeting DSOD dam safety
requirements.
STABILITY OF THE DAM DURING EARTHQUAKE
Information Used in the Stability Evaluation of the Dam
A comprehensive evaluation of the stability of the dam during an earthquake was
performed using a computer model of the dam. This stability evaluation was performed
for the dam with the proposed higher water level, as well as for the dam with the existing
reservoir level. In order to evaluate the stability of the dam during an earthquake, several
key pieces of information were developed. This information includes the following.
• The location of the nearest active faults. An active fault is currently defined by the
Division of Safety of Dams to show evidence of earthquakes occurring on them in the
past 35,000 years. Faults with inconclusive evidence of activity, or that cannot be
shown to be inactive, are treated as being active for the purposes of the evaluation of
dam safety.
• The size (or magnitude) of the largest earthquake that could occur on these faults.
This largest earthquake is referred to as the Maximum Credible Earthquake (MCE),
and is the largest earthquake that is thought possible to occur on that fault. The MCE
is the "design earthquake."
• The amount of ground shaking at the dam from the design earthquake. The larger the
size or magnitude of the earthquake, and the closer the fault is to the dam, the greater
the ground shaking will be at the dam.
The results of the evaluation concluded that the closest and most significant active fault is
the Rinconada fault, located about one mile to the west, close to Pozo Road. The design
earthquake that could be generated on this fault is a magnitude 7-%2, which is a large
earthquake. As an example, the Magnitude 6.8 1989 Loma Prieta earthquake caused
significant damage in San Francisco, and it occurred about 60 miles away from San
Francisco. The combination of such a large earthquake located so close to the dam
results in very strong ground shaking at the dam.
The performance of the dam under this strong shaking was analyzed for the current and
proposed raised reservoir levels using a computer model that estimates the stresses in the
dam caused by the earthquake. The stresses were then compared with the strength of the
concrete of the dam. An assessment was made whether the dam will maintain sufficient
integrity to prevent the sudden release of the reservoir.
Stability of the Dam with the Raised Reservoir Level
The results of the computer analysis indicate that for the case of the raised reservoir level,
the stresses induced in the dam by the Maximum Credible Earthquake on the Rinconada
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fault would be much greater than the strength of the concrete in many portions of the
dam. As such,the analyses indicate that the dam is not strong enough to withstand strong
ground shaking with the increased load from a raised reservoir. The dam, under the
proposed reservoir raise,does not satisfy modem seismic performance evaluation criteria
such as those currently used by DSOD.
Stability of the Dam with the Current Reservoir Level
The computer analyses were also completed for the dam with the reservoir at its current
maximum level. The analysis indicated that stresses could exceed the strength of the
concrete at some locations of the dam,but with very short time durations. As such,the
overall stress levels and distribution were found to remain within an acceptable range
relative to the safety of the dam. The dam,under the existing maximum reservoir level,
satisfies modem seismic performance evaluation criteria such as those currently used by
DSOD.
Comparison with 1990 Analyses
The results of previous analyses(completed in 1990)indicated that the dam,with the
maximum reservoir level raised by 19.3 feet,would be stable during the design
earthquake. The current analyses show that the dam would not be strong enough to
withstand the design earthquake. The reasons for this difference are largely due to the
differences in estimations of earthquake ground motions used in the analyses, details of
which are discussed below.
A previous seismic hazard analysis was performed by Woodward Clyde Consultants for
Salinas Dam in 1988 (submitted to the City in the 1989 Interim and 1990 Revised
Report). In this report, an MCE of magnitude (Ms) 7.4 resulting from an earthquake on
the Rinconada fault at a distance of about one mile was selected. This evaluation was
based solely on available data and no field investigations were performed. An earlier
study by the Corps of Engineers in 1977 concluded that the Rinconada fault was not
active based on two trenches excavated across the fault. Thus the judgment that the fault
was active in the 1988 study was conservative and was based principally on recording of
seismicity in the vicinity of the fault. As stated in the 1990 report, very little seismologic
and geologic information is available on the Rinconada fault. Considering the above, a
median (or 50%percentile) peak horizontal acceleration of 0.65g was selected. This
approach was judged to be consistent with DSOD procedures based on the information
known at that time.
In this current evaluation of Salinas Dam, an analysis of aerial photographs and a field
reconnaissance was performed along portions of the Rinconada fault. The analysis found
evidence of active faulting. In addition, the fault consists of three segments, and previous
assessments of the fault included that up to 50%of the fault length could rupture during
an earthquake. However,the Landers earthquake (Magnitude 7.3, 1992) showed that
fault rupture could jump to several segments. Therefore, the likelihood of an earthquake
occurring close to the dam was increased from the 1990 study. Considering this finding,
a more conservative ground motion(mean+ 1, or 84th percentile) of 1.1 Og (which is 70%
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greater than the 1990 value of 0.65g)was selected for use in the analysis. Based on the
updated information, it was judged that DSOD requires use of these higher seismic
values.
STABILITY OF THE DAM DURING FLOODING
Standardized procedures are used to estimate the maximum amount of flooding which
could occur at a dam. The most recent National Oceanic and Atmospheric Administration
(NOAA) guidelines(referred to as HMR No. 58,published in 1998) were used to
estimate probable maximum amount of rainfall that could occur over the Santa Margarita
Lake watershed. The amount of rainfall was then used to calculate the design flood,
referred to as the Probable Maximum Flood, or PMF.
The computer model was used to check the stability of the dam with higher reservoir
loads during the design flood. It was found that the dam could safely withstand the
design flood under the raised reservoir scenario.
For both the existing maximum reservoir and the proposed raised reservoir level,the
PMF would overtop the dam. With the raised reservoir, overtopping flow would be
greater and occur over a longer duration compared to the current maximum reservoir
level. The overtopping would not damage the dam itself,however,the rock at the base of
the dam could be eroded. This erosion could lead to undermining of the dam foundation,
which could lead to damage to the dam. Methods to prevent erosion damage from
occurring are relatively simple,and include placing reinforced concrete at the areas
where erosion could occur.
RIGHT ABUTMENT STABILITY
During the construction of the dam, measures were incorporated in the design to
strengthen the rock mass on the right abutment, which was thought to be prone to
movement during an earthquake. The DSOD has asked that the stability of the rock
foundation at the right side of the dam be evaluated. Based on our evaluation of the data,
our analysis results indicate that the rock mass is stable.
COORDINATION WITH THE DIVISION OF SAFETY OF DAMS AND THE
CORPS OF ENGINEERS
Very good coordination and understanding of the project has been developed with the
DSOD and the Corps of Engineers. Meetings have been held with the DSOD, Corps of
Engineers, City and County to discuss dam safety issues. As part of the study, an interim
report was prepared and submitted to DSOD in November 1999. The purpose of the
document was to present basic analysis parameters and evaluation criteria proposed for
use in the seismic analysis evaluation of Salinas Dam. In February 2000, DSOD sent a
letter to the City confirming that the earthquake ground motions proposed for use in the
analyses were satisfactory.
POTENTIAL DAM STRENGTHENING MEASURES TO ALLOW RAISE IN
MAXIMUM RESERVOIR LEVEL
Remedial construction has been performed on a number of concrete dams in the U.S. in
recent years to strengthen them to resist damage from strong earthquakes. On the basis of
our own experience in the design of strengthening measures for concrete arch dams in
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California(such as the 190-foot high Gibraltar Dam near Santa Barbara),the most
straight-forward approach to strengthen the Salinas Dam would be to place a concrete
buttress against the downstream face of the dam. This would"stiffen"the dam, and
result in satisfactory stability of the dam. This approach could be combined with
construction of a concrete barrier in the spillway, which would eliminate the need for the
spillway gates and reduce overall construction costs.
Another approach, which would result in less ground disturbance, would be to thicken the
arch by placing additional concrete on the downstream face of the dam to stiffen the arch
shell and improve its overall stability. We are considering this approach for a 100-foot
high arch dam in California. An additional analysis was performed as part of our work to
evaluate this strengthening concept considering a modified dam with an overall average
concrete thickness which is approximately double that of the existing dam. The results of
this analysis indicate that this approach would be technically feasible providing that a
high quality bond can be achieved between the new and existing concrete. The actual
required thickness of additional concrete would depend on a number of factors including
constructibility-related considerations. However, although this approach may not be
appropriate for Salinas Dam because of the site-specific factors such as strong
groundshaking, it is recommended that its viability be reviewed in more detail at an
appropriate time.
In summary,there are a number of options that can be considered for strengthening the
dam. The overall cost,technical feasibility and environmental feasibility will govern the
feasibility of these options.
SUMMARY AND CONCLUSIONS
• The dam with a raised reservoir level does not satisfy modem seismic performance
evaluation criteria such as those currently used by DSOD.
• There are a number of options that can be considered for strengthening the dam. The
overall cost,technical feasibility and environmental feasibility will govern the
feasibility of these options.
• The dam with the existing maximum reservoir level satisfies modern seismic
performance evaluation criteria such as those currently used by DSOD.
• The design flood will overtop the dam for the existing and raised reservoir cases, but
can be safely remediated by relatively easy methods.
• The dam can safely withstand the probable maximum flood (PMF) loading under the
raised reservoir scenario.
• Whether the reservoir is raised or not, it is recommended that the areas that could be
damaged by overtopping during the design flood leading to erosion be covered with a
suitably designed concrete blanket.
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URS -
July 5, 2000
Project No: 66-099SB037.02
Mr. Gary Henderson, Water Division Manager
City of San Luis Obispo
955 Morro Street
San Luis Obispo, CA 93401
Subject: Salinas Reservoir Expansion Project-Phase 1, Dam Engineering
Dam Strengthening Alternatives
Dear Mr. Henderson:
In accordance with your request, we are pleased to provide you with two conceptual
approaches, along with ballpark cost estimates, for seismically strengthening Salinas
Dam. These approaches are based on a 19-foot increase in the maximum reservoir level.
Remedial construction has been performed on a number of concrete dams in the U.S. in
recent years to strengthen them for critical seismic loading conditions. Our proposed
strengthening approaches were developed on the basis of our experience in the design of
strengthening measures for concrete arch dams in California under the California
Department of Water Resources, Division of Safety of Dams (DSOD)jurisdiction. As
such, the most straightforward and effective method of strengthening Salinas Dam would
be to construct a roller-compacted concrete (RCC) buttress against the downstream face
of the dam. This approach has been successfully implemented on several arch dams in
California. The addition of a buttress would change the thin arch dam into a gravity
structure. A second approach would be to thicken the arch by placing additional
concrete on the downstream face of the dam. The dam thickening would "stiffen"the
arch shell and thereby increase its structural capacity during an earthquake. Very
preliminary analysis indicates that this approach would be technically feasible providing
that adequate shear transfer capacity can be achieved between the new RCC materials
and the existing concrete.
This letter report presents a description of these two strengthening approaches, referred to
herein as "RCC Buttress" and "Arch Thickening". The description includes precedent,
design features, and relative advantages and.disadvantages of each approach. Ballpark
cost estimates are included for use in feasibility and planning studies.
Please note that:
• Both approaches are based on downstream additions so as to maintain the reservoir in
operation during the construction.
• Both approaches are based on methods accepted, or under favorable review, from
DSOD.
URS Corporation
500 12th Street. Suite 200
Oakland. CA 94607-4014
Tel: 510.893.3600 0
Fax: 510.874.3268
URS -
Mr. Gary Henderson
City of San Luis Obispo
June 30,2000
Page 2 of 8
RCC BUTTRESS
CONCEPT
The roller-compacted-concrete (RCC) alternative involves construction of an RCC
buttress against the downstream face of the existing dam. The objective of this design is
to change the dynamic response of the dam and thus reduce the dynamic stresses induced
in the dam under earthquake loading. The addition of the RCC buttress transforms the
current arch dam into a curved gravity structure.
PRECEDENT
RCC has been used for structural upgrades of several concrete arch and multiple arch
dams in California as well as in other States. In nearly all cases the need for upgrading
has involved strengthening to address seismic loadings substantially higher than
originally designed for. The use of an RCC buttress for arch dam rehabilitation and
strengthening has gained general acceptance from Federal and State regulatory agencies
in general and from DSOD in particular. Two arch dams strengthened using an RCC
buttress'and which are most similar to Salinas Dam are Gibraltar Dam in California and
Santa Cruz Dam in New Mexico. Both of these dams are larger than Salinas Dam(195-
feet and 151-feet maximum height respectively) although both are single curvature thick
arch structures whereas Salinas Dam is a double curvature thin arch structure. The
significance of this difference is discussed below. Gibraltar Dam came under the
jurisdiction and both FERC and DSOD accepted the design. Construction of Gibraltar
Dam was completed in 1991 and at Santa Cruz Dam in 1990. Recent examples of
multiple arch dams in California strengthened using a downstream buttress include
Littlerock Dam in Palmdale, and Bear Valley Dam in San Bernardino County. In
addition, URS is currently finalizing the design of a partial height RCC buttress at Weber
Dam, a 3-bay arch dam in the Sierra foothills east of Sacramento.
CONCEPT FEATURES
The layout of the buttress section would be accomplished using conventional design
methods for concrete dams such as those used by the U.S. Bureau of Reclamation and the
U.S. Army Corps of Engineers.
The layout was conservatively assumed, based upon stability and constructability
considerations, and no analyses were performed. For the purposes of estimating cost,the
buttress section is assumed to extend the full height of the overflow section of the arch.
During preliminary design the possible use of a partial height buttress could be
investigated, which if feasible would significantly reduce construction costs.
Conceptual designs of the various components of the design include the following:
1. A minimum crest width of 20 feet to facilitate construction. This provides the
minimum width for construction equipment to place, spread and compact the RCC at
the top of the buttress section.
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W Gary Henderson
City of San Luis Obispo
June 30,2000
Page 3 of 8
2. A downstream slope of 0.7H: 1.OV based upon previous design experience and on the
use of a formed downstream face. An unfaced downstream face will be limited to the
angle of repose of the RCC materials which would result in an upstream slope closer
to 0.8H: 1.OV.
3. The downstream face will be designed as a stepped spillway. The surface of the steps
will be conventional concrete that would be placed concurrent with the RCC
placement. The steps act as roughness elements creating turbulence and entraining
air in the flood flows in order to dissipate energy and minimize flow accelerations and
terminal velocity. With reduced energy and lower flow velocity at the toe of the
buttress,the erosive power of the floodwater is substantially reduced and the need for
extensive erosion protection of the bedrock can be eliminated. Two-foot high steps
are proposed as a compromise between hydraulic efficiency and ease of construction.
4. A drainage gallery will be included within the buttress. Drain holes for relief of
foundation pore pressures will be installed from the gallery and used to reduce the
amount of uplift acting on the base of the buttress. The gallery would also serve as
access for post construction inspection and, if required, foundation grouting.
5. The buttress could extend across the spillway, thus eliminating the need for spillway
gates, and leading to some overall reductions in cost.
PROS AND CONS
PROS
1. Provides the most positive strengthening for the dam. Curved gravity dams are
extremely resistant to structural failure and have performed well during seismic
shaking. This concept would remove any possible argument that could be made as to
the seismic stability of the dam under the raised reservoir condition.
2. The RCC buttress-strengthening concept is accepted by DSOD.
3. There are several precedents to the use of an RCC buttress in California to seismically
strengthen arch or multiple-arch dams.
4. Can readily accommodate a future raise.
5. Eliminates the need for the erosion protection slab on the abutments and in the
channel beneath the arch crest. The use of a stepped downstream face as a secondary
or auxiliary spillway dissipates much of the energy in the overtopping flows.
6. Any potential problems associated with the adverse structural geology on the right
abutment are not a significant factor in the stability of a curved gravity structure.
7. Eliminates the need for spillway gates, which would remove the cost of gates from
the total construction cost estimate.
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URS -
Mr.Crary Henderson
City of San Luis Obispo
June 30,2000
Page 4 of 8
CONS
1. The appearance of the facility, while pleasing, will be significantly different.
2. The extent of the buttress needs to be reviewed considering project description and
other aspects of the EIR.
3. The overhanging or negatively sloping downstream face will restrict how close
construction equipment can come to the face of the arch resulting in use of more
conventional (i.e. expensive) "interface concrete" (ie, concrete placed between the
overhang of the existing dam, and the new RCC)than would otherwise be necessary.
This was not an issue at either Gibraltar dam or Santa Cruz Dams, which have
positively sloping downstream faces.
4. Aggregate for the RCC can not be produced on site and will need to be imported(the
Santa Margirita"Hansen" quarry is relatively close).
5. No spillway gate could restrict reservoir operational flexibility, although the intended
operational approach is to operate the reservoir with the gates always maintaining a
full reservoir.
COST ESTIMATE
A conceptual-level estimate of the cost of construction of the RCC alternative is
presented in Table 2. The estimated construction cost, in 2000 dollars, is $8.7 million.
Quantities and unit prices shown in Table 2 may change, and the number of work items
will increase, as design work progresses and the required work becomes better defined.
Consequently the conceptual level estimate should be considered to have a range of
minus 15 percent to plus 30 percent, which equates to a range of$7.4 million to $11.3
million. The estimates do not include design or construction management costs, nor do
they include the spillway related costs associated with raising the reservoir level.
Potentially the three biggest factors affecting the cost are:
1. Price and availability of imported aggregate rock. The unit price for producing RCC
shown in Table 2 is based on importing aggregate rather than producing it on site. A
moderate haul distance (10 miles) and an approximate cost for 1'/z aggregate(from
Hansen Aggregates) were assumed in assessing a unit price. However, Hansen
Aggregate's quarry in Santa Margirita only produces a 1'/z x 3/4 aggregate which is too
clean for RCC production. Their crushing plant and screens would need to be
modified to produce aggregate with a substantially higher fines content. A
conservative mix design comprising 200 pounds of cement and 100 pounds of
pozzolan per cubic yard of RCC was assumed.
2. Site Logistics. Having sufficient area available close to the dam in which to set up a
batch plant and for aggregate storage. The cost of in-place RCC will increase
substantially if off site production of RCC is necessary. The estimated cost is based
07/05/00 XALawton%Aren WJC
Arro+c���r Z
Mr.Crary Henderson
City of San Luis Obispo
June 30,2000
Page 5 of 8
on being able to set up a batch plant immediately downstream of the dam. Space
would have to be created immediately adjacent to the outlet control building, along
with use of space at the intersection of the access roads to the toe of the dam and the
top of the right abutment. A more detailed analysis is needed to properly evaluate
construction logistics.
3. Access to the dam. Transporting the RCC from the batch plant to the buttress can be
accomplished by a variety of methods including using trucks, a conveyor belt or
lifting with cranes. Access for construction equipment and RCC on to the buttress is
restricted on the right abutment by the spillway and on the left abutment by the step
topography. The spillway must remain serviceable during construction, effectively
limiting vehicular access from the right abutment. The environmental impacts (and
economics) associated with cutting haul roads at various elevations on the steep
topography of the left abutment may effectively rule out the use of haul trucks to
transport the RCC. Such access restrictions may lead to the use of commonly used
conveyor systems. If use of neither conveyors nor trucks proves feasible, the RCC
could be placed by crane(tower crane or cable) although the setup cost and slow
production rate would increase placement cost. The estimated cost is based on being
able to use either a conveyor belt system or haul trucks.
Allowances were made for water management downstream of the worksite during
construction and for relatively minor modifications to the existing outlet system to
incorporate it into the buttress. However, the extent of these items is not defined at a
conceptual level and these may substantially impact the final cost.
An allowance for unlisted and unidentified items, equal to 25%of the subtotal, was
made.
For review purposes, we compared the total unit cost(estimated total cost divided by the
volume of RCC) for a Salinas Dam buttress with costs for some recent RCC projects, in
order to assess the reasonableness of our conceptual estimate. The comparison summary
is presented in Table 3. The comparison projects include both new construction and
rehabilitation of embankment and concrete gravity dams.
• The costs(except for Weber which is still in design) are an average of the bids
received (ignoring extreme numbers) including the engineers estimate.
• None of the projects (except again for Weber Dam, which includes extensive outlet
works modification) have any unusual features that would distort the unit costs.
• The unit cost for Salinas is approximately $190 per cubic yard, which places it at the
low end of the $260 to $300 plus range for the four rehabilitation projects. These
four projects are however much smaller than Salinas and the lower unit cost for
Salinas reflects economies of scale.
07/05/00 x:uawwo\M=gZ-M1A
fl'�-'cl�cNMeN't' Z
URS
I& Gary Hendason
City of San Luis Obispo
June 30,2000
Page 6 of 8
• Furthermore,the cost of the RCC and facing/interface concrete in the Salinas estimate
is approximately 50% of the total projected cost, which is in line with the proportions
reported by Hansen and Reinhardt (Roller Compacted Concrete Dams,199 1)
ARCH THICKENING
CONCEPT
This alternative consists of thickening the dam on the downstream side. The thickened
dam would have moderately increased stiffness and rigidity (relative to a buttressed
dam), which would reduce the fundamental period of the dam, thereby increasing the
dynamic response. However,the stresses generated within the dam would be reduced
from their current levels since a larger structural section is resisting the dynamic forces.
Preliminary analyses using the finite element model of the dam indicate that thickening is
technically effective at reducing stresses to levels that satisfy modern seismic
performance evaluation criteria.
PRECEDENT
The concept is currently in the final design phase for the seismic upgrading of San
Clemente Dam, a gravity arch dam located in Monterey County. San Clemente is under
the jurisdiction of DSOD, and the State has worked closely with URS in implementing its
design.
There are examples of concrete-gravity and masonry-gravity dams being raised by
placing mass concrete against the downstream face. However, in the majority of
instances the dams were designed to accommodate such a modification by incorporating
a stepped face in the original design to facilitate shear transfer with the overlay concrete.
DESIGN FEATURES
The approach involves placing conventional concrete against the downstream face of the
dam to increase the structural capacity of the arch. The thickness of the new concrete
would be approximately the same as the original thickness at each elevation and location
along the dam profile (ie, the thickness of the dam would double). With the reservoir
elevation raised by the proposed 19 feet, our preliminary analyses indicate that to avoid
excessive overstressing of the during the MCE(on the Rinconada fault based on 840i
percentile motions), the thickness of the arch portion of the dam would need to be
approximately doubled.
For dynamic stresses to be reduced to acceptable levels, sufficient shear transfer must be
achieved between the new and existing concrete in order to produce a monolithic section.
07/05/00 XALawton\=en9L_14 C
rrac��4' Z
URS
W.Gary Henderson
City of San Luis Obispo
June 30,2000
Page 7 of 8
PROS AND CONS
PROS
1. The appearance of the dam would not be substantially altered.
2. The footprint of the strengthened dam would remain within the area of disturbance
identified in the EIR.
CONS
1. The option is not feasible if the reliable shear strength provided by concrete-to-
concrete bond is insufficient to resist shear stresses generated along the interface.
Unless the generated shear stresses are low, as may be the case with a thin overlay,
providing adequate shear strength is generally not possible using the concrete-to-
concrete bond strength. Extracting the interface shear stresses from the computer
model of the thickened dam is not straightforward and was not done as part of this
work Typically, a stepped pattern on the interface is necessary to act as shear keys in
order to achieve a reliable shear transfer mechanism. However, a stepped face is only
achievable if the dam is constructed that way to, for instance, allow thickening to
occur as part of a future dam raise.
2. If concrete bond (i.e. adhesion) is found to provide adequate shear strength, the
surface of the existing concrete must be very carefully prepared. Adequate surface
preparation is very sensitive to quality control exercised during construction. The
bond strength achieved is markedly degraded if specified procedures are not precisely
followed.
I The preliminary analyses did not address the impact that doubling the mass of the
arch may have on the effectiveness and stability of the thrust blocks, particularly on
the right abutment.
4. The anchored concrete slab over the bedrock surface downstream of arch will still be
necessary to prevent the dam from being undercut by flood flows overtopping the
dam crest.
5. There is minimal precedent for this type of strengthening.
COST ESTIMATE
The cost estimate for the arch thickening is very conceptual, and is based on the
assumption that the required bond strength can be developed and that the quantity of
additional concrete (or the thickness of the additional concrete) is approximately equal to
the quantity of the existing concrete.
07/05/00 XALawtoolstren91-4bC
U -
Mr.Gary Henderson
City of San Luis Obispo
June 30,2000
Page 8 of 8
Assuming the technical and design issues can be satisfactorily resolved, a ball-park
estimate of the cost of construction of the arch thickening alternative is $6.5 million. The
estimate should be considered to have a range of minus 15 percent to plus 30 percent,
which equates to a range of$5.6 million to $8.5 million. The estimate do not include
design or construction management costs, nor does it include the spillway related costs
associated with raising the reservoir level.
SUMMARY
Although the cost estimate for the arch thickening approach is lower compared to the
buttress approach, the buttress approach is recommended for the following reasons:
❑ It is well accepted by the DSOD based on our recent design practice;
❑ The arch thickening concept, while appropriate for dams with relatively small seismic
loading, my not be appropriate for the Salinas Dam project because of the relatively
high seismic loading.
We would be glad to discuss these concepts with you in further detail at your
convenience.
Very truly yours,
URS GREIN'ER WWOODWARD CLYDE
Gil M. Lawton
Project Manager
Attachments (3)
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TABLE 3. UNIT COST COMARISION
RCC 6 Concrete
Dam State Type Volume TOTAL Unit Cost
Peterson Dam Colorado New 10, 000 $180
Tiehack Dam Wyoming New 115, 000 $115
Bullard Dam* Orgeon New 10, 000 $230
Dupage Reservoir* Illinois Embankment 12, 000 $283
Mona Reservoir Utah Embankment 3, 000 $266
Weber Dam California Arch 10, 000 $350
Rush Dam Kansas Arch 5,000 $288
Salinas Dam Estimate Arch 50, 000 $194
* Flood control dam
1-20