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Design/Build Approach Big Success for Pine Brook RCC Dam
Del A. Shannon, P.E.1, Black & Veatch Corporation
Jeff Allen, P.E.2, E & C Services
Robert de Haas, Pine Brook Water District
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| View of the upstream face during the initial
filling of Pine Brook Dam. |
The design/build approach has been successfully used formally for
the first time in Colorado to expedite the design and construction
of a new dam. This process began when Pine Brook Water District
– a small water district serving only 400 customers, located
two miles (3.2 km) northwest of Boulder, Colorado – sought
to build a dam with an extremely aggressive schedule and budget.
The 400 customers of the Pine Brook Water Districts reside above
the elevation to which the City of Boulder provides water service.
For the last 10 years the Pine Brook Water District has suffered
from severe drought conditions. The District’s surface water
source on Four Mile Creek has gone dry several times since 2002,
with the most recent stint occurring in September of 2006 and lasting
nearly a month. Prior to construction of Pine Brook Dam, the District
would have two to three weeks of water storage, creating extreme
water restriction situations. With completion of the dam the District
has a full year’s water supply when the reservoir is full,
with no water restrictions.
Because of aggressive schedule and budget constraints, the district
determined that the design/build approach would be the most advantageous
to complete this project. The district selected ASI RCC, Inc. (contractor)
and TCB, Inc. (engineer) to accomplish this project. Considerations
included geotechnical aspects, flood hydrology, RCC mix design,
dam layout, seepage cutoff and collection, outlet works, instrumentation,
and aesthetics. By working as an integrated team the group was able
to quickly address design issues and minimize costly studies and
evaluation of multiple alternatives. Construction began only eight
and a half months after initiation of the design process.
Dam Design
The new dam is a roller-compacted concrete (RCC) gravity structure
approximately 86-feet-high (26.2 m) at its maximum section with
a crest length of approximately 560 feet (170.7 m). The dam creates
a 100 acre-foot (12.3 hectare-m) reservoir with a surface area of
approximately 4 acres (1.6 hectares). The dam is designed to safely
pass the inflow design flood (IDF) equal to the probable maximum
flood (PMF) event. The structure includes an upstream parapet wall
designed to concentrate the flow during the IDF to the central 290
feet (88.4 m) of the dam. This section is called the ‘emergency
spillway’ section. Appurtenant structures include a service
spillway riser structure with an uncontrolled-drop inlet and a conduit
with an energy dissipater.
The RCC structure is founded on bedrock. A 10-foot (3.0-m)-wide
key was built into the weathered bedrock below the base of the dam
as a seepage cutoff. The depth of the key varied approximately from
5 feet (1.5 m) to 10 feet (3.1 m). The downstream face of the RCC
is unformed RCC and the upstream face is air-entrained conventional
concrete facing with specified compressive strength of 3,000 psi
(20.7 MPa) at 28 days.
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| A view showing crest of dam, parapet
wall, and unformed uncompacted downstream face. |
The non-overflow section consists of a vertical upstream face,
a 3.83-foot (1.17 m)-high parapet wall, and a 0.75H:1V downstream
face. The crest is 15 feet (4.6 m) wide. The emergency spillway
section is similar to the non-overflow section except that the crest
and top of parapet wall elevations are 2.5 feet (0.76 m) lower than
the elevations at the non-overflow section. A 6-foot (1.8 m)-wide
and 0.5-foot (0.15 m)-deep low-flow notch was constructed in the
emergency spillway to focus low flows through the spillway at one
location so they can be more easily identified and handled.
Approximately six months after initial filling of the dam in 2007,
the downstream face of the dam will be covered by a soil cover with
a slope of 2H:1V or flatter. The downstream soil cover was not incorporated
into the structural design of the dam but was important for economic,
environmental and aesthetic purposes.
No stilling basin for the emergency spillway was designed. Though
turbulent flow conditions are expected at the dam toe, foundation
erosion or undermining is not expected. Stability analyses demonstrated
that the dam will be safe for the PMF event without the soil backfill.
The service spillway is a cast-in-place concrete drop inlet and
riser structure founded on RCC and anchored to the upstream face
of the dam. The outlet conduit discharges flood water through the
RCC dam and downstream soil cover to an energy dissipation structure
at the toe of the downstream soil cover. The service spillway can
pass normal flows up to 500 cfs (14.2 m3/sec). Higher flows will
be passed over the emergency spillway.
The outlet works consist of one 12-inch (300-mm) outlet to provide
emergency drawdown and two 6-inch (150-mm) diameter steel pipes
with intakes located at different elevations to provide raw water
flows to the new potable water treatment plant.. An inlet screen
is installed at each of the 6 inch (150 mm) intakes. The outlet
works are encased in concrete through the dam and soil blanket.
Only the 12 inch (300 mm)emergency outlet is operated from the upstream
face, the 6 inch (150 mm) intakes are operated from below the toe
of the dam by standard isolation valves and the flows are controlled
with the water treatment plant also located immediately below the
dam.
The structure was designed to resist full hydrostatic uplift, so
it was not necessary to install foundation drain holes to relieve
uplift pressure. Seepage through the dam foundation will drain to
the downstream side of the dam. After the soil embankment is placed
on the downstream side of the dam, drainage will be collected through
a drainage system. While all dam foundations have some level of
seepage, excessive seepage can be a problem and can lead to failure.
Modeling results predict that the seepage through the foundation
of the Pine Brook Dam should be too minor to erode or damage the
rock foundation or lead to dam failure. If actual seepage rates
are larger than anticipated, and present a safety hazard or an operational
concern, a grout curtain will be added.
Seepage control was provided by adequate lift bonding and minimizing
cold joints between RCC lifts. Cold joints less then 14 hours required
no special treatment. Cleaning and washing the lift surface was
required for joints 14 to 36 hours old. Older joints required bedding
mortar to bond consecutive lifts. Bedding mortar mix consisted of
2,800 lb/yd3 (1,660 kg/m3) sand, 500 lb/yd3 (297 kg/m3) cement,
and 300 lb/yd3 (178 kg/m3) water.
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| A close-up view of water stop, conventional
concrete for upstream facing, and contraction joint materials. |
Crack-control inducers were installed and seepage at the induced
cracks was controlled by the use of conventional concrete facing
system and water stops on the upstream face.
RCC contraction joint locations were adjusted upon completion
of the excavation based on the bedrock topography and major grade
breaks encountered. A total of six contraction joints were initially
planned, but a total of nine contraction joints were eventually
installed.
RCC Mix Design
Based on stability analyses, an RCC mix producing a one-year unconfined
compressive strength of 1500 psi (10.3 MPa), tensile strength of
75 psi (517 kPa) , and cohesion of 300 psi (2.1 MPa) were specified.
Freeze-thaw durability concerns were addressed by the use of air-entrained
concrete upstream facing and the soil cover on the downstream face.
The selected RCC mix consisted of 2,170 lb/yd3 (1,287 kg/m3) on-site
aggregate crushed to 3-inch (75 mm) minus; 1,532 lb/yd3 (909 kg/m3)
imported Colorado Department of Transportation Class 4 base, 160
lb/yd3 (95 kg/m3) Type I/II cement, 100 lb/yd3 (59 kg/m3) fly ash,
and 234 lb/yd3 (139 kg/m3) water. The design/build team quickly
concluded that a design based on lower design strengths and conservative
cross-section would provide flexibility in aggregate selection and
proportions. The District’s concerns and permit restrictions
made on-site aggregate development very attractive, though not necessary.
Space and budget concerns led to on-site crushing of about 55 percent
of the needed aggregates.
Construction and Cost
The design of the Pine Brook Dam was initiated in January 2005
and completed in June 2005. After incorporating the State Dam Safety
Engineer’s comments, the project was approved for construction
on September 22, and concrete placement for the dam began the next
week.
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| View of upstream facing formwork during construction.
Note water stops and steel rods anchoring the concrete facing
to the RCC. |
RCC placement began the last week of October and continued until
the first week of December when RCC construction was suspended due
to cold weather. RCC placement resumed in mid-February 2006 and
was completed in mid-April. The project required approximately 36,000
yd3 (27,500 m3) of RCC; 1,600 yd3
(1,220 m3) of air-entrained concrete for upstream facing;
and 1,400 yd3 (1,070 m3) of abutment/leveling/dental
concrete. The outlet works and principal spillway riser and conduit
were constructed during the suspension of RCC placement.
RCC materials were mixed using an on-site compulsory mixer. A 100-foot
(30.5-m) long conveyor belt conveyed the RCC to haul trucks and
the trucks transported the product to the point of placement. A
dozer spread the RCC and vibratory rollers compacted the material
in 12-inch (300-mm) lifts.
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| Equipment used to transport, spread and compact
RCC. |
The total project cost (engineering and construction) for the design
and construction of the Pine Brook Dam was approximately $4.5 million.
The unit cost for RCC including material processing, mixing and
placement costs was approximately $75/yd3 ($98/m3).
While it is difficult to compare these costs to those of other projects,
it is believed these costs are significantly lower than projects
of similar size and scope. Additionally, it is believed that the
project schedule was reduced by as much as 70% when compared to
similar projects. The price efficiency and schedule benefit was
realized, in part through contractual approach, simplified design,
and productive team coordination.
In recognition of the innovative efforts undertaken by the design/build
team to design and construct the high hazard dam, using the design/build
approach, in only 18 months, Pine Brook Dam received U.S. Society
on Dams' Award of Excellence in the Constructed Project.
The award was presented during USSD’s conference in March
2007.
1 Mr. Del Shannon was a Senior Project Manager with
TCB, Inc. prior to joining Black & Veatch.
2 Mr. Jeff Allen was President of ASI RCC, Inc. prior
starting E & C Services.
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Credits
Owner/Construction Manager: Pine Brook Water District,
Boulder, Colorado
Contractor: ASI RCC, Inc., Buena Vista, Colorado
Designer: TCB, Inc. Denver, Colorado
QC Testing: CTL/Thompson, Denver, Colorado
RCC Mix Design: ASI RCC, Inc., Buena Vista, Colorado
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