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Soil-Cement Case Study
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> Moss Creek Dam
Large Soil-Cement Drop Structure Solves Major
Erosion Problem
John L. Rutledge, P.E., Vice President
Freese and Nochols, Inc.
The Moss Creek Dam is located on Moss Creek east of Big Spring,
Texas. It is owned by the City of Big Spring, but operated by the
Colorado River Municipal Water District, as part of their water
supply system for the west Texas area. In April of 2000, a four
to eight inch (100 to 200 mm) rainfall fell over a 26 square mile
(67 sq. km) drainage area in less than 6 hours. The runoff was later
equated to approximately the 25-yr flood.
The primary spillway for the dam consists of a 350-foot (1 07-m)
wide uncontrolled concrete weir in the left abutment that provides
for an 11-foot (3.3-m) drop. The channel that directs the flow back
to the original channel has a steep, 3% grade for its 1,500-foot
(457-m) length. Prior to this event, the spillway had operated only
once in the 25 years since its construction and suffered only minor
erosion damage. In 2000, however, the erosion damage was dramatic,
with gullies as deep as 25 feet (7.6 m) formed in the channel. An
asphalt access road was also destroyed. The eroded material was
deposited downstream, blocking much of the channel and causing the
floodwaters to back up within 2 feet (0.6 m) of the District's pumps.
Though much of the exit channel was severely damaged, the embankment
and the spillway structure were not threatened during the event.
However, it was clear that major repairs would be needed to avoid
future risks to the spillway.
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| View of drop structure and exit channel. |
Several alternatives were reviewed in the initial analysis. The
final recommendation was to construct a large soil-cement drop structure
that would be located and shaped to minimize the required excavation.
It was designed as an oval bowl with a narrow exit channel, forcing
higher tailwater to properly contain the hydraulic jump within the
uniquely shaped basin. The configuration of the oval bowl shaped
structure focused lower flows through the central portion of the
basin and allowed higher flows to drop into the basin from the sides.
The final maximum height of the basin was designed as an average
35-foot (10.7-m) drop. The long exit channel was partially lined
with additional soil-cement and included two small soil-cement drop
structures to effect better grade control and more appropriate flow
velocities.
The plan for the soil-cement placement called for one-foot (0.3-m)
steps with approximate 45 degree compacted faces on all of the slopes
of the drop structure bowl. This configuration balanced the desired
dissipation of some of the flow energy with the need to simplify
construction techniques and lower costs by avoiding formed steps.
In the exit channel, soil-cement was plated on the bottom and on
the slopes in order to hold down the construction quantities. Subsurface
drainage was provided by two long, horizontal drains installed along
the face of the slope underneath the soil cement with downstream
outlets located beyond the expected hydraulic jump area.
Construction for the project began in early November 2001. Excavation
for both the stilling basin bowl and the exit channel and the simultaneous
construction of a new embankment adjacent to portions of the exit
channel continued through December and into early January of 2002.
During this time, a mix design for the soil-cement was prepared
using the contractor's proposed aggregate. The specifications provided
for a minimum 1,500 psi (10.4 MPa) compressive strength with wet-dry
and freeze-thaw tests showing less than 10% loss. Based on the laboratory
results, a design of 8% cement was used. This provided average laboratory
strength of about 2,000 psi (13.8 MPa) with minimal losses from
the two durability tests.
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| Placing soil-cement for drop structure and
exit channel. |
Soil-cement placement began in March 2002 in the downstream channel.
Placement and spreading was performed by a dozer or motor grader,
when possible. Compaction then followed with a vibratory roller.
The specifications called for average 98% of maximum standard Proctor
density and field density results during construction averaged about
100%. The downstream channel and the stilling basin floor were completed
in March and placement of the stilling basin bowl steps began in
early April 2002. Placement procedures included the placement of
soil cement material into a spreader box with a front-end loader.
The 12-in. (300-mm) lifts were then compacted using a vibratory
roller. A motor grader was used to form the 1:1 step faces. The
1:1 outside faces were then compacted using a rubber tire attached
at an angle to the motor grader blade. Compacted densities of the
step face were consistent with the remainder of each step and met
the required density. Soil-cement placement was completed in early
May. Slightly less than 10,000 cubic yards (7,645 m3)
were placed in about a month.
Laboratory strengths for the soil cement were generally about 2,000
psi (13.8 MPa) at 28 days compared to the specified minimum strength
of 1,500 psi (10.4 MPa). Cores taken during construction verified
the needed strength with field strengths ranging from about 1,700
to 2,200 psi (11.7 to 15.2 MPa) after more than 28 days.
The contractor, Price Construction of Big Spring, performed the
construction at a cost of $914,000. The cost of the soil-cement
in place was $45.76 per cubic yard ($59.85/3) for 9,900
cubic yards (7,569 m3) placed. The new drop structure
and exit channel for the spillway have been in place for more than
three years and appear to be in excellent shape, though the spillway
has not operated during that time.
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More on Soil Cement:
Overview
Embankment Slope Protection
Bank Protection/Levees
Drop and Grade Control Structures
Liners
FAQs
Southern California Case Study
Moss Creek Dam Case Study
Rueter-Hess
Dam and Reservoir Case Study
RCC/Soil Cement Contractor Directory
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