PROJ00382

• Contract/PO #: C153685
• Contractor Name: Monument, Town of
• Contract Type: Loan
• Water District: 10
• County: El Paso
• Bill Number: SB 94-029
• Loan Projects - Doc Type: Feasibility Study

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Chapter IV - Formulation of Alternatives

· The proposed filter systems are standard construction practices and the County and Town

will receive more qualified bidders and the construction cost will be considerably lower

compared with the core trench or an upstream liner.

· According to existing records, the existing dam was constructed with a "puddle trench"

excavated to bedrock. The location and other construction details of this cutoff are not

known.

· Standard practice dictates that it is more effective to filter and properly collect embankment

and foundation seepage on the downstream side than to control seepage on the upstream face

of an embankment dam. Numerous reported case histories have shown that a small defect in

the upstream liner could significantly reduce the effectiveness of the barrier.

· The EI Paso County Agricultural Extension Service and the Colorado State University

Cooperative Extension were contacted regarding the portion of the embankment that would

need to be excavated to remove all of the major roots in the existing trees on the

embankment. According to these sources, the roots are predominantly located in the top 6 to

18 inches of the soil, and usually do not reactl a depth greater than 3 to 7 feet. Because of the

very large sizes of the existing trees, we are recommending that a minimum of 10 feet of

embankment be removed as part of the clearing and grubbing process. Since the roots

typically spread out well beyond the dripline, and because the entire embankment is covered

with trees, we also recommending that the entire embankment be excavated to a minimum

depth of 10 feet so that all of the existing roolS are removed in the embankment.

Embankment Alternative EI - Chimney/Blanl<et Drain System

For this alternative, as shown in Figure IV-6, the top portion of the embankment is

excavated to elevation 6920.0, and a minimum of 10 feet is excavated in the downstream slope

of the embankment. All of the existing trees and shrubs on the embankment will be cleared and

all the roots will be grubbed. A 3-foot-thick chimney drain will be constructed up to elevation

6920. The chimney drain is connected to a 3D-foot-wide horizontal blanket drain, consisting of2

feet of drain gravel sandwiched between two I-foot-thick filter/drain sand layers. A 6-inch PVC

slotted toe drainpipe is embedded in the drain gravel layer. The internal drains will filter and

Monument Lake Dam Feasibility Study

26

BDYLE