2007-08-10_PERMIT FILE - C1980007 ‎(3)‎

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 8/10/2007
• Doc Name: Design Report for Alteration of Monument Dam
• Section_Exhibit Name: Exhibit 76 Report April 2007
• Media Type: D
• Archive: No

' Table 5 Summary of Seepage Analysis under Existing Conditions at Dam (Before
Dam Modifications)

iezometer
Tip
Elevation
feet Measured
Total
Head
feet Computed
Total
Head
feet
Diff.
(M-C)
feet

tratum

orin

Cross-
Section
PZ-06-01 7313.5 7327.5 7328.7 -1.2 Soft Sediment SB-06-02 B
PZ-06-02 7279.5 7397.0 7297.3 -0.3 Colluvium SB-06-02 B
PZ-06-03 7247.2 7248.4 7249.4 -1.0 Weathered
Bedrock SB-06-02 B
PZ-06-04 7310.4 7327.9 7326.9 1.0 Colluvium SB-O6-03 C
PZ-06-05 7286.4 7327.0 7320.8 6.2 Colluvium SB-06-03 C
PZ-06-06 7262.4 7314.1 7315.4 -1.3 Colluvium SB-06-03 C
PZ-06-07 7287.0 7299.8 7298.2 1.6 Colluvium SB-06-04 C
PZ-06-08 7269.0 7292.9 7297.3 -4.4 Colluvium SB-06-04 C
PZ-06-09 7251.0 7290.0 7296.2 -6.2 Colluvium SB-O6-04 C
The SEEP/W computer output for the seepage analysis is included in Appendix C.
8.1.4 Summary of Seepage Results
The groundwater flow regime was modeled for the dam. The model was calibrated such that total
heads estimated in the model were matched as closely as possible with the measured values in the
installed piezometers. The results of the seepage analysis, which for both existing and proposed
conditions are included in Appendix C, are described below.
It can be seen that for existing conditions at Cross-Section B, significant downward flow is observed
at the location of soil boring SB-06-02: Also, a split in the phreatic surface is observed. This is
expected considering the presence of the highly pervious bedrock layer and the fact that the lowest
piezometer (PZ-06-03) has been reading zero porewater pressure since it was installed in July 2006.
The phreatic surface does not reach the downstream face, which is consistent with field observations
by Barr personnel in which no seepage was seen coming from the downstream face at this portion of
the dam.
The seepage for the proposed geometry at Cross-Section B shows a similar pattern. The heads in the
soft sediment layer were lowered by about 15 feet, primarily due to the head loss across the upstream
clay blanket. The soft sediment layer is the location of the upper piezometer (PZ-06-O1). The heads
measured in the middle piezometer (PZ-06-02) are predicted to drop by about 10 feet, and those in
the bottom piezometer (PZ-06-03) are not predicted to change with the proposed modifications.
It can be seen that for existing conditions at Cross-Section C, downward flow is also observed at the
location of soil boring SB-06-03 and nearly hydrostatic conditions at the location of soil boring SB-
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