PROJC00141

• Contract/PO #: C150135
• Contractor Name: Silt Water Conservancy District
• Contract Type: Loan
• Water District: 0
• County: Garfield
• Bill Number: MC3
• Loan Projects - Doc Type: Feasibility Study

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d. No observation or measurement of suspended material in the seepage water has
been reported.

However, we doubt that the above considerations would constitute sufficiently
conclusive evidence that fOlUJdation/embankment material could not be carried by the
seepage water for the following reasons:

a. Even if there are no continuous joints through the seepage aquifer, that does not
necessarily preclude piping of soil along the seepage paths. Ob'liously there Me one
or more continuous paths for the seepage flow. If these paths consists of a series of
interconnected but differently oriented joints and bedding planes, the resulting
circuitous seepage routes would reduce the potential for transportation of
embankment/folUJdation material but would not eliminate that possibility.

b. It is possible that seepage may not exclusively be following natural joints and
planes. Construction of the outlet tunnel may have created fractures or opened
existing joints.

c. Unless seepage discharges are carefully monitored for suspended material, the slow
removal of material canied by seepage water may not be visually identifiable.

d. The fact that the dam and reservoir has operated over a long period of time with
little or no expressions of piping soil loss does not necessarily assure that removal of
embankment/foundation material is not occuning and will not occur for a number
of reasons:

i. Cavities in tile embankment and/or foundation can develop slowly over a long
period of time as a result of slow, long term removal of material. It is possible
that this removal of material can occur intennittently under certain periodic flow
conditions. Relatively large cavities can develop without collapsing and causing
sUlface expressions.

ii. A catastrophic piping-related failure would not necessarily be exclusively the
result of embankment/folUJdation material being carried through joints in the
bedrock. It is possible that the slow development of cavities as described above
could progress, increasing pore pressures and seepage forces in the
embankment and foundation soils surrounding the cavities. Possible long-term
serious consequences of this process could include forcing seepage to the
surface or instability of the downstream embankment.

iii. There are conditions in addition to erosion of the sandstone aquifer that could
result in the enlargement of seepage paths. The Martin and Wood report noted
that the local formations are of marine deposition and that there are indications
of significant removal of soluble material. This dissolution of material has the
potential of significantly increasing the size of seepage paths and, therefore,
increasing the potential for removal of embankment/foundation material over