PERMFILE106953

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 69 Section C I. Construction Approach & Construction Monitoring Program
• Media Type: D
• Archive: No

• Reduction of Drivin Fg orces
The forces tending to cause movements downslope are essentially gravitational, therefore a simple
approach to mitigating slope failures is to reduce the mass of material involved in the failure.
Techniques for this include flattened slopes, benched slopes, reduced excavation depths, and
surface and subsurface drainage.
Benching and thereby flattening cut slopes in known landslide azeas aze alternatives that have
already been adopted into the construction plans for the Sylvester Gulch facilities and aze part of
the `go-slow, observational approach'. Bench height and width will be determined at the time that
the initial cuts aze made.
Modifications to the line and grade of the access road to reduce the required cuts can be considered
as detailed site specific surveying is accomplished.
Because of its high stabilization efficiency in relationship to design and construction costs,
drainage of surface water and groundwater is the most widely used and generally most successful
slope stabilization method. Of all possible mitigation alternatives to be considered, proper
drainage is probably the single most important. Drainage will reduce the weight of the mass
tending to cause the slope failure and increase the strength of the material in the slope.
The most likely drainage condition that will be encountered in the construction of the Sylvester
Gulch facilities is surface water seeping into or infiltrating into the head of the cut. Diversion
ditches and interceptor drains are widely used to ensure that surface waters are carried away from
the slopes.
• Because seepage forces act to increase the driving force on a landslide, control of subsurface water
is of major importance. Subsurface drainage as a method of lowering the groundwater table within
the unstable mass consist of drainage blankets and trenches, drainage wells, and horizontal drains.
Drainage trenches are incorporated into the design of the potentially unstable cut slopes in
Sylvester Gulch. Additional drains and interceptors can be installed as the cuts are advanced and
water is encountered. Drainage wells and horizontal drains can be drilled into the slopes from the
construction benches and from the toe of the slope if the site specific groundwater conditions
require such treatment.
Increase in Resistin Fg orces
The third general method for stabilizing cut slopes is to increase the resisting forces. Although
techniques vary widely, they generally function by either:
• Applying a resisting force at the tce of a landslide, or
• Increasing the internal strength of the materials in the failure zone so that the slope remains
stable without external assistance
Both approaches will be considered in the event of slope failures during construction.
A number of methods have been developed that increase the resisting forces at the tce of a sliding
mass. These include various methods of adding mass to the toe area and various structural
retention systems.
Methods used to increase the internal strength of the mass include subsurface drainage, a wide
. variety of reinforcing systems, and miscellaneous methods such as chemical and thermal treatment