PERMFILE61031 - Laserfiche WebLink

Home Browse Search

• REPAIR A french drain, consisting of 6-inch slotted pipe bedded in 1 1/2" washed rock, was constructed along the length of the upper scarp, within the failure surface. The water collected by the french drain is conveyed down the slope to the access road ditch in a 6-inch diameter ADS pipe. It is proposed to construct a similar drainage structure within the toe of the slide and that water which is collected will be conveyed along with the water from the upper drain, to a point below the entire slope onto the valley floor and eventually to the sediment pond. The purpose of the upper drainage feature is to keep any additional water from entering the slide's failure surface, and ultimately preventing excess pore pressures from building up in the subsurface soils. The lower drainage feature will aid in lowering the already high pore pressures within the slide by the collection and handling of the subsurface water present in the failure • surface. By conveying all the collected water to the valley floor, the chance of inducing any elope instability below the slide will be reduced. As of April 13, 1993, the upper drain was conveying a flow of 4 gallons per minute from the slide's failure surface. As of May 29th, the drain was conveying 2 gallons per minute. As of August 3rd, the flow was reported to have reduced to a trickle. This flow history is evidence that the upper drain is at a location that should assure the ongoing interception of near surface groundwater flow which originates from upland snow melt and precipitation and has effectively lowered the groundwater table from the previous level. RECOMMENDATIONS It is recommended that the reconstruction of the portal access road be delayed until the slide area has drained sufficiently. The • reconstruction required for the access road will consist of filling 21-10