Laserfiche WebLink
owner requires greater confidence in the potential effectiveness of the repair /stabilization measures <br />discussed below, further investigation, landslide characterization, analysis and design will be needed. <br />The following areas were identified as appropriate for repairs: <br />"Oil Well Access Road Slide" This is a fairly small slump in the Pond 6 area above an existing county <br />road that leads to an oil well. The toe of the slide has encroached into the ditch above the road, disrupting <br />surface drainage and causing water to accumulate in the ditch. The current plan consists of regrading the <br />slide and reopening the ditch, installing a trench or V- shaped drain through the horizontal midpoint of the <br />slide mass, and embedding large boulders along or near the base of the slide mass, above the ditch, to act <br />as a toe buttress or retaining wall. <br />"Pump House Slide" The mine operator is planning to reclaim several switchbacks above the Pond 6 <br />slide area. The "Pump House Slide" has a narrower shape and more of a mudflow morphology than other <br />landslides in the area. Rough grading to create surface drainages in the toe area had been performed just <br />before our visit. Reestablishing vegetation and improving surface drainage above the slide area to reduce <br />infiltration are appropriate strategies to help stabilize this area. <br />"Pit Ramp Slide" This area contains mapped mud flow and landslide deposits and unstable slopes <br />(Madole, Colton, and Miller), and has been the site of ongoing slope failure and repairs over the last <br />approximately 8 years. The scarp and toe features have recurred at least three times in virtually the same <br />location within several years after repairs have been made. The head scarp appears to correspond to a cut <br />excavated into the original slope to create a haul road. When the haul road was reclaimed, a fill prism <br />was placed within the road cut. It appears that this fill prism, soil and topsoil placed on the slope below <br />the original road, and possibly some of the shallow in situ soil and /or rock are unstable. Erosion gullies <br />have developed across the surface of the slide in several areas. Although the slide has been regraded <br />several times, it appears that the underlying failure surface continues to be weakened by perched water <br />(shallow subsurface water from surface infiltration). <br />The repair plan that we discussed in the field consists of excavating two trench drains (either lined or <br />filled with rock), regrading the toe bulge, gently knocking down the vertical head scarp, seeding, and <br />weed control. One trench drain is planned along existing erosion gullies near the horizontal midpoint of <br />the slide mass, and another in the wet area near the western toe bulge above the road ditch. The trenches <br />should extend through the slide mass and into the original road surface at the head scarp area, and will <br />become shallower as they approach the ditch. We discussed angling the drains 30 ° -45° laterally as they <br />descend the slope in an effort to capture more surface water. If the drains are angled, fully penetrate the <br />slide mass (extending below the failure surface), and are filled with hard, angular rock, they should act as <br />shear keys and increase the overall strength of the slide mass, thereby reducing, but not eliminating, the <br />risk of future movement. <br />Numerous slides surrounding Ponds 16 and 16A. Ponds 16 and 16A are located at the base of a <br />mapped, pre- mining mudflow that encompasses the area surrounding drainage PM -2A. NWCC <br />investigated the Pond 16 slides and presented recommendations in a "Site Observations and Slope <br />Stabilization Recommendations, Pond 016 Area Slides" report, dated October 28, 2010. An approved <br />repair plan, TR -73, is in place for this area. The plan includes stabilizing the slides and slumps through <br />improved surface and subsurface drainage. Pond 16A will be backfilled and the Road J/K curve will be <br />relocated toward the west, over the backfilled area; underdrains will be installed from a cluster of <br />abandoned artesian wells and the Pond 16 slide to new surface drainage PM -2; and the area below the <br />shrub plot will be excavated to intercept shallow subsurface flow currently emanating from the area <br />surrounding the distinctive spoil spring cavity. Excavations to intercept subsurface flow along the base of <br />