Avalon Canyon Landslide Repairs

Major storm damage, including hydraulic soil erosion, ground movements, and a broken 30 inch diameter storm drain, occurred in Avalon Canyon as a result of the 1997/1998 winter storms and exceptionally high rainfall.

Planning for repairs was particularly challenging because of weak geologic units and streets, homes, and churches perched atop the unstable area. GEI served as geotechnical consultant on a team led by Jacob Associates, Inc. to investigate the cause of the failure and design repairs.

During the winter of 1997/1998 the storm drain failed, causing extensive erosion at the point of the pipe break and producing deep gully erosion down slope. The heavy rainfall throughout February and March 1998 continued the erosion at the head wall of the canyon and gully erosion down slope, undercutting and destabilizing the existing slopes. Comparison of topographic surveys before and after the storm damage indicated that approximately 100,000 cubic yards of soil had been eroded. This erosion triggered slope movements, eroding portions of the service road leading from Avalon Drive to the base of the canyon, and undermining portions of Avalon Drive at the head of the canyon. The repair of Avalon Canyon required about four times the volume of soil eroded during the storms of 1997/1998 to flatten, buttress and stabilize the slopes.

GEI’s geotechnical investigation and evaluation provided the required data and information for the Jacobs/GEI project team to design repair alternatives which could be constructed before the beginning of the winter season.

Additional work included advanced slope-stability analyses and on-site field observations during Phase 1 slide repairs.

After conducting investigations, GEI concluded that the canyon head wall and south side slopes could be stabilized, and erosion and landsliding improved, by reconstructing and fl attening the slopes. These techniques increased slope stability by about 60 percent. The reconstruction is also expected to increase the seismic resistance of the slopes under earthquake shaking.

GEI also recommended that the regraded slopes be revegetated to manage and control surface water erosion. Surface water runoff will be routed into pipes or ditches to withstand the expected high flow velocities.