Landslides

Detailed Geotechnical Engineering Analysis

Large, new ground cracks developed at the top of a cut slope over 100 feet high in the Seacliff Subdivision in January 2023. A consultant advised the City of Richmond that a big, deep landslide could be occurring, and the City ordered the evacuation of 15 homes at the base of the hillside. AKA was engaged by the Homeowner’s Association to assess the hillside and comment on the emergency work being performed. AKA initiated survey monitoring of the slope and a large crack in a retaining wall, performed a LiDAR survey, and conducted drone flights. In addition, AKA reviewed historic information regarding the slope on aerial photographs and past consultant reports. As the data was provided to the City, the evacuation order was lifted. Detailed studies performed the following summer and fall included subsurface exploration in the areas of ground crack, video surveys of buried storm drain pipes on the slope, additional drone flights, and stability analyses. This work will guide remedial work on the slope in the future.

Preliminary Geologic and Geotechnical Study

AKA has extensive data and specialized expertise pertaining to urban landslides, particularly the series of large active and potentially active landslides that exist on the western flank of the Berkeley Hills. The active Blakemont landslide straddles the border between El Cerrito and Kensington and is about 3,000 feet long and 700 feet wide. Most residences located over the landslide have experienced damage due to landslide movements. AKA conducted a preliminary study for the Blakemont Property Owners Association to delineate a proposed boundary for a planned Geologic Hazard Abatement District (GHAD) and develop conceptual approaches and cost estimates for landslide mitigation. The scope of our study entailed reviewing historic aerial photography and existing geologic/geotechnical reports, mapping surface conditions, and investigating subsurface conditions with deep borings, exploratory trenches, and inclinometers. The results of our preliminary study indicate that the active portion of the landslide is relatively shallow, averaging less than about 25 feet thick, and that lowering groundwater levels could stop or significantly reduce future landslide movement under static (non-earthquake) conditions. Based on this finding, AKA developed a detailed and constructible conceptual mitigation design utilizing deep subdrains coupled with surface drainage improvements to permanently lower groundwater to a level that would result in acceptable stability.