Photo overlooking the Pacific from the Asilomar Boulevard Landslide
Looking south east from the head of the active portion of the Asilomar Boulevard Landslide

Key Issues

  • Movement on the Asilomar Boulevard Landslide has damaged the roadway and threatens homes and associated infrastructure above and below the landslide

Solutions

  • Conduct a comprehensive field investigation to characterize geotechnical and hydrogeologic conditions in the vicinity of the active landslide
  • Design a landslide stabilization plan incorporating both geotechnical and hydrogeologic mitigation measures

Hydrogeologic Investigation and Dewatering System Design for Asilomar Boulevard Landslide

Pacific Palisades, Los Angeles County, California

A field investigation program was conducted to support design of a landslide stabilization plan.

Recent movement on the younger landslide deposits of the Asilomar Boulevard Landslide Complex in the Pacific Palisades area prompted the City of Los Angeles Department of Public Works (Bureau of Engineering - Geotechnical Engineering Division) to engage Ninyo & Moore and Todd Groundwater to evaluate alternative geotechnical solutions. Ninyo & Moore, the prime contractor, developed a geotechnical stabilization plan and Todd Groundwater conducted a hydrogeologic investigation to evaluate the feasibility of operating a well-based dewatering system to complement the preferred geotechnical alternative, which also included a soldier pile wall with tie-back anchors and grading.

The scope of work for the hydrogeologic investigation included:

  • Design and installation of test dewatering and monitoring wells using the hollow-stem auger method
  • Performance and evaluation of multi-well aquifer pumping tests
  • Development and calibration of a groundwater flow model
  • Simulation of dewatering the upper portion of the active landslide
  • Development of technical specifications and a cost estimate for a conceptual well dewatering system.

Available geologic information, including documented landslide movement, suggests that hydrogeologic conditions along Asilomar Boulevard are complex and highly variable. Accordingly, a combination of variable-discharge (step-drawdown) and constant-discharge pumping tests was used to characterize hydrogeologic conditions across the active portion of the landslide. Data collected from the step-drawdown tests provided critical aquifer and well hydraulic information to design the constant-discharge tests. In turn, data collected from constant-discharge tests were analyzed using appropriate well hydraulic equations to estimate aquifer hydraulic properties, including aquifer transmissivity, hydraulic conductivity and storativity.

The numerical groundwater flow model was calibrated to determine the potential effectiveness and required flow rates of dewatering wells. The model was calibrated to site-wide, steady-state conditions and to the dynamic conditions of the aquifer tests.

Aerial photo and cross-sectional diagrams of the landslide area
Aerial photo and cross sections with model-simulated dewatering well drawdowns

Results of the field program and modeling indicated that a five-well dewatering system along the north side of Asilomar Boulevard would enhance the stability of the landslide and complement the preferred geotechnical solution.

A conceptual dewatering system design including well technical specifications, general electrical and conveyance system design, performance testing, and instrumentation was developed. A preliminary cost estimate (including engineering fees, contractor labor, and materials) was also provided.