Challenge

The Portuguese Bend landslide is one of several historically active and dormant landslides that now form the Altamira Landslide. It has been studied since at least 1956. An early attempt by the US Army Corps of Engineers to mitigate the landslide with caissons failed. Geo-Logic Associates was asked to develop a mitigation measure in 2016. Evaluation of over 500 studies generated over a 70-year period revealed that artesian pressure is the main cause of landsliding.

Solutions

Upon completion of a feasibility study, GLA developed a solution that called for installation of artesian-pressure-relieving directionally drilled wells approximately 1,400 feet deep that was selected. A “proof-of-concept” study completed in 2023 involved three-dimensional stability and seepage analyses and demonstrated that the proposed solution was both technically feasible and the most economical. Soon thereafter, based upon what GLA developed and presented, the Federal Emergency Agency (FEMA) awarded the largest mitigation grant in its history to the City. Shortly before hydrauger installation, movement of the Altamira Landslide commenced however. Areas well beyond the Portuguese Bend landslide were affected. Over 680 acres were designated as the “disaster area” and the mitigation of the Portuguese Bend landslide was put on hold. For the Altamira Landslide, GLA developed and implemented emergency mitigation measures. These included tentative installation of 11 deep dewatering wells up to 340 feet deep along the toe of the Altamira Landslide.

Results

Emergency mitigation measures for the Altamira Landslide significantly reduced landsliding, practically to a standstill. In-construction monitoring showed a rapid decrease in groundwater pore pressures at various levels within the landslide, confirming the GLA hypothesis that artesian pressure is the major cause of instability.

… the City was just informed that we are a recipient of a $23.3 million grant from FEMA for the Portuguese Bend Landslide Project. …we made many last-minute requests, all of which you [Geo-Logic] accommodated. The information you and your team provided was critical to the success of the grant application… we could not have done it without you and we are very grateful.”

— Ramzi Awwad – City of Rancho Palos Verdes Director of Public Works