LandslideMitigation InfrastructureSafety DeepLandslide StabilisationEfforts

Uncovering the Deepening Crisis: Rancho Palos Verdes in California Faces New Landslide Threats

Land movement, intensified by storms, created significant cracks in the pavement and inflicted damage on nearby homes along Dauntless Drive in Rancho Palos Verdes. Source: Los Angeles Times (image by Brian van der Brug)

The coastal city of Rancho Palos Verdes has long struggled with the persistent threat of land movement, but recent findings have revealed that the situation is far more complex and perilous than previously understood. The discovery of a deep, active landslide plane beneath the Portuguese Bend area, believed to be part of the ancient Altamira landslide complex, has forced city officials to reassess their approach to stabilizing the region. This new understanding of the geological conditions is crucial for addressing the mounting challenges facing the community.

The Unfolding Geotechnical Challenge

Earlier this year, geotechnical investigations uncovered a deep and active landslide plane at depths ranging from 245 to 345 feet, far below the previously monitored slip plane of the Portuguese Bend landslide. This discovery has upended previous emergency stabilization efforts, as the newly identified slip plane is not only deeper but also moving at a faster rate. The ancient Altamira landslide, previously considered dormant, is now active and threatens to expand, potentially affecting a much larger area and exacerbating the already severe damage to infrastructure and homes.

Geological Map depicting the active and inactive (''ancient'') landslides. Source: Los Angeles Times (image by California Geological Survey)

Geologists and engineers working on the project have stressed the urgency of addressing this deeper landslide. The focus has now shifted to a pilot plan involving the drilling of vertical wells to pump out groundwater, which is known to be a primary driver of the land movement. However, much remains unknown about the full extent of the Altamira landslide complex and its potential to impact additional neighborhoods beyond those already affected.

A Community in Crisis

The implications of the ongoing land movement are profound. Over 130 homes in the area have already had their gas shut off due to safety concerns, and utility companies have warned that electricity could be next. The land movement has caused extensive damage to homes, roads, and utilities, and has even prompted a lawsuit from residents against local officials and agencies. The lawsuit alleges that inadequate management of water resources has contributed to the oversaturation of the hillsides, triggering the recent acceleration of the landslide.

Despite the challenges, there is a small glimmer of hope. Recent measurements indicate that the overall rate of land movement has slightly decelerated, though the area is still shifting at a rate far faster than normal. City officials are continuing to push for state assistance and exploring all possible avenues to mitigate the damage, but the situation remains dire.

Continuous movement damaging road infrastructure and house foundations. Source: Latest Report of the Rancho Palos Verdes city council

Rancho Palos Verdes is now at a critical juncture. The discovery of the deeper landslide plane has highlighted the need for a more comprehensive and unified approach to managing the multiple landslides in the area. As the city grapples with the unfolding crisis, the focus will remain on safeguarding the community and preventing further destruction.

Click here to check the last's week 'unprecedented' ground movement from the Ancient Altamira landslide complex due to back-to-back years of heavy rain.

Sources: latimes.com, granicus.com

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Sam Gonzales

Sep, 13, 2024 Having reviewed n inspected SFR development Geotechnical reports 15+ years for a municipal public works, deep seeded ancient non investigated landslides are common in northern California once hillside excavation begins. Borings to define landslide limits n excavation, confirmation by a Certified Engineering Geologist mandated. Hydraguars alone won't solve this problem. Unloading n placement of the material skillfully during excavation is critical. Goodluck to all involved.