MULTIPLE SLOPE FAILURE MECHANISMS RESULTING IN EPISODIC SEA CLIFF RETREAT AT THE SOUTHWEST MARINE FISHERIES SCIENCE CENTER, LA JOLLA, CALIFORNIA

The stability of a three-story reinforced concrete building complex located at the edge of a near-vertical, 65 meters high sea cliff has been geologically investigated since its construction in 1963. These previous studies failed to fully recognize the interplay of slope failure mechanisms because they lacked detailed field characterization of the cliff.

Since construction, the cliff edge has retreated a total of 16 meters to within 6 meters of a building corner. The most recent and one of the largest retreats occurred in 1997-1998. The increased threat of this condition motivated our investigation.

Our analysis of historical photographs and topographic maps shows that movement rates are episodic and not evenly distributed over time or location. The most recent failures have occurred in apparent association with large El Nino events: e.g. 2 years following the 1986-1987 El Nino and, again, during the very strong 1997-1998 event. These re-occurring periods of heavy rainfall and attendant higher levels of ground water facilitate slope instability.

An innovation of our investigation was to use fixed-ropes to examine the geologic structures exposed in the near-vertical cliff face. Excavations along the base of the cliff previously indicated that the sedimentary rock was cut by a system of NE-trending faults and other associated joints and fractures. Large diameter borings located between the complex and cliff-edge failure zone further exposed a system of open fractures and clay seams trending beneath the buildings.

We conclude that slope instability is due to: (1) block slides along gently seaward-dipping clay seams; (2) shallow slumping in the upper 17 meters of soils; (3) topple blocks and rock falls in the conglomerate middle cliffs along numerous near-vertical faults; and (4) wave erosion of the talus slope along the base of the cliff. The interaction of these different mechanisms, enhanced by infiltrating water from natural and other sources, controls the rate of landslides and bluff edge retreat.

Previous investigations often concluded with a focus on average retreat rates, which, in light of the episodic retreat process, are very misleading. This investigation shows that the collection and examination of more detailed field data can greatly influence conclusions and lead to a more accurate slope-stability assessment.