Perspectives on the Design of Deep Basement Walls in Southern California before and after Earthquakes

Buildings with deep basements became more commonplace in Southern California because of the elimination of a maximum building height limitation of 13 stories in the City of Los Angeles in the late 1950s. With the tall buildings often came deep basement structures to accommodate for the required parking for these larger occupancy structures. Southern California geotechnical practice provided design pressures for basement walls that were based on active earth pressure conditions rather than at rest pressures. Design of deep basements in Southern California that considered seismic earth pressures were not common until after the mid-1990s. Design for at rest pressures for basement walls in Southern California did not occur until the adoption of the California Building Code in 2007 that reflected the earth pressure requirements of the International Building Code that specified at rest earth pressures for restrained walls. There have been a significant number of tall buildings with deep basements constructed in Southern California with depths of up to 60 feet and even up to 100 feet below ground surface. Many of these were completed before the 1994 Northridge earthquake and some even before the 1971 San Fernando earthquake. Strong motion instrumentation was installed in some of these buildings. This paper will review the earthquake motions recorded in the basements of selected buildings in these two earthquakes and also examine the earth pressures which these buildings were designed for and comparisons with the present practice are made.