Seismic Earth Pressures on Deep Building Basements

The International Building Code and ASCE 7-05 require that earth retaining structures and basement walls be designed for seismic earth pressures. Although there are many documented failures of retaining structures during earthquakes, almost all are associated with some form of soil-related failure in loose or poorly compacted soils in waterfront or marine locations or associated with embankments, slope instability or liquefaction. On the other hand, there have been no reports of damage to building basement walls as a result of seismic earth pressures in recent United States earthquakes including the 1971 San Fernando, 1987 Whittier Narrows, 1989 Loma Prieta and 1994 Northridge earthquakes, or in the 1995 Kobe, Japan or 1999 Chi Chi, Taiwan earthquakes. However, despite the absence of compelling damage or failure due to seismic earth pressures, inclusion of seismic earth pressures is required in the design of earth retaining structures and basement walls in the current United States building code. Most geotechnical engineers estimate seismic earth pressures using the Mononobe-Okabe method of analysis developed in the 1920s based on model tests of walls with sand backfill on a small shake table. The results from the original MononobeOkabe method have been compared to more recent tests which allow superior geometric and material property scaling using wall and soil models shaken in a centrifuge. The centrifuge tests strongly suggest that the Mononobe-Okabe methodology does not properly model full scale conditions and may be extremely conservative in the predicted seismic earth pressures. In addition, many geotechnical engineers are uncertain about the various inputs to the Mononobe-Okabe method which adds more unpredictability in the reported results. The applicability of the Mononobe-Okabe method to non-sandy backfill is also an issue. Based on the recent research, provisional recommendations for the design of building basement walls are presented, and the impact on the structural design of the basements is discussed.