The Earthquake Engineering Online Archive

Documentation and analysis of field case histories of seismic compression during the 1994 Northridge, California, earthquake

Stewart, Jonathan P.; Smith, Patrick M.; Whang, Daniel H.; Bray, Jonathan D.

PEER-2002/09, Pacific Earthquake Engineering Research Center, University of California, Berkeley, 2002-10, 223 pages (400/P33/2002-09)

Seismic compression is defined as the accrual of contractive volumetric strains in unsaturated soil during strong shaking from earthquakes. While ground deformations from seismic compression have been reported in the literature, it contains few case histories in which the amount of ground deformation was known accurately from pre- and post-earthquake surveys. In this report, two such case histories are documented in detail and analyzed. Both case studies involve deep canyon fills in Santa Clarita, California, an area strongly shaken by the Northridge earthquake (peak accelerations on rock approximately equal to 0.3-0.7 g). The performance of the fills was quite different. In one case (denoted Site A), ground settlements up to approximately 18 cm occurred, which damaged a structure, while in the other case (Site B), settlements were less than approximately 6 cm. One important thrust of the present work involved cyclic simple shear laboratory testing of four reconstituted soil samples from the two subject sites. These samples all have fines contents near 50% (such that the fines fraction controls the soil behavior), but have varying levels of fines plasticity. Each specimen was compacted to a range of formation dry densities and degrees of saturation. The results significantly extend the seismic compression literature, which has consisted primarily of laboratory testing of clean uniform sands. The test results show that seismic compression susceptibility increases with decreasing density and increasing shear strain amplitude. Saturation is found to be important for soils with plastic fines but relatively unimportant for soils with nonplastic fines. Comparisons are made of test results for soils with and without fines. The objectives of analyses performed for the two sites were (1) to investigate the degree to which seismic compression can explain the observed ground displacements and (2) to evaluate the sensitivity of calculated settlements to variability in input parameters as well as the dispersion of calculated settlements given the overall parametric variability. The analysis procedure that is used decouples the calculation of shear strain from that of volumetric strain. The shear strain calculations involved one- and two-dimensional ground response analyses employing site-specific dynamic soil properties and a suite of input motions appropriate for the respective sites. Volumetric strains are evaluated from the shear strains using material-specific models derived from the simple shear laboratory test results.

Available online: http://peer.berkeley.edu/publications/peer_reports/reports_2002/0209.pdf