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Report on Research Conducted Under Grant No Pacific Earthquake Engineering Research Center Documentation and Analysis of Field Case Histories of Seismic Compression during the 1994 Northridge, California, Earthquake Jonathan P. Stewart Patrick M. Smith Daniel H. Whang University of California, Los Angeles and Jonathan D. Bray University of California, Berkeley PEER 2002/09 OCT. 2002 Documentation and Analysis of Field Case Histories of Seismic Compression during the 1994 Northridge, California, Earthquake Jonathan P. Stewart, Patrick M. Smith, and Daniel H. Whang Department of Civil and Environmental Engineering University of California, Los Angeles Jonathan D. Bray Department of Civil and Environmental Engineering University of California, Berkeley A report on research conducted under Grant No. 1434-HQ-GR-00037 from the U.S. Geological Survey, National Earthquake Hazards Reduction Program; Grant No. 9701568 from the National Science Foundation; and the Pacific Earthquake Engineering Research Center’s Program of Applied Earthquake Engineering Research of Lifeline Systems PEER Report 2002/09 Pacific Earthquake Engineering Research Center College of Engineering University of California, Berkeley October 2002 ABSTRACT USGS/NEHRP Award No. 1434-HQ-98-GR-00037 DOCUMENTATION AND ANALYSIS OF FIELD CASE HISTORIES OF SEISMIC COMPRESSION DURING THE 1994 NORTHRIDGE, CALIFORNIA EARTHQUAKE PI: Jonathan P. Stewart Civil & Environmental Engineering Department University of California Los Angeles, CA 90095-1593 Tel: (310) 206-2990 Fax: (310) 206-2222 [email protected] 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 ≈ 0.3–0.7 g). The performance of the fills was quite different. In one case (denoted Site A) ground settlements up to ∼18 cm occurred, which damaged a structure, while in the other case (Site B) settlements were < ∼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 of test results for soils with and without fines suggest that for many cases, fines decrease seismic compression potential relative to clean sands. For soils with fines, it appears that seismic compression is most pronounced when the fines are nonplastic, or when the fines are plastic and the soil has a clod structure. We iii observe clod structures in plastic soils compacted dry of the line of optimums or at low densities, but not in nonplastic soils. 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 de-couples 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. Parametric variability in all significant model parameters is estimated, and the analyses are repeated according to a logic tree approach in which a weight is assigned to each possible realization of the model parameters. The analyses results provide probabilistic distributions of shear strain, volumetric strain, and settlement, the last of which can be compared to observed field settlements. Calculated ground settlements at Site B match observations between the 30th and 70th percentile levels. At Site A, the analyses successfully predict the shape of the settlement profile along a section, but the weighted average predictions are biased slightly low (match occurs at the 50th to 70th percentile level). We speculate that the underprediction likely results from imperfect knowledge of site stratigraphy and/or underestimation of volumetric strains from the laboratory tests as a result of the non-reproducibility of the field soil’s clod structure. Sensitivity studies reveal that the mean value of calculated settlements is highly sensitive to shear strain amplitude and compaction condition, while the standard deviation is mostly strongly influenced by variability in the shear strains. The median and standard deviation of shear strains, in turn, are strongly influenced by the site shear wave velocity profile, ground motion characteristics, and the method of site response analysis (i.e., 1-D versus 2-D). The various sources of parametric variability combine to form a coefficient of variation of about 0.5 to 1.0, being closer the low end of the range if 2-D analyses are performed (∼0.5–0.7) and the upper end of the range if 1-D analyses are performed (∼0.8–1.0). iv NONTECHNICAL PROJECT SUMMARY We investigate ground settlements at two canyon fills shaken by the Northridge, California, earthquake. The settlements are found to result from seismic compression, a process by which volumetric strain accrues in unsaturated soil during strong earthquake shaking. Insights into soil parameters affecting seismic compression are gained through a simple shear laboratory testing program involving soils from the sites. Back-analyses of site performance are able to capture the observed field settlement patterns. The analyses also provide insight into the ground motion and site parameters that significantly influence the seismic compression susceptibility of a fill section. v ACKNOWLEDGMENTS Support for this work was provided by the U.S. Geological Survey, National Earthquake Hazards Reduction Program, Award No. 1434-HG-98-GR-00037; a CAREER grant from the National Science Foundation (NSF Award No. 9733113); the Pacific Earthquake Engineering Research Center’s Program of Applied Earthquake Engineering Research of Lifeline Systems with funding from the Pacific Gas and Electric Company (PEER award number Z-19-2-133-96); and the UCLA Department of Civil & Environmental Engineering. This work made use of Earthquake Engineering Research Centers Shared Facilities supported by the National Science Foundation under Award #EEC-9701568. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. The authors would like to thank the following individuals who provided materials that were of assistance in this research program: Del Yoakum of Geosoils, Jim Roberts of Jacobs Engineering, Mike Sisson, and Jerod Cascadden. We acknowledge the technical assistance to the laboratory testing program provided by Michael Riemer of the Pacific Earthquake Engineering Research Center. The supportive work of UCLA undergraduate and graduate students Trolis Niebla, Mathew Moyneur, and Annie Kwok is also acknowledged. vii CONTENTS ABSTRACT.................................................................................................................................. iii NONTECHNICAL PROJECT SUMMARY ..............................................................................v ACKNOWLEDGMENTS .......................................................................................................... vii TABLE OF CONTENTS ............................................................................................................ ix LIST OF FIGURES .....................................................................................................................xv LIST OF TABLES .....................................................................................................................xxv 1 INTRODUCTION..............................................................................................................1 2 PRIOR OBSERVATIONS OF THE SEISMIC PERFORMANCE OF COMPACTED FILLS.......................................................................................................5 2.1 Historic Observations...............................................................................................5 2.2 Observations from the 1994 Northridge Earthquake...............................................6 2.2.1 Damage Distribution and Typical Fill Deformation Features ...........................6
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