Seismic compression is defined as the accrual of contractive volumetric strains in unsaturated compacted soil during earthquake shaking. Existing seismic compression analysis procedures are based on laboratory test results for clean uniform sands, and their applicability to compacted soils with fines is unclear. We evaluate seismic compression from cyclic simple shear laboratory testing of four compacted soils having fines contents that are sufficiently large that fines control the soil behavior, but possessing varying levels of fines plasticity. Each soil material is compacted to a range of formation dry densities and degrees-of-saturation. The test results show that seismic compression susceptibility decreases with increasing density and decreasing shear strain amplitude. Saturation is also found to be important for soils with moderately plastic fines (plasticity index, PI » 15), but relatively unimportant for soils with low plasticity fines (PI » 2) across the range of saturations tested (³54 %). The saturation effect appears to be linked to the presence or lack of presence of a clod structure in the soil, the clod structure being most pronounced in plastic soils compacted dry of the line-of-optimums or at low densities. Comparisons of test results for soils with and without low- to moderately-plastic fines suggest that fines can decrease the seismic compression potential relative to clean sands.

The nees@UCLA mobile field laboratory was utilized to collect forced and ambient vibration data from a four-story reinforced concrete (RC) building damaged in the 1994 Northridge earthquake. Both low amplitude broadband and moderate amplitude harmonic excitation were applied using a linear shaker and two eccentric mass shakers, respectively. Floor accelerations, interstory displacements, and column and slab curvature distributions were monitored during the tests using accelerometers, linear variable differential transformers (LVDTs) and concrete strain gauges. The use of dense instrumentation enabled verification of common modeling assumptions related to rigid diaphragms and soil-structure-interaction. The first six or seven natural frequencies, mode shapes, and damping ratios were identified. Significant decreases in frequency cor responded to increases in shaking amplitude, most notably in the N-S direction of the building, most likely due to preexisting diagonal joint cracks that formed during the Northridge earthquake. DOI: 10.1193/1.2991300

We describe the characteristics of a simple shear apparatus capable of applying realistic multidirectional earthquake loading to soil specimens. This device, herein termed the Digitally Controlled Simple Shear (DC-SS) apparatus, incorporates features such as servohydraulic actuation and true digital control to overcome control limitations of some previous dynamic soil testing machines. The device is shown to be capable of reproducing sinusoidal and broadband command signals across a wide range of frequencies and amplitudes, although the device has limited control capabilities for very small command displacements (less than approximately 0.005 mm). The small deformation limitation results from noise introduced to the control system from analog-to-digital conversion of feedback signals. We demonstrate that bidirectional command signals can be accurately imparted with minimal cross coupling, which results from an innovative multiple-input, multiple-output digital control system. The capabilities of the device are demonstrated with a series of broadband tests on unsaturated soil specimens subjected to uni- and bidirectional excitation.

Seismic compression is defined as the accrual of contractive volumetric strain in unsaturated soil during strong shaking by earthquakes. We document and analyze two case histories (denoted school site and site A) of ground deformation from seismic compression in canyon fills strongly shaken by the Northridge earthquake. Site A had ground settlements up to about 18 cm, which damaged a structure, while the school site had settlements up to about 6 cm. For each site, we perform decoupled analyses of shear and volumetric strain. Shear strain is calculated using one-dimensional and two-dimensional ground response analyses, while volumetric strain is evaluated from shear strain using material-specific models derived from simple shear laboratory testing that incorporates important effects of fines content and as-compacted density and saturation. Analyses are repeated using a logic tree approach in which weights are assigned to multiple possible realizations of uncertain model parameters. At the school site, predicted settlements appear to be unbiased. At site A, the analyses successfully predict the shape of the settlement profile along a section, but the weighted average predictions are biased slightly too low. We speculate that the apparent site A bias can be explained by limited resolution of the site stratigraphy, bias in laboratory-derived volumetric strain models, and/or uncertainty in the estimated earthquake-induced settlements.

This paper describes a set of dynamic field tests performed on the Alfred Zampa Memorial Bridge (AZMB), also known as the New Carquinez Bridge, which is located 32km northeast of San Francisco on interstate Highway I-80. The AZMB, opened to traffic in November 2003, is the first suspension bridge in the United States with an orthotropic steel deck, reinforced concrete towers and large-diameter drilled shaft foundations. The dynamic field tests described herein were conducted just before the bridge opening to traffic. They included ambient vibration tests, mainly wind-induced, and forced vibration tests based on controlled traffic loads and vehicle-induced impact loads. Four different controlled traffic load patterns and seven different vehicle-induced impact load configurations were used in the forced vibration tests. The dynamic response of the bridge was measured through an array of 34 uni-axial and 10 tri-axial force-balanced accelerometers deployed along the whole length of the bridge. These dynamic field tests provided a unique opportunity to determine the dynamic (modal) properties of the bridge in its as-built (baseline) condition with no previous traffic loads or seismic excitation. Such properties could be used to validate and/or update the finite element models used in the design phase of this bridge. They could also be used as baseline for future health monitoring studies of this bridge. At the end of the paper, the ambient vibration test data were used to identify the bridge modal parameters (natural frequencies, damping ratios and mode shapes) using the data-driven stochastic subspace identification method.