[S07-7] Site effects II

[S07-7-02] Liquefaction Monitoring and Observations of Excess Pore Pressure Generation During Strong Motion

^○Jamison Steidl (University of California, Santa Barbara, USA)

Unique dense observations of excess pore pressure generation and dissipation during earthquakes are presented from multiple borehole arrays within ~100 meters of each other in the confined 5-meter thick silty sand layer at the Wildlife Liquefaction Array (WLA), Imperial Valley, California. In what is the most densely instrumented liquefaction array anywhere in the world, continuous recordings of acceleration and pore pressure are providing in situ empirical evidence documenting the range of ground motion and dynamic strain levels at which the onset of excess pore pressure and nonlinear soil behavior begins, augmenting previous case history data, and cyclic tri-axial and centrifuge laboratory testing. The observations with depth within the liquefiable layer shows the excess pore pressure develop quickly at the top of the layer just below the impermeable clay cap, and then migrate towards the bottom of the layer as the ground motions subside and the pressure dissipates over the course of minutes to hours depending on the level of ground motion. Confirmation of this behavior is provided by monitoring multiple locations within ~100 meters of each other, with 8 transducers at one location and 5 transducers at a second location (both operating for more than a decade now), and 5 transducers at a third location (since 2014). The 5 sensors at the third location densely cover the upper two meters of the liquefiable layer, while the sensors at the other two locations sample the top, middle, and bottom of the layer. A comparison of these recordings to demonstrate the spatial variability both with depth in the liquefiable layer as well as within the 100-meter areal distance at the field site with be presented. This spatial variability is analyzed with respect to both the fine-scale heterogeneous soil properties (percent fines, relative density, saturation) as well as with respect to installation methods and site improvements to potentially increase liquefaction resistance.