GOALI: Dynamic Soil Properties - Effects of Construction-induced Stress Changes

Northwestern University, Evanston IL

Investigators

Abstract

Frequently, a project in a seismically active zone must consider conditions at multiple strain levels. In design, the stability and limiting deformations for the structure are evaluated at relatively high strain levels. The effects of ground shaking on the completed structure are evaluated on the basis of stiffness at small strain levels. These ?dynamic? and ?static? stiffnesses are based on tests conducted prior to construction and thus only reflect the stress conditions prior to construction of the structure. However, the effects of a seismic event normally impact the structure after it has been constructed. Conditions in the soil are altered from their initial condition, and thus the question arises as to the effects of the stress changes on the ?dynamic? properties of the affected soil. The design and construction of the Port of Anchorage Expansion Project provides a test bed to evaluate these concepts and extend the state-of-the-art. This research project is a joint effort between Northwestern University and GeoEngineers, Inc. of Redmond, Washington, the geotechnical consultants for the project. The expansion project consists of constructing nearly 2 miles of a new open-cell wharf facility. The 90-ft-high wharf will consist of a series of interconnecting sheet pile cells that form a continuous retaining wall along the face but are ?open? at the ends (landside). Construction of the large wharf structure will greatly alter the stresses, and thus the dynamic properties, in the foundation soils beneath and seaward to the structure. To evaluate the dynamic properties applicable to the foundation soils after construction, these stress changes must be considered explicitly. The scope of work includes conventional field and laboratory testing, field cross-hole seismic testing, seismic CPT probes, thin-walled tube sampling of the clay, triaxial stress probe tests with internal measurements of axial and radial deformations and synchronous measurements of shear wave velocity with bender elements in 3-dimensions. Finite element analyses of wharf construction and simulation of the design seismic event will also be conducted. Performance monitoring of wharf construction will permit comparison of predictions with full-scale performance, and parametric studies will be conducted to define when these construction effects are important. Ultimately, a testing protocol will be developed to relate conventional field and laboratory results to detailed laboratory results. This collaboration provides a unique opportunity to relate conventional data collected during site investigation, research quality stress-strain data and state-of-the-art finite element simulation with full-scale performance of a geo-structure to advance the state-of-the-art in geotechnical earthquake engineering. In particular, advances in understanding the effects of loading on the wave propagation characteristics of incrementally-nonlinear materials are relevant to this field. Information gained in this research will directly apply to all other finite element modeling projects where deformations under static and seismic loading conditions are important, particularly in applications to static problems wherein it is important to include constitutive responses over very small to larger strains. Both graduate and undergraduate students will play an integral part in this research. The close collaboration between GeoEngineers, Inc. and Northwestern University provides an exciting opportunity for students to participate in a major design and construction project. This work will be excellent experience for them, whether they choose to pursue an academic/research career, or a career in engineering practice.