Geotechnical Engineering - Research

Landslides and debris flows cause significant damage and loss of life around the world each year. In the US alone, the annual economic costs of landslides can be estimated conservatively to be between $1 and $2 billion, with an associated 25 to 50 yearly casualties. To help protect people, infrastructure, and lifelines against such effects it is critical to have engineered structures that are capable of resisting the loads that are induced during these events. However,determining such loads can be very challenging---landslides and debris flows are highly dynamic and inherently complex in nature. The true extent of this complexity can be better understood by considering some of the key challenges that arise in attempting to model a landslide or debris flow. A general implementation must include the ability to capture the transitions between solid-like and fluid-like states, phase interaction modeling, appropriate volume constraints, boundary condition generalization, constitutive modeling, all within the context of motions with complexities on many scales. The purpose of this work is to develop a robust numerical modeling framework capable of accommodating these key phenomena with the primary initial goal of predicting loads on protective structures. The basic approach is based on adapting and extending the Material Point Method (MPM), which is well-suited to modeling large deformation, flow-like phenomena.

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Pile foundations are commonly used to support heavy column loads from buildings, bridges, and other structures.  The response of pile foundations during earthquakes is a complicated soil-foundation-structure interaction problem.  The seismic response of a pile foundation, however, can strongly affect the seismic performance of the structure it supports.  As a result, pile foundations must be carefully designed in order to ensure the reliability and economy of the structure.

The concepts of performance-based earthquake engineering can be used to provide more consistent and objective evaluations of system performance in areas of widely-varying seismicity.  Previous research has shown that consistent application of many conventional seismic hazard evaluation procedures can lead to inconsistent actual likelihoods of failure, and that this inconsistency can be eliminated using performance-based concepts.  The objective of this project is to apply performance-based concepts to the evaluation of pile group performance, and to develop design procedures in a load and resistance factor (LRFD) framework.

Finite element analyses of pile groups are being performed using the OpenSees computational platform.  Ground motions representing small to large earthquakes recorded at short to long distances are being applied to a three-dimensional lumped mass system representing a typical highway bridge.  The resulting foundation loads are applied to various pile groups in various soil conditions.  The resulting dynamic displacements are related to peak loads and to pseudo-statically obtained displacements.  The results will allow development of load and resistance factors with both force-based and displacement-based checking procedures.

In this project a procedure was established to appropriately account for the presence of a liquefied layer on the analysis of pile foundations subject to liquefaction--induced lateral spreading. For this purpose three--dimensional finite element models, considering a single pile embedded in a soil continuum, were used to compute representative py curves for various combinations of soil profile and pile diameter.  Comparison of the py curves resulting from homogenous and layered soil profiles, in which a liquefied layer was located between two unliquefied layers, was used to identify reductions in the ultimate lateral resistance and initial stiffness of the py curves representing the unliquefied soil due to the presence of the liquefied layer. These reductions were characterized in terms of a reduction factor which follows an exponential decay model.  Dimensionless parameters were proposed as a means of implementing appropriate reductions for an arbitrary soil profile and pile diameter.

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Soil liquefaction is an important seismic hazard that has caused extensive damage to buildings, bridges, dams, wharves, and lifelines in past earthquakes.  It has caused buildings to sink into the ground, bridges and dams to collapse, and buried tanks and pipelines to pop out of the ground.  It has also caused devastating landslides in both natural and man-made ground, and has destroyed major port facilities.

In current practice, the complex, transient ground motion produced by an earthquake is typically represented by a single point – its maximum value – and a measure of the duration of the motion. Such peak-amplitude-based parameters provide no information on whether liquefaction occurs early or late in an earthquake, which limits their effectiveness in predicting the effects of liquefaction.  This project is investigating the use of evolutionary measures of ground motion intensity, i.e., measures that build up with time over the duration of a ground motion, which can allow the missing dimension of time to be brought into the liquefaction hazard evaluation problem, will be investigated in this project. 

To generate data with which to identify optimum evolutionary intensity measures, a series of centrifuge tests, in which instrumented model soil profiles are subjected to in-flight shaking, are being performed using the RPI geotechnical centrifuge.  At the same time, we are performing cyclic simple shear tests using real-time, transient loading histories in Washington and in collaboration with the University of Texas.  The data from these tests are being analyzed and interpreted to compare the relative abilities of old and new intensity measures to predict liquefaction.

Below: RPI geotechnical centrifuge

Worldwide, seismically induced rock-slope failures have been responsible for approximately 30% of the most significant landslide catastrophes of the past century. They are among the most common, dangerous, and still least understood of all seismic hazards. Rock-slope failures differ fundamentally in two key respects from landslides in unconsolidated, soil materials:

• rock-slope stability is controlled principally by discontinuities in the rock mass
• owing to their potentially large volumes, high velocities, long travel distances, and impact forces, the consequences of rock-slope failures can be severe.

While earthquake-induced soil landslides have been well studied, there is little fundamental research on the more common and often more significant problem of rock-slope stability under seismic conditions. As a result, the current state-of-the-practice for assessing the seismic stability of rock-slopes lags behind that of soil slopes and typically involves either qualitative assessments (i.e., relative hazard assessment using descriptive parameters) or highly simplified quantitative analyses (i.e., pseudostatic methods). Neither of these approaches captures the key mechanisms driving rock-slope failure, or the consequences of failure.

This research seeks to advance the fundamental understanding of the rock-slope failure process under seismic conditions through a fully integrated program of physical and discrete element method numerical simulations. The resulting improved knowledge will drive the development of improved rock-slope failure assessment guidelines, analysis procedures, and predictive tools. In addition to markedly improving the basic understanding of seismic rock-slope failures, this research will drive the shift from currently employed qualitative assessment procedures to modern performance- and risk-based methodologies.

Topographic effects refer to the modification and amplification of seismic ground motion in the vicinity of topographic features such as hillsides, ridges, and canyons. Although it is widely recognized that topographic amplification can elevate seismic risk, there is currently no consensus on how to reliably quantify its effects, which has precluded development of acceptable guidelines on how to account for this phenomenon in practice, thus leaving an important factor contributing to seismic hazard unaccounted for in routine design.

This research integrates knowledge about topographic effects gained from:

• centrifuge model testing (using the NEES geotechnical centrifuge at the University of California, Davis) of topographic features
• field data acquired with temporary, locally-dense instrumentation arrays recording frequent and predictable stress-induced mining seismicity in a mountainous region of Utah
• rigorous numerical modeling studies
• statistical analyses of the Next Generation Attenuation strong ground motion data base

It is envisioned that this work will result in:

• an order-of-magnitude increase in the amount of high quality data on topographic amplification
• greater fundamental understanding of this phenomenon
• quantification of topographic effects on ground motions
• improved attenuation relationships that account for topographic amplification
• widely adopted guidelines and provisions to account for this seismic hazard in practice