MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094
Seismic Vulnerability of the Highway System

Task C2-4: Pile Group Foundation Design for Liquefaction-Induced Lateral Displacement - Centrifuge and Analytical Studies

Subject Area: Special Bridges - Ground Motion & Geotechnical Studies
 Research Year 3

Ricardo Dobry, Rensselaer Polytechnic Institute
Geoffrey R. Martin, University of Southern California

Objective

The primary objective of this task is to develop design guidelines for large pile group foundations susceptible to liquefaction-induced lateral displacement. This will be done through centrifuge model tests, supplemented by analytical studies, intended to simulate lateral spread loading of closely spaced pile groups. The Japanese Road Association (JRA) has published a limit equilibrium approach to lateral loading, based on limited Kobe earthquake case history data. This approach will be re-evaluated based on a series of 12 centrifuge tests, and re-evaluation of the Japanese case history data and JRA procedures.

This two-year centrifuge-based study has three specific objectives: (1) to establish a database of centrifuge results directly useful to calibrate the parameters needed for p-y and limit equilibrium analysis of pile groups exposed to both liquefied and nonliquefied soil pressures, thus supplementing the corresponding information already available from previous RPI centrifuge tests for single piles; (2) to extract guidelines for limit equilibrium and p-y analyses of pile groups from these centrifuge tests; and (3) to reevaluate the JRA limit equilibrium procedure for pile groups, on the basis of (1) and (2), plus reevaluation of the original 1995 Kobe case histories and analyses used to develop this procedure.

Approach

In Research Year 3, the work will consist mainly of conducting the first six centrifuge model experiments, as shown in the following table, and initiating corresponding interpretations and analytical modeling. The remaining centrifuge tests will be conducted in Research Year 4.

* The liquefiable sand will be densified within and around the pile group to simulate the effect of pile driving

The Year 3 tests constitute an expansion of similar centrifuge models tested in the last few years, where an instrumented pile foundation model is subjected to liquefaction and lateral spreading caused by in-flight shaking at the base of a "small" (46x25cm in plan dimensions) laminar box container. These previous tests involved mostly single end-bearing piles in either two- or three-layer soil profiles, with the bending response of the pile in two-layer profiles controlled by the lateral pressure of the liquefied layer, and the pile response in three-layer profiles controlled mainly by the lateral pressure of the shallow nonliquefied layer. The main thrust of these tests is the use of a "large" (70x35cm) laminar box that can test 2x2, 3x3 and 4x4 pile groups. The basic soil, pile, and shaking parameters will be kept the same as in previous RPI centrifuge tests. That is, the 6m-thick prototype liquefiable layer will be formed by fine Nevada sand saturated with water and having a relative density of about 40%, with total soil profile depths of 8m and 10m, end-bearing pile models of these same lengths, prototype pile diameter d = 0.6m; with the pile-soil models tested at a centrifuge acceleration of 50g, and subjected in flight to a prototype peak base acceleration of 0.3g with a shaking duration causing a permanent free field lateral ground displacement of about 0.8m.

Test 1 corresponds to a row of three piles joined by an embedded pile cap, with shaking perpendicular to the row, to study the response, equivalent liquefied soil pressure, and p-y curves of the simplest possible closely spaced pile system. Tests 2 and 3 correspond to a 2x2 square pile group in the small and large laminar boxes, respectively. Tests 3 through 8 are all square 2x2 or 3x3 pile groups in the large laminar box and covering the 2-layer and 3-layer soil profiles. In Tests 5 and 8, the 3x3 pile groups are repeated but with the liquefiable sand layer densified within the groups and around it to simulate the effect of pile driving. The pile cap will be embedded in the liquefiable layer for the case of the 2-layer profile, and it will be embedded in the shallow nonliquefiable layer for the case of the 3-layer profile.

In addition, contact will be established with the Japanese researchers who studied the case history data of the Kobe earthquake and developed the JRA lateral loading design procedure for pile groups, in preparation for a re-analysis of these data and procedures.

In Research Year 4, the work will consist of will consist of:

• Additional interpretations and analyses of the centrifuge results in order to develop guidelines for limit equilibrium and p-y lateral spreading analyses of pile groups, following methodologies developed at RPI, USC and other research centers in previous years;
• Conducting centrifuge Tests 7 and 8, plus four additional centrifuge experiments, as needed, to fill gaps in knowledge and generalize further the guidelines (these additional tests may include a repeat of some pile group tests with a viscous pore fluid; spacings other than 3d; pile groups including batter piles; rectangular pile group tests; and testing of friction rather than end-bearing pile groups);
• Reevaluation of the JRA limit equilibrium procedure for pile groups.

Technical Challenges

The primary technical challenge will be to determine how best to combine centrifuge results and case history data to formulate a design procedure for pile groups subjected to liquefaction-induced lateral ground deformation.

Products

• Report documenting the results of all centrifuge tests, including comparisons, interpretations, and analyses.
• Design guidelines on the application and use of of p-y and limit equilibrium analyses, consistent with the examined case histories and the centrifuge test results.