This award is an outcome of the NSF 09-524 program solicitation "George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research (NEESR)" competition and includes the University of California-San Diego and New Mexico Tech (subaward). This project will utilize the NEES equipment site at the University of California-San Diego. The dynamic response and performance of earth retaining structures has traditionally been measured by testing small physical models, which have yielded valuable information but represent a compromise with respect to field structures. What is needed for a better understanding of the earthquake performance of soil retaining walls is full-scale testing. The objective of the project is to perform a unique experimental investigation of the earthquake performance of full-scale (10 m) reinforced soil retaining walls constructed using realistic materials and methods. Considering that these walls will be several times taller than for any previous research, a key focus of the proposed work will be the influence of wall height on overall system response and distribution of dynamic forces in soil reinforcement. Other focus areas will be dynamic earth pressure on the back of the wall, effects of dynamic loading on load transfer mechanisms between soil and reinforcing elements, and permanent wall deformations after dynamic loading. The proposed tests will be conducted using the NEES@ UCSD large high performance outdoor shake table (LHPOST). One or more full-scale (10 m) retaining walls will be tested on the LHPOST. These walls will be constructed using realistic soil types, compaction methods, reinforcement, and facing elements. In order to conduct these full-scale tests, a unique large soil confinement box will be designed and constructed. The box will measure 6 m × 12 m × 10 m high and will closely approximate plane-strain test conditions. The rear boundary condition for the box can take three configurations: rigid back boundary, energy absorbing back boundary and a unique stress-controlled/free-displacement back boundary. In addition to retaining walls, the soil confinement box will have many additional future applications including soil levees, unreinforced and reinforced soil slopes, landfill cover systems, and bridge abutments. The intellectual merit of this project has the potential to transform the way in which reinforced soil retaining walls are designed in earthquake engineering practice. Lessons learned from this testing are expected to lead to more rational seismic design procedures, which in turn are expected to reduce the seismic risk for this important component of our national and global civil infrastructure. It is expected that, once constructed, the proposed soil confinement box will provide extensive broader impacts to the geotechnical earthquake engineering research and professional communities. At UCSD, this facility will lead to exciting educational experiences for graduate student researchers, undergraduate student research assistants, and students enrolled in UCSD courses, and provide for exciting outreach opportunities to showcase current advanced engineering research to the greater Southern California area. Data from this project will be archived and made available to the public through the NEES data repository. This award is part of the National Earthquake Hazards Reduction Program (NEHRP).
Extensive experimental research has been conducted using physical models to characterize the behavior of geotechnical (i.e., soil-type) structures, such as retaining walls and foundations, under earthquake loading. These tests have yielded valuable information but, due to size and weight limitations of shaking tables and geotechnical centrifuges, have necessarily used small-scale models. As a result, the data obtained from these tests have been compromised with respect to the behavior of actual field structures. To circumvent this difficulty, the project investigators have recently designed and constructed a Large Soil Confinement Box (LSCB) for seismic testing of full-scale geotechnical structures at the University of California-San Diego (UCSD). The LSCB is attached to the top of the Large High Performance Outdoor Shake Table (LHPOST) at UCSD's Englekirk Structural Engineering Center (Fig. 1, Fig. 2) and consists of an outer steel frame and inner walls of high strength concrete panels. The LSCB is a rigid rectangular box with an open roof that covers the footprint of the LHPOST and has interior dimensions of 10.1 m (length), 7.6 m (height), and 4.6 m or 5.8 m (width), depending on assembly configuration. The LSCB can hold large geotechnical test specimens inside and shake them back-and-forth to simulate the loading of an actual earthquake with lateral accelerations up to 0.7g. Using the LSCB, seismic performance tests have been conducted on a 6 m-tall reinforced soil retaining wall (Fig. 3) and a pair of rocking bridge foundation specimens (Fig. 4). These specimens were constructed using realistic materials and methods, including soil placed in layers and rolled using a field-scale compactor, and subjected to a range of dynamic motions and scaled earthquake records. Landmark tests such as these can be used to improve seismic design procedures and refine numerical models used to assess seismic performance for a wide variety of geotechnical structures, such as retaining walls, foundations, levees, and bridge abutments. The equipment developed for this project, the data obtained for a reinforced soil retaining wall and two rocking bridge foundations, and future research to be conducted using the same equipment, is expected to improve the earthquake safety of our national and international civil engineering infrastructure in high seismic regions, such as California.
