Project:  Time Dependent Analysis and Behavior of Geosynthetic-Reinforced Structures

PI(s):  Robert D. Holtz, Pedro Arduino, and Kuen Lin

Sponsor:  NSF

Project Description   The objectives of the proposed research are to (1) develop a new special "bi-material" finite element to model a GRS interface region; (2) develop procedures for testing geosynthetic reinforced lower quality soils subject to creep movement; develop appropriate testing protocols for determination of soil and geosynthetic properties as well as composite parameters required for design; and (3) modify existing GRS wall and slope design procedures to appropriately consider lower quality backfill materials, foundation and wall/slope movements, drainage, and other long-term wall and slope performance issues.

The research will utilize both analytical and experimental methods to achieve these objectives. The analytical work includes the development of an accurate and efficient special "bi-material" finite element to model a GRS interface region. Within the "bi-material" element, the integral forms of nonlinear viscoelastic constitutive relations will be used for both soils and reinforcing materials. Appropriate stress and displacement conditions along the interface between the soil and the reinforcement will be imposed. Variational theorems in visco-plasticity will be used to derive element equations for the special bi-material GRS element. The special element will be integrated into an existing finite element code. Parametric studies will be conducted to investigate the effect of soil and geosynthetic material properties and geometry on the performance of GRS structures. Simple design criteria will be established based on the analytical and experimental studies.

The experimental work will primarily utilize the Unit Cell Device developed by Boyle (1995) for obtaining the in-soil modulus of GRS at small strains. The device is unique in that it relies on the soil to cause the load to develop in the geosynthetic reinforcement, and this induced load is directly measured. The UCD will be utilized to validate the special finite element derived in the analytical part of this research. Up to five typical reinforcing geosynthetics and three compacted cohesive soils will be subjected to sustained loading. A limited study of stress relaxation as well as the effective stress conditions at the soil-reinforcement interface will also be made.

Although the research as proposed is interesting from a fundamental soil-geosynthetic composite behavior point of view, for the results to be accepted by designers and regulators, the entire GRS wall-slope system must be considered. Design procedures must be modified to effectively consider lower quality backfill materials, their soil, geosynthetic, and soil-geosynthetic interaction properties, drainage, creep, movements of the foundation and the face of the wall or slope, and other long-term design and performance issues. Design at working stresses for such systems should be approached from a limit state viewpoint, which is a similar approach to LRFD being adopted by many structural codes.

Potential impact on advancing knowledge with the successful completion of this research can be very large in terms of its positive influence on GRS wall and slope construction costs. The results will be directly implemented in our graduate-level courses and short courses we currently teach on a regular basis. The project will also contribute to the education of three graduate students in Civil and Environmental Engineering and in Aeronautics and Astronautics. The involvement in interdisciplinary research will be of great benefit to their professional development.

For more information send e-Mail to: holtz@u.washington.edu