The shearing behavior of a soil-structure interface governs the response of many geotechnical systems, ranging from laboratory and insitu tests to piled foundations and tunnel jacking. Our group, through collaboration with other researchers, has examined a number of aspects related to the behavior, evolution, and characterizations of soil-structure interfaces. Topics examined include:
The latter topic has been examined in-depth through collaboration with David White (Cambridge University) and Prof. Mark Randolph (Univ. of Western Australia) and is presented below. Additional information on the topics above is available through our publications.
The shaft resistance of piled foundations is known to degrade with cyclic loading, although the governing mechanism is not well understood. High profile and costly failures of offshore foundations have arisen from unexpected substantial interface friction degradation. Cyclic degradation in sands is attributed to a decrease in the normal stress due to cumulative contraction of the soil within the shear zone contacting the interface with cycling. These characteristics can be replicated in the laboratory through cyclic interface direct shear tests with a constant normal stiffness (CNS) confinement condition wherein the volumetric change within the interface shear zone is coupled to the far-field response.
Using a custom interface shear device monotonic and cyclic interface shear tests were performed. Conventional external measurements of specimen deformation were augmented by particle image velocimetry (PIV) measurements of internal specimen deformation, as viewed through a transparent window. PIV is a velocity-measuring technique that has been recently adapted for planar soil deformation measurements (White et al. 2001) and provided displacement measurement resolution of ± 0.5 mm.
The results from the monotonic test based on global measurements are shown below.
The video of complied images can be view clicking the following link (link here to video - with warning of file size). Analysis with PIV enables quantification of the deformations within the shear zone. As an example, the cumulative vertical and horizontal displacement profiles as a function of distance from the interface can be determined.
Examination of these and similar figures enables determination of shear band evolution and thickness, local shear and volumetric strains, dilation rates, etc.
Substantial insights can be gained into cyclic interface shear behavior as well. The results from a cyclic test on uncemented silica sand (medium density, cycling +/- 1mm) based on global measurements are shown below.
The compiled digital image videos for this and other tests can be viewed by clicking on the following links.
Analysis with PIV enables evaluation of shear band evolution and thickness, local shear and volumetric strains, dilation rates, etc. within each individual cycles as well as cumulatively throughout 45 cycles. Examples of reduced data, again for uncemented silica sand (displacement control +/- 1 mm) are shown below.
Revelation of the uniformity and evolution of the interface shear zone has enable extension of the critical state concept to the modeling of behavior within the interface shear zone. The continuous changes in void ratio within the shear band can be tracked given knowledge of the initial void ration and volumetric changes within the shear band as determined by PIV. The normal stress is monitored through the external vertical load cell. Based on these measurements, a simple model for this contraction is developed and linked to the decay in normal stress and hence the limiting loss of interface friction. The critical state concept as applied to interface shear behavior is conceptually presented below, followed by the framework calibrated against test results for uncemented silica sand.
The results presented above provide a brief overview of our recent activities in this area.Ongoing research is focusing on the effect of initial specimen density, particle shape, mineralogy, particle breakage, cycling magnitude, initial normal stress, and stiffness on the interface shear behavior. Efforts include further validation and calibration of the interface critical state concept against these parameter variations.
More Information? Jason T. DeJong