Alejandro Martinez

Experimental Study of Shear Zones Formed at Sand/Steel Interfaces in Axial and Torsional Axisymmetric Tests

The interface shear behavior of granular materials is central to many engineering applications, including the performance of structures like deep foundations, landfills, and retaining walls. Consequently, it is paramount to understand the behavior of construction material-soil interfaces involved in these applications. Furthermore, it has been shown that the study of interface behavior, in the laboratory and in-situ, can provide robust information about the soils properties and engineering performance. This paper presented laboratory evaluations of micro and meso-scale shear deformation of medium-sized sands aimed at developing an improved fundamental understanding of granular-continuum stress-strain behavior. A comparison of interface testing results from two different shear directions—axial and torsional—demonstrated that the evolution and progression of shear zone formation was affected differently by changes in the interface surface roughness and particle angularity. In particular, it was observed that torsional shear is a more dilative process that induces a larger degree of soil shearing and is greatly affected by particle angularity. Studies of shear-induced volume changes also revealed that the influence zone for torsional shearing is larger than that for axial shearing, with soil dilation occurring inside the shear zone in contact with the material counterface and soil contraction in a surrounding outer zone. Fundamental micromechanical processes that aim to explain the differences between the behavior of axial and torsional tests are proposed.

Axisymmetric Shearing of Sand-Steel Interfaces Under Axial and Torsional Loading

Interfaces of soils with man-made materials play a major role in geotechnical systems, from deep and shallow foundations to landfills and retaining walls. Accordingly, it is of paramount importance to understand the behavior of interfaces involving different construction and natural materials. For soil-man made material surface interfaces, a linear increase in strength occurs with increasing surface roughness until the interface shear strength reaches that of the soil. Any subsequent increases in surface roughness result in no additional change of observed system interface strength. In this paper a series of laboratory axisymmetric interface shear tests involving axial and torsional loading are presented. This study involves interface shear tests between Cone Penetration Test (CPT) friction sleeves with different roughnesses and different sands. CPT sleeves with surface roughness ranging from that of the conventional smooth sleeve to those with texture elements of 1 mm in height are tested with sub-rounded and sub-angular medium sized sands. This laboratory study is part of an effort to improve soil characterization and pile design by means of the CPT that utilizes multi-sleeve modules. The next generation system expands the use of friction sleeves with different roughnesses to include response measurement under both axial and torsional loading.

Advances in Development of Axial-Torsional Multi-Sleeve Penetrometer for Extra-Terrestrial Studies

Recent advances in the development of a subsurface characterization device that combines axial and torsional loadings are presented in this paper. The multisleeve penetration system uses interchangeable friction sleeves with a controlled surface roughness that provides unique information regarding geotechnical properties such as shear and interface behavior of granular media. Experimental laboratory tests performed in an axisymmetric device allow for both axial and torsional loading modes to be investigated without the disturbance effects of the penetration process. Studies of the global behavior in both axial and torsional shearing directions allow for direct comparison and fundamentally robust interpretation of the results. Furthermore, micro-scale studies provide useful information regarding the interface shear behavior of the granular media during testing. The results show the potential of the axialtorsional multi-sleeve device to provide information about soil and regolith properties such as shear strength, structure and state of stress as well as practical issues such as ease of excavation and mobility of vehicles. Discrete Element Method simulations provide further insight about the micro-mechanical processes involved in both axial and torsional shearing tests.

Axial-Torsional Multi-Sleeve Friction Penetration System for Lunar Subsurface Studies

The multi-sleeve friction penetration system is an in-situ testing device that is derived from the well established cone penetration test. It incorporates a series of friction sleeves with varying surface texture in addition to the standard smooth friction sleeve located directly beneath the tip. The multiple measurements made with this device allow it to provide new insight into soil type and stratigraphic variations as well as insitu shear strengths as a function of sleeve texture height. This paper describes a next generation version of this device that incorporates torsional load sensing capabilities in addition to the standard axial load sensing capabilities. In this manner, the effects of different vertical and horizontal stress states on measured sleeve stresses can be explored. This device offers significant benefits as an alternative to the portable bevameter, which was used in the past to measure the mechanical response of lunar soils under normal and shear loading conditions that were likely to be imposed by various wheel and track systems.

Hypoplastic Simulation of Axisymmetric Interface Shear Tests in Granular Media

Modelling the soil-structure interface behaviour is an important but often neglected part of a holistic geotechnical modelling approach. The Mohr-Coulomb model, which is often used, is in many cases not a suitable choice to represent the complex phenomena taking place in the localized interface shear zone. Recently developed interface models formulated into the hypoplastic framework have demonstrated their ability to model the barotropy and pyknotropy in the interface shear zone. It is especially important for the models to consider the dependence of the interface shear zone on the initial void ratio to correctly simulate the changes in normal stress around cylindrical inclusions. This paper presents the results of a number of axisymmetric interface shear simulations using a hypoplastic interface and soil model. The simulation results are compared to experimental results obtained from axisymmetric interface shear tests performed between a medium dense sand and surfaces of varying roughness. The surfaces used in these experiments consisted of sheets of sandpaper glued on metal sleeves, and had average surface roughness values between 0.001 and 0.290 mm. The experimental and modelling results are compared considering the stress-deformation responses. Based on this assessment, recommendations are provided regarding the use of the hypoplastic interface shear models.

Interface Shear Response of JSC-1A, GRC-3, and JSC-Mars1 Regolith Simulants

AbstractThe response of soil–structure interfaces is of primary importance for the construction and design of infrastructure because all loads, whether static or dynamic, are transferred to the gro...