SMALL-DIAMETER TDR CABLES FOR MEASURING DISPLACEMENT IN PHYSICAL SOIL MODELS

This paper describes the selection and testing of small-diameter cables for detecting and measuring, in real-time, the development of internal shear zones within laboratory and centrifuge soil models. Visual methods are normally used for measuring displacements in centrifuge models. Placement of Plexiglas sides allows observation of displacements at the end(s) by means of spherical markers placed in the soil, grids marked on the Plexiglas, layers of colored sand or similar means. Another method utilizes dry spaghetti as displacement markers; the spaghetti softens from moisture in the soil and deforms with the displacement pattern of the soil. The advantages of using TDR in physical soil models are that the data is provided in real-time and continuous readings are possible because data acquisition is rapid. Eight braided coaxial cables have been selected for evaluation in a modified direct shear device. Cable diameters range from 1.5 to 2.7 mm, which are significantly smaller than cables normally employed for field measurements. Specialized blocks have been manufactured to fully confine each cable in the shear box, while creating a shear zone of a known, controllable thickness. During direct shearing, voltage reflection and shear load is measured as a function of shear displacement. The most effective cables will have a low stiffness and generate high voltage reflections at small shear displacements.

C.E. Pierce
Assistant Professor, Department of Civil and Environmental Engineering
University of South Carolina
Columbia, South Carolina

C.M. McAlister
Graduate Research Assistant, Department of Civil and Environmental Engineering
University of South Carolina
Columbia, South Carolina