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Title: Stresses and shearing resistance in soil beneath a rigid wheel
Authors: Al-Hussaini, Mosaid M.
Gilbert, P. A.
Keywords: Computer applications
Soil-wheel interaction
Rigid wheels
Vehicle mobility
Shear properties
Soil stresses
Publisher: Soils and Pavements Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Technical Report
Abstract: The problem of vehicle mobility is a complex one in that a rigorous analytical treatment of the mechanics involved between vehicle characteristics, such as geometry, size, and driving forces, and the properties and responses of soil that supports a moving vehicle has not been developed. In general, immobilization problems occur as a result of the loss of or excessive demand for traction created by a combination of sinkage, slope, forces applied, obstacles, and environmental conditions. Immobilization problems associated with poor traction due to weak soils or slippery surfaces are not limited to rigid or pneumatic-tired wheels, but also affect powered track wheels with grousers. Two approaches have been followed in attempting to solve the soil-wheel interaction problem: an analytical approach based on the elastic solution of a plane strain problem, and an experimental approach based on the relationship between the shear and normal stresses that may occur within the vicinity of or at the interface of the soil and the wheel of a moving vehicle. The analytical solution is based on the assumption that the stresses within the soil are the results of the tangential and radial stresses created by a wheel partially embedded in soil with an impending rotation. The Airy stress function was used in representing the stresses within the soil in terms of analytic functions. The Schwarz-Christoffel equation was used to transform the geometry and the boundary condition of the region beneath the wheel and to match them with the stress functions. The Cauchy integral equation was applied on the transformed boundary conditions to obtain the shear and normal stress at any point within the region of the soil-wheel system. A computer program for reducing the results and obtaining numerical values of the stresses at any point within the vicinity of the wheel was also written. It is believed that the analytical solution developed will permit the evaluation of stresses within the soil beneath a wheel that result from various combinations of radial and tangential stresses. The experimental investigation was designed to investigate the shear stresses and traction forces that may exist between a model rigid wheel or tire wheel and the supporting soil. CH material (Vicksburg buckshot clay) was compacted and tested in an annular torsion shear machine. Four types of specimens were tested in this study: the first series of tests consisted of shearing homogeneous soil specimens; in the second series, the soil was sheared against smooth rubber; in the third series, the soil was sheared against polished stainless steel; and in the fourth series, a nonhomogeneous soil specimen, with upper and lower halves having different water contents, was sheared. The initial normal stresses used in the test program were 5, 15, and 30 psi, and all specimens were sheared under constant-volume conditions at rates of shear deformations of 0.002, 0.2, and 2.0 in./min. Test results showed that nonhomogeneous soil specimens with upper and lower halves having water contents of 26 and 16 percent, respectively, were stronger than homogenous soil specimens with a uniform water content of 26 percent and that the strengths of both types of soil specimens were higher than those at the interface of smooth rubber or polished stainless steel and soil. Test results showed that the strength of nonhomogeneous soil specimens was higher than that of homogeneous soil specimens with uniform water content, and the strengths of both types of soil specimens were higher than those at the interface of smooth rubber or polished stainless steel on soil. The results also showed that the shear resistance developed between soil and rubber is much higher than that developed between soil and polished steel. The shear stress at failure for all test series showed an increase with increasing initial normal stress and increasing rate of shear deformation. The shear stresses at the interfaces of smooth rubber and soil and also of polished steel and soil dropped sharply when the plane of contact was covered with a film of water. The peak shear stress obtained from each test series was plotted as a function of the corresponding normal stress in order to facilitate its use in analytical solutions.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Report

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