Soil bearing tests using a spherical penetration device HR-117, 1967

(1967) Soil bearing tests using a spherical penetration device HR-117, 1967. Transportation, Department of

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Abstract

The behavior of soil under stress is generally more complex than, for example metals; the latter are homogeneous and have a better defined elastic-plastic boundary, whereas soil beneath a foundation is seldom homogeneous, large variations in strength often occurring in both the vertical and horizontal directions. Soil properties also vary due to differences in moisture content, bulk density, internal structure, and the way in which stress is applied. In any soil engineering problem the most important task is the determination of strength properties of soil. Three widely used methods are shear tests, bearing tests and penetration tests. Usually these tests are for specific design purposes. Shear tests include the unconfined compression test, shear box tests and triaxial tests. These tests are made on comparatively small samples in the laboratory to determine the ultimate bearing capacity of the soil mass, stability of embankments and cuts, earth pressures on retaining walls and sometimes thickness of pavements. The usual objective of shear tests is to determine cohesion and angle of internal friction under loading and drainage conditions similar to those that will occur in the soil mass. These constants are then used in conjunction with theories of stress distribution or theory of plastic failure in the soil mass. Because of hte small sample size, these tests in effect determine the strength properties only at a point in the soil mass. A number of tests on samples from different points must be made in order to obtain an overall evaluation of the strength of the soil mass. An alternative short-cut method is to utilize rapid field tests for design and quality control, even though basic soil parameters are not directly indicated. Plate bearing is a commonly used field test made by loading on the surface of the soil mass. Circular plates of various sizes, of the order of several square feet, are loaded in increments, much as occurs during construction. Each applied load causes deflection which is partly elastic and partly due to the compression of the soil. A perimeter shear factor is taken into account by using several different sizes of plates with varying perimeter/area ratios. Failure surfaces are shown in Figure 1. The principal uses of bearing tests are to determine stresses within concrete pavements by means of Westergaard's analysis, to design pavements taking into account subrade and subbase strength, to test the stability of existing road and airfield pavements, to predict settlement and bearing capacity under foundations, and to determine the elastic or deformation properties of soils in situ.