Unconfined - University of Missouri Columbia Civil and...

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University of Missouri – Columbia Civil and Environmental Engineering UNCONFINED COMPRESSION TEST PURPOSE The purpose of this laboratory is to determine the unconfined compressive strength of a saturated, cohesive soil sample. We will measure this with the unconfined compression test which is an unconsolidated undrained (UU or Q-type) test in which the lateral confining pressure is equal to zero (atmospheric pressure). We will also determine the undrained shear strength by three other methods: the laboratory shear vane, the torvane and the pocket penetrometer. INTRODUCTION The unconfined compression test is by far the most popular method of soil shear testing because it is one of the fastest and cheapest methods of measuring shear strength. The method is used primarily for saturated, cohesive soils recovered from thin-walled sampling tubes. The unconfined compression test is inappropriate for dry sands or crumbly clays because the materials would fall apart without some kind of lateral confinement. To perform an unconfined compression test, the sample is extruded from the sampling tube. A cylindrical sample of soil is trimmed such that the ends are reasonably smooth and the length-to- diameter ratio is on the order of two. The soil sample is placed in a loading frame on a metal plate, by turning a crank (or motor driven), the operator raises the level of the bottom plate. The top of the soil sample is restrained by the top plate, which is attached to a calibrated proving ring. As the bottom plate is raised, an axial load is applied to the sample. The operator turns the crank at a specified rate so that there is constant strain rate. The load is gradually increased to shear the sample, and readings are taken periodically of the force applied to the sample and the resulting deformation. The loading is continued until the soil develops an obvious shearing plane or the deformations become excessive. The measured data are used to determine the strength of the soil specimen and the stress-strain characteristics. Finally, the sample is oven dried to determine its water content. The maximum load per unit area is defined as the unconfined compressive strength, q u . In the unconfined compression test, drainage of the soil pores is not allowed from the soil sample either during set-up or during the shearing process. It is assumed that the water content remains constant during the entire test. Without a change in water content (or void ratio), the effective stress of the sample does not change. For tests of constant effective stress, it is appropriate to express the strength in terms of total stress. APPARATUS The loading frame consists of two metal plates. The top plate is stationary and is attached to the load-measuring device. The bottom plate is raised and lowered by an electric motor or by means of a crank on the front of the loading frame. After the soil sample has been placed between the
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University of Missouri – Columbia Civil and Environmental Engineering
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Unconfined - University of Missouri Columbia Civil and...

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