Tag: Soil Tests

California Bearing Ratio (CBR) Test on Soils

California Bearing Ratio (CBR) Test on Soils

California Bearing Ratio (C.B.R.) Test which is an ad hoc penetration test developed by the California State Highways Dept. (USA) for the evaluation of sub-grade strengths. It is a measure of the shearing resistance of a soil to penetration under controlled density and moisture conditions. The strength of a soil is found by causing a Plunger (or piston) of standard size to penetrate a specimen of the soil prepared to the density and moisture Conditions of the soil to be tested in a standard mould. The resistance to penetrations measured and then expressed as a percentage of the known resistance to penetration of the plunger in a crushed aggregate. The California Bearing Ratio Test can be made on nearly all soils ranging from clay to fine sand. This test is generally used for the design of road pavements.

California Bearing Ratio Test

Vane Test Procedure – Shear Strength of Soils

Vane Test Procedure – Shear Strength of Soils

The Vane test is a field shear test for clays in which a vane consisting of two or four blades fixed at right angles, is attached to the end of rod and pushed into the soil at the bottom of a borehole. The torque required to cause rotation or shear the soil is measured. This torque is approximately equal to the moment developed by the shear strength of the clay acting over the surface of the cylinder with a radius and height equal to that of the vanes. The vane test has the advantage over the unconfined compression test in that the shear strength of a soft and sensitive clay at considerable depths (say up to about 30 m) can be determined insitu without obtaining undisturbed samples. This test is still in the process of development.

Triaxial Compression Test of Soils Procedure

Triaxial Compression Test of Soils Procedure

Triaxial Compression test is used where more precise values of the cohesion and angle of internal friction of a soil are required than determined from a shear box test. The specimen of the soil is subjected to three compressive stresses at tight angles to one another, and one of these stresses is increased until the specimen fails in shear. The test differs from the shear box test in that the stresses determine the plane of shear failure which is not predetermined.

Triaxial Compression Test

In this test a cylindrical specimen usually 40 mm dia. and 75 mm long is enclosed in a thin rubber membrane and is subjected to radial fluid (water or glycerine) pressure. Increasing axial stress is applied at the top until failure occurs. The test is repeated with different pressures and the results are plotted in the form of Mohr’s circles. The triaxial apparatus is probably the most useful for research into the fundamental properties covering the strength of soils but is elaborate.

The undrained triaxial test is, in general, used as a basis for estimating bearing capacity, earth pressure and slope stability of cohesive soils. Unconfined compression test is used for predominantly clayey soils which are saturated or nearly saturated.