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Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

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Chapter 4 LABORATORY TESTING FOR SOILS<br />

the box, the upper half being restrained by a proving ring or load cell which is used to record the<br />

shear load. The sample is not sealed in the shear box; it is free to drain from its top <strong>and</strong> bottom<br />

surfaces at all times.<br />

The cross-sectional area over which the specimen is sheared is assumed to remain constant during<br />

the test.<br />

The direct shear test has been used to carry out undrained <strong>and</strong> drained shear tests, <strong>and</strong> to<br />

determine residual strength parameters. Morgenstern <strong>and</strong> Tchalenko (1967) reported the results of<br />

optical measurements on clays at various stages during the direct shear test, <strong>and</strong> it is clear that at<br />

peak shear stress <strong>and</strong> beyond, failure structures (Reidels <strong>and</strong> thrust structures) are not coincident<br />

with the supposed imposed horizontal plane of failure. In addition, the restraints of the ends of the<br />

box create an even more markedly non-uniform shear surface. Since the direction of the failure<br />

planes, the magnitude <strong>and</strong> directions of principal stresses <strong>and</strong> the pore pressure are not<br />

determinable in a normal shear box experiment, its results are open to various interpretations (Hill<br />

1950), <strong>and</strong> this test is now rarely used to determine undrained or peak effective strength<br />

parameters. Triaxial tests may be performed more conveniently <strong>and</strong> with better control.<br />

March 2009 4-11

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