ECE 2311 SOIL MECHANICS II batch two_2.pdf

# ECE 2311 SOIL MECHANICS II batch two_2.pdf - ECE2311 Soil...

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ECE2311 Soil Mechanics II Mungathia©2013 Page 1 ECE 2311: SOIL MECHANICS II COHESION It is possible to make a vertical cut in silts and clays and for this cut to remain standing, unsupported, for some time. This cannot be done with dry sand which, on removal of the cutting implement, will slump until its slope is equal to an angle known as the angle of repose. In silts and clays, therefore, some other factor must contribute to shear strength. This factor is called cohesion and results from the mutual attraction existing between fine particles that tend to hold them together in a solid mass without the application of external forces. In terms of the Mohr diagram this means that the strength envelope for the soil, for undrained conditions, no longer goes through the origin but intercepts the shear stress axis (see figure below). The value of the intercept, to the same scale as n σ , gives a measure of the unit cohesion available and is given the symbol c or c u. Coulomb’s law of soil shear strength It can be seen that the shear resistant offered by a particular soil is made of the two components of friction and cohesion . Friction resistance does not have a constant value but varies with the values of normal stress action on the shear plane whereas cohesive resistance has a constant value which is independent of the value of n σ . Mohr-Coulomb Failure Criteria This theory states that a material fails because of a critical combination of normal stress and shear stress, and not from their either maximum normal or shear stress alone The Mohr–Coulomb failure criterion represents the linear envelope that is obtained from a plot of the shear strength of a material versus the applied normal stress. τ u c φ σ A cohesive soil, subjected to undrained conditions and zero total normal stress will still exhibit a shear strength, c u

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ECE2311 Soil Mechanics II Mungathia©2013 Page 2 In 1766 Coulomb suggested that the equation of the strength envelope of a soil could be expressed by straight line equation: φ σ τ tan + = c f Where f τ = Shear stress at failure i.e the shear strength c = Unit cohesion σ = Total normal stress on failure plane φ = Angle of shearing resistance The equation gave satisfactory predictions for sands and gravels , for which it was originally intended, but it was not so successful when applied to silts and clays . The reasons for this are that the drainage conditions under which the soil is operating together with the rated of the applied loading have a considerable effect on the amount of shearing resistance the soil will exhibit. None of this was appreciated in the 18 th century and this lack of understanding continued more or less until 1925 when Terzaghi published his theory of effective stress .
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