ECE 2311_Shear Strength.pdf - ECE 2311 Soil Mechanics II...

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ECE 2311: Soil Mechanics II Shear Strength and Failure i TABLE OF CONTENTS 2 SHEAR STRENGTH AND FAILURE ....................................................................... 1 2.1 Mohr Coulomb ................................................................................................... 1 2.2 Shear tests .......................................................................................................... 5 2.2.1 Direct shear test .............................................................................................. 5 2.2.2 Simple shear test ............................................................................................. 6 2.3 Triaxial test ........................................................................................................ 8 2.4 Water stresses ................................................................................................... 10 2.4.1 Elastic response ............................................................................................ 12 2.4.2 The influence of dilatancy ............................................................................. 14 2.5 Undrained tests ................................................................................................. 16 2.6 Stress paths ....................................................................................................... 18 2.6.1 Stress parameters .......................................................................................... 18 2.6.2 Triaxial test ................................................................................................... 20
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ECE 2311: Soil Mechanics II Shear Strength and Failure 1 2 SHEAR STRENGTH AND FAILURE One of the characteristic properties of soils is the fact that increasing shear stresses induce progressively increasing angular deformations, until total failure occurs. An example of this type of failure is the landslide, where a huge portion of soil slides down over the supporting soil or bedrock. This type of failure often occurs after heavy rainfall, building up water pressures and reducing the effective stresses. Shear failure of course also may occur in artificial structures such as dikes and embankments. This chapter focuses on the determination of the shear stresses under failure. Further attention will be paid to some laboratory tests for the determination of the failure parameters, the influence of the water pressure and dilatancy. Finally, the concept of stress paths will be discussed. 2.1 Mohr Coulomb It is plausible to assume that failure occurs if on a certain plane in the soil the shear stress becomes too large. This results in sliding of soil along this plane. No sliding occurs along any other plane, since the resistance of the soil against sliding on those planes is still sufficient. Coulomb used the analogy of a sliding rigid block on a smooth base in order to find the following critical shear stress: tan f c (2.1) where  is the effective normal stress on the plane, c the cohesion and the angle of internal friction . Now it is assumed that when on a certain plane the shear stress is smaller that f (the subscript f means failure) the deformations are very small and elastic, or even can be neglected. However, when the critical value f on a plane is reached the deformations are unlimited. It should be clear that the stresses in (2.1) are effective stresses representing the  1  xx  1  3  3  yx  xy  yy 3 1 y x Fig. 2.1: Stresses on arbitrary perpendicular planes.
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ECE 2311: Soil Mechanics II Shear Strength and Failure 2 stresses transmitted through the soil skeleton. Therefore, many authors write c and  to emphasise that these quantities relate to effective stresses. From solid mechanics, it is known that the stresses in a certain point acting on several planes can be expressed in the principal stresses and the angle between the plane and the principal directions. The stresses on two planes with normal vectors in x- and y- direction, making an angle with respectively the largest and smallest principal stress direction (Fig.
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