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L15-stress_strain

# L15-stress_strain - 1 15 STRESS-STRAIN BEHAVIOUR OF SOILS...

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15. STRESS-STRAIN BEHAVIOUR OF SOILS 15.1 The behaviour of sands In practice sands are usually sheared under drained conditions because their relatively high permeability ensures that excess pore pressures are not generated. This behaviour can be investigated in a variety of laboratory apparatus. We will consider the behaviour in simple shear tests. The simple shear test is similar to the shear box test but it has the advantage that the strain and stress states are more uniform enabling us to investigate the stress-strain behaviour. The name simple shear refers to the plane strain mode of deformation shown below: τ σ dx H dz γ xz γ xz = dx/H ε z = - dz/H = ε v For this deformation there are only two non-zero strain components, these are the shear strain, γ xz = dx/H, and the normal strain ε z = dz/H. The volume strain, ε v = ε z . For sands the two most important parameters governing their behaviour are the Relative Density, I d , and the effective stress level, σ′ . The Relative density is defined by where e max and e min are the maximum and minimum void ratios that can be measured in standard tests in the laboratory, and e is the current void ratio. This expression can be re-written in terms of dry density as and hence Sand is generally referred to as dense if I d > 0.6 and loose if I d < 0.3. d I = e - e e e max max min d s w G 1 + e γ d dmin d dmin dmax I 1 1 1 1 1

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15.1.1 Influence of Relative Density The influence of relative density on the behaviour can be seen in the plots below for tests all performed at the same normal stress . τ γ γ ε v Dense (D) Medium (M) Loose (L) D M L e γ D M L The following observations can be made: All samples approach the same ultimate conditions of shear stress and void ratio, irrespective of the initial density Initially dense samples attain higher peak angles of friction ( φ′ = tan -1 ( τ / σ′ ) ) Initially dense soils expand (dilate) when sheared, and initially loose soils compress 2
15.1.2 Influence of Effective Stress Level The influence of stress level can be seen in the plots below where the two dense samples have the same initial void ratio, e 1 and similarly the loose samples both have the same initial void ratio e 2 . τ γ γ ε v D 2 L 2 D 2 L 2 e D 1 L 1 D 1 L 1 σ 1 σ 2 τ τ σ′ = tan φ′ ult CSL γ D 2 L 2 D 1 L 1 σ The following observations can be made: The ultimate values of shear stress and void ratio, depend on the stress level, but the ultimate angle of friction ( φ′ ult = tan -1 ( τ / σ′ ) ult ) is independent of both density and stress level Initially dense samples attain higher peak angles of friction ( φ′ = tan -1 ( τ / σ′ )), but the peak friction angle reduces as the stress level increases.

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L15-stress_strain - 1 15 STRESS-STRAIN BEHAVIOUR OF SOILS...

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