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Unformatted text preview: 1 SETTLEMENT IN CLAY SUMMARY Strains due Surface Load: Surface loads cause a change in the stresses within the soil mass and induce two types of soil strain. Linear Elastic, Homogeneous, Isotropic: E = Young's modulus Poisson's ratio, x y al longitudin lateral = = Normal Strain, ( ) ) ( E 1 z y x x + - = Volumetric Strain, ( ) z y x v + + = Shear Strain, G xy xy = Bulk Stress, + + = 3 z y x Shear Modulus, ) 1 ( 2 E G + = Bulk Modulus, ) 2 1 ( 3 E K v - = = Note that for Saturated Clay = 0.5 and K = (water is incompressible) Settlement: A clay layer under a surface load may "settle" (strain) significantly. This settlement under load has three basic components. "Immediate settlement" occurs rapidly and is distortional - due to rotational strains. The settlement component with the largest magnitude, the "consolidation settlement" , is due to volume change and may continue for many years because the pore water must be squeezed from the soil. After the primary phase of consolidation is complete, a long term creep or "secondary consolidation" may continue for hundreds of years. In general, the total settlement is given by; = i + c + s where = total settlement i = immediate settlement c = primary consolidation settlement s = secondary consolidation (creep) (immediate settlement, i , is sometimes referred to as distortional settlement, d .) yz x y z xy zx x y z x y z zx xz xy yx yz zy x y z 3-D soil element with & normal ( ) & shear ( ) stress Normal strain ( ) due to normal stress Rotational strain ( ) due to shear stress s C Time i 2 Immediate Settlement ( i ): This settlement is due to rotational strain (distortion) within the soil not a change in volume. (Note that if no shear strain, say a blanket load, then no immediate settlement.) Immediate settlement is typically much smaller in magnitude than c and occurs rapidly, i.e. during construction. It is affected by: 1. footing rigidity 2. footing shape 3. location beneath footing 4. soil stiffness parameters E and 5. soil layering 6. presence of rigid layer (rock) o Compute I Using Theory Of Elasticity Superposition is valid: For relatively uniform clay strata E and can be assumed constant and, for sufficiently low stress levels, elastic theory gives satisfactory estimates of i . o Uniformly Loaded Flexible Footing A flexible loaded area deforms into a dish shape, producing a uniform reaction (e.g. a tank). The settlement, i , under the corner of a flexible rectangular loading is one half of the settlement at the center....
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- Fall '08