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Lecture 6

# Lecture 6 - 15 Consolidation Settlement Time Recall that...

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ρ 15 Consolidation Settlement: Recall that the settlement of cohesive soil under load is mostly due to consolidation: ρ = ρ i + ρ c + ρ s where ρ = total settlement ρ i = immediate settlement ρ c = (primary) consolidation settlement ρ s = secondary compression (creep) Primary Consolidation: o soil is a three phase system (soil-water-air) o cohesive soils (clays) have a low fluid conductivity o water is essentially incompressible o an external load applied to a (saturated) clay is initially supported by an increase in water pressure ( Δ u > 0) without volume change ( Δ e = 0) o as the excess water pressure seeps out, volumetric and structural changes occur o consolidation is therefore time-dependent o if the soil has previously experienced a stress greater than the applied load (i.e Overconsolidation Ratio = p C / σ ' 0 > 1), the volume change will be reduced o Fluid-Filled Cylinder with Spring Analogy: P = P SPRING + P WATER Δ u = P/A (B) Load Applied P SPRING = 0 P WATER = P Δ u = 0 (A) Equilibrium P SPRING = 0 P WATER = 0 (D) Full Consolidation P SPRING = P P WATER = 0 Δ u = 0 ρ Time Δ u < P/A (C) Partial Consolidation P > P SPRING > 0 P > P WATER > 0

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ρ 16 o Graphical presentation of Fluid-Filled Cylinder with Spring Analogy: o When a load is applied to a saturated clay soil, it is initially carried by the pore water as an excess pore water pressure (over hydrostatic), Δ u. The applied load, due to a foundation, increases the pore pressure already there. o With time, water will move out of the clay layer to the surface, or to some drainage boundary. The rate of this movement is primarily a function of the soil permeability and the layer thickness.
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