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L12-elastic_settlement - 1 12 SETTLEMENTS OF STRUCTURES...

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12. SETTLEMENTS OF STRUCTURES 12.1 The settlement process An important task in the design of foundations is to determine the settlement, this is shown schematically in Figure 1. Maximum Settlement Soil Layer As discussed earlier the skeletal soil material and the pore water are relatively incompressible and any change in volume can only occur due to change in the volume of the voids. For the volume of the voids to change, pore water must flow into or out of a soil element. Because this cannot happen instantaneously when a load is first applied to a soil there cannot be any immediate change in its volume. For one-dimensional conditions with no lateral strain this implies that there is no immediate vertical strain and hence that the excess pore pressure is equal to the change in vertical stress. However, under more general conditions both lateral (or horizontal) and vertical strains can occur. Immediately after load is applied there will be no change in volume, but the soil deformations will result in an initial settlement. This is said to occur under undrained conditions because no pore water has been able to drain from the soil. With time the excess pore pressures generated during the undrained loading will dissipate and further lateral and vertical strains will occur. Ultimately the settlement will reach its long term or drained value. When the load is first applied to the soil there will be a tendency for the more highly stressed parts of the soil to compress and thus for there to be a reduction in the volume of the voids. The pore water will respond to this tendency towards a decrease in volume by undergoing an increase in pore water pressure and so initial excess pore water pressures will develop. Subsequently there will be a flow of water from regions of high excess pore water pressure to regions of low excess pore water pressure, and the load induced excess pore water pressures will dissipate. This is the process of consolidation, and during this process the soil will undergo a settlement which varies with time. Ultimately after a long period of time all the excess pore water pressures will have dissipated and the settlement of the soil will cease and it will reach its long term or drained settlement (the term drained is used because all excess pore water pressures have dissipated and there will be no further drainage of water from the voids although the voids will still remain saturated). The process of consolidation is shown schematically in Figure 2. It should be stated that the process described above represents a simplification because some soils tend 1 Fig. 1 Settlement of a loaded footing
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to creep. For such soils there will be additional creep settlements even though the effective stress does not change.
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