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When a saturated clay-water system is subjected to an external pressure, the pressure applied is
initially taken by the water in the pores resulting thereby in an excess pore water pressure. If
drainage is permitted, the resulting hydraulic gradients initiate a flow of water out of the clay mass
and the mass begins to compress. A portion of the applied stress is transferred to the soil skeleton,
which in turn causes a reduction in the excess pore pressure. This process, involving a gradual
compression occurring simultaneously with a flow of water out of the mass and with a gradual
transfer of the applied pressure from the pore water to the mineral skeleton is called consolidation.
The process opposite to consolidation is called swelling, which involves an increase in the water
content due to an increase in the volume of the voids.

Consolidation may be due to one or more of the following factors:

  1. External static loads from structures.
  2. Self-weight of the soil such as recently placed fills.
  3. Lowering of the ground water table.
  4. Desiccation.
    The total compression of a saturated clay strata under excess effective pressure may be
    considered as the sum of
  5. Immediate compression,
  6. Primary consolidation, and
  7. Secondary compression.
    The portion of the settlement of a structure which occurs more or less simultaneously with the
    applied loads is referred to as the initial or immediate settlement. This settlement is due to the
    immediate compression of the soil layer under undrained condition and is calculated by assuming
    the soil mass to behave as an elastic soil.
    If the rate of compression of the soil layer is controlled solely by the resistance of the flow of
    water under the induced hydraulic gradients, the process is referred to as primary consolidation.
    The portion of the settlement that is due to the primary consolidation is called primary
    consolidation settlement or compression. At the present time the only theory of practical value for
    estimating time-dependent settlement due to volume changes, that is under primary consolidation
    is the one-dimensional theory.
    The third part of the settlement is due to secondary consolidation or compression of the clay
    layer. This compression is supposed to start after the primary consolidation ceases, that is after the
    excess pore water pressure approaches zero. It is often assumed that secondary compression
    proceeds linearly with the logarithm of time. However, a satisfactory treatment of this phenomenon
    has not been formulated for computing settlement under this category.

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