Normally Consolidated and Overconsolidated Clays

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A clay is said to be normally consolidated if the present effective overburden pressure pQ is the
maximum pressure to which the layer has ever been subjected at any time in its history, whereas a
clay layer is said to be overconsolidated if the layer was subjected at one time in its history to a
greater effective overburden pressure, /?c, than the present pressure, pQ. The ratio pc I pQ is called the
overconsolidation ratio (OCR).
Overconsolidation of a clay stratum may have been caused due to some of the following factors

  1. Weight of an overburden of soil which has eroded
  2. Weight of a continental ice sheet that melted
  3. Desiccation of layers close to the surface.
    Experience indicates that the natural moisture content, wn, is commonly close to the liquid
    limit, vv;, for normally consolidated clay soil whereas for the overconsolidated clay, wn is close to
    plastic limit w .
    Fig. 7.7 illustrates schematically the difference between a normally consolidated clay strata
    such as B on the left side of Section CC and the overconsolidated portion of the same layer B on the
    right side of section CC. Layer A is overconsolidated due to desiccation.
    All of the strata located above bed rock were deposited in a lake at a time when the water level
    was located above the level of the present high ground when parts of the strata were removed by
    erosion, the water content in the clay stratum B on the right hand side of section CC increased
    slightly, whereas that of the left side of section CC decreased considerably because of the lowering
    of the water table level from position DQDQ to DD. Nevertheless, with respect to the present
    overburden, the clay stratum B on the right hand side of section CC is overconsolidated clay, and
    that on the left hand side is normally consolidated clay.
    While the water table descended from its original to its final position below the floor of the
    eroded valley, the sand strata above and below the clay layer A became drained. As a consequence,
    layer A gradually dried out due to exposure to outside heat. Layer A is therefore said to be
    overconsolidated by desiccation.
Normally Consolidated and Overconsolidated Clays 1

Diagram illustrating the geological process leading to overconsolidation of clays (After Terzaghi and Peck, 1967)

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