a. Preconsolidation. Stresses exceeding the present effective vertical
pressure of overburden produce preconsolidation (1) by the weight of
material that existed above the present ground surface and that has been
removed by erosion, excavation, or recession of glaciers, (2) by capillary
stresses from desiccation, and (3) by lower groundwater levels at some time
in the past.
b. Underconsolidation. Compressible strata may be incompletely
consolidated under existing loads as a result of recent lowering of
groundwater or recent addition of fills or structural loads. Residual
hydrostatic excess pore pressure existing in the compressible stratum will
dissipate with time, causing settlements.
c. Evaluation of Existing Conditions. Determine consolidation
condition at start of construction by the following steps:
(1) Review the data available on site history and geology to
estimate probable preconsolidation or underconsolidation.
(2) Compare profile of preconsolidation stress determined from
laboratory consolidation tests (Chapter 3) with the profile of effective
over-burden pressures.
(3) Estimate preconsolidation from c/P+c, ratio, where c is the
cohesion (q+u/2,) and P+c, is the preconsolidation stress, using laboratory
data from unconfined compression test and Atterberg limits (see Chapter 3).
(4) If underconsolidation is indicated, install piezometers to
measure the magnitude of hydrostatic excess pore water pressures.
d. Computation of Added Stresses. Use the elastic solutions (Chapter
4) to determine the vertical stress increment from applied loads. On
vertical lines beneath selected points in the loaded area, plot profiles
of estimated preconsolidation and effective overburden stress plus the
increment of applied stress. See Figure 3 for typical profiles. Lowering
of groundwater during construction or regional drawdown increases
effective stress at the boundaries of the compressible stratum and
initiates consolidation. Stress applied by drawdown equals the reduction
in buoyancy of overburden corresponding to decrease in boundary water
pressure. In developed locations, settlement of surrounding areas from
drawdown must be carefully evaluated before undertaking dewatering or well
pumping.
Section 3.
INSTANTANEOUS SETTLEMENT
1. IMMEDIATE SETTLEMENT OF FINE-GRAINED SOILS. Generally, the
instantaneous settlement results from elastic compression of clayey soil.
For foundations on unsaturated clay or highly overconsolidated clay, the
elastic settlement constitutes a significant portion of the total
settlement.
7.1-209
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