Environmental Engineering Reference
In-Depth Information
10.5.3
Predicting Collapse Potential
Preliminary Phases
Data Collection
A preliminary knowledge of the local geology from a literature review aids in anticipating
soils with a collapse potential, since they are commonly associated with loess and other
fine-grained aeolian soils, and fine-grained valley alluvium in dry climates. The suscepti-
bility of residual soil is difficult to determine from a normal literature review. Rolling ter-
rain with a moderately deep water table in a climate with a short wet season and a long
dry season should be suspect.
Landform Analysis
Loess and valley alluvium are identified by their characteristic features described in
Chapter 7. Residual soils with collapse potential may show a lack of surface drainage
channels indicating rainfall infiltration rather than runoff, especially where the soils are
known to be clayey. Unexplained collapse depressions may be present.
Explorations
Test Borings and Sampling
Drilling using continuous-flight augers (no drilling fluid) will yield substantially higher
SPT values in collapsible soils than will drilling with water, which tends to soften the soils.
A comparison of the results from both methods on a given project provides an indication
of collapse potential. Undisturbed sampling is often difficult in these materials.
Representative samples, suitable for laboratory testing, were obtained with the Denison
core barrel in the Negev Desert of Israel (Figure 2.67).
In residual soils, a “soft” upper zone with low SPT values will be encountered when
drilling fluids are used, often underlain by a hard zone or crust which may contain
limonite nodules or concretions.
Test pits are useful for close examination and description of the soils in the undis-
turbed state, in situ natural density tests, and the recovery of block samples for laboratory
testing.
Simple Hand Test
A hand-size block of the soil is broken into two pieces and each is trimmed until the
volumes are equal. One is wetted and molded in the hand and the two volumes then
compared. If the wetted volume is obviously smaller, then collapsibility may be suspected
(Clemence and Finbarr, 1980).
Field Load Tests
Ground saturation by ponding or using bottomless tanks is useful for evaluating collapse
and subsidence where very large leakage may occur, as through canal linings.
Full-scale or plate load tests , for the evaluation of foundation settlements, should be per-
formed under three conditions to provide comparative data at founding level:
1.
Soils at natural water content when loads are applied.
2.
Loads applied while soil is at the natural water content until the anticipated
foundation pressure is reached. The ground around and beneath the footing is
then wetted by pouring water into auger holes. (In very dry climates and clayey
soils, several days of treatment may be required to achieve an adequate level of
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