Page 114 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 109

Figure 5.5
Quicklime
stabilization of a
highly expansive
B b horizon with a
series of soil
borings. The
proximity to the
valley slope
allowed drainage
of the perched
groundwater
table and
aggravated the
condition.

so this is the location where expansive clay problems are most likely to occur.
The house in Fig. 5.5 is on an eroded hillside where the exposed paleosol clay
undergoes wet-dry cycling. The condition was further aggravated by a leaky floor
drain.

5.5 GROUNDWATER AND SOIL COLOR

5.5.1 Definition of a Groundwater Table
It sometimes is assumed that a groundwater table represents the saturation
elevation in a soil, but that is not correct, because soil above the water table also
can be saturated by capillary action, as water can be drawn up into a small glass
tube. The amount of capillary rise is small in sand, but can be substantial when
pores are finer, as in clay. The groundwater table is defined as the level to which
groundwater will rise in an open boring where capillary action has no effect.

5.5.2 Importance of a Groundwater Table
Which weighs more, a bucket of sand suspended in air or the same bucket of
sand suspended in water? The answer is obvious, or at least it was to Archimedes,
who noted that the weight of an object submerged in water is reduced by the
weight of the water that is displaced. Similarly, buoyant forces on soil that is
submerged below a groundwater table cause it to weigh considerably less than the
same soil that is not submerged.

If the level of the groundwater table changes, so does the weight of the soil. That is
why lowering a groundwater table by pumping from wells can cause areas to

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