Environmental Engineering Reference
In-Depth Information
Environmental Factors
Basic Factors
Water-table depth:
For a soil to develop substantial heave it must lie above the static water
table in a less than saturated state and have moisture available to it. Moisture can origi-
nate from capillary action or condensation as well as in the form of free water.
Climate:
Highly expansive soils are found in climates from hot to cold. Long periods
with little or no rainfall permit the water table to drop and the soils to decrease in mois-
ture. The soils dry and shrink, large cracks open on the surface, and fissures develop
throughout the mass, substantially increasing its permeability. In Texas, during the dry
season, these cracks can extend to depths of 20 ft. Rainfall or other moisture then has easy
access for infiltration to cause swelling.
Topography
affects runoff and infiltration. Poorly drained sites have a higher potential
for ground heave than slopes.
Environmental Changes Cause Surface Movements
Decrease in moisture, causing shrinkage and fissuring, results from:
Prolonged dry spells or groundwater pumping producing a drop in the water
table
●
Growth of trees and other vegetation producing moisture loss by transpiration
●
Heat from structures such as furnaces, boiler plants, etc., producing drying
●
Increase in moisture, causing swelling and heave, results from:
Rainfall and a rise in the water table
●
Drilling holes, such as for pile foundations, through a perched water table that
permits permeation into a lower clay stratum (ENR, 1969)
●
Retarding evaporation by covering the ground with a structure or a pavement
●
Thermo-osmosis, or the phenomenon by which moisture migrates from a warm
zone outside a building area to the cool zone beneath the building
●
Condensation from water lines, sewers, storm drains, and canals
●
Removal of vegetation that increases susceptibility to fissuring and provides
access for water
●
Time Factor
Usually, a year or more passes after construction is completed before the effects of heave
are apparent, although heave can occur within a few hours when the soil has sudden
access to free water as from a broken water main or a clogged drain.
A plot of yearly rainfall and heave as a function of time for a house in the Orange Free
State in South Africa is given in
Figure 10.43.
It shows almost no heave for the first year,
then a heave of 11.6 cm occurring over a period of 4 years, after which movement essen-
tially stops. The long-term effect is the result of the slow increase, due to natural events, of
moisture content beneath a covered area.
10.6.3
Swelling in Rock Masses
Marine and Clay Shale
Characteristics
As described in
Section 6.7.3,
montmorillonite is a common constituent of marine and clay
shale; therefore, these shales have a high swell potential. They are commonly found dis-
