Environmental Engineering Reference
In-Depth Information
Hanging wall
FIGURE 8.34
Differential water pressures along the sides of a normal or
dip-slip fault can be extremely hazardous to tunnels
approaching from the hanging-wall side.
Foot wall
side of the fault, piezometric pressures were relatively low and controllable by draining,
whereas on the footwall side the piezometric level was high and perched (Figure 8.34).
When the tunnel penetrated the clay gouge, 3000 yd
3
of crushed material and water were
suddenly released into the tunnel.
Corrosion of concrete tunnel linings can be high where water flows over gypsum or
other sulfates and contains solutions of sulfuric salts, as described below.
Pavements
High groundwater tables and capillarity provide the moisture resulting in “pumping” or
frost-heaving of pavements in fine-grained granular soils, or heaving from swelling clays.
Water Quality
Importance
Two basic aspects of water quality are:
The desired quality of water supply depends upon its intended use, e.g., potable
water, industrial (pure for pharmaceutical use, much less pure for cooling), or
irrigation.
●
Corrosive effects on construction materials, primarily concrete, are influenced by
water quality.
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Soluble Salts
Normally, salts are found in solution in all groundwater. Concentrations of soluble salts
are dependent on water movement, temperature, and origin. As groundwater passes
through rocks and soils, their minerals are subject to dissolution; sedimentary rocks are
generally more soluble than are the igneous and metamorphic types. The salts occur in the
form of carbonates, bicarbonates, and sulfates, principally of Ca, Na, and Mg. In calcare-
ous rocks, the water is enriched with carbonates; in ferruginous rocks, with iron oxides.
The principal agent causing solution is CO
2
, resulting from rainwater infiltrating through
surface organic materials.
Some definitions are as follows:
Hard water contains a high level of carbonates, of the order of 50 g of CaCO
3
for
1000 L or water. Pipes become encrusted and soap does not foam.
●
Soft water contains a low level of carbonates.
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