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Groundwater and engineering
Groundwater can also have dramatic implications for engineering and geotechnical studies. The study of groundwater is essential for engineers who construct dams, tunnels, water conveyance channels, mines, and other structures. Groundwater must be considered whenever the stability of slopes is important, whether the slope is natural or constructed. Groundwater must also be taken into account when devising measures to control flooding. In all of these situations, groundwater flow and fluid pressure can create serious geotechnical problems.
Groundwater, for example, may create structural weaknesses in dams, or it may flow underground right around the structure as it did at the Jerome Dam in Idaho. Water flowed so efficiently through the rock formations surrounding the reservoir that the dam would hold no water, even though it was structurally sound.
In another case, when geological exploration was being carried out in preparation for the construction of the Revelstoke Dam in British Columbia, geologists and engineers were concerned about an old landslide on the bank of the proposed reservoir. They suspected that the water held in the reservoir could increase groundwater pressures enough to make the slide unstable. The solution was to increase drainage around the slide to ensure that groundwater pressures did not increase. In 1963, these same conditions at the Vaiont reservoir in Italy caused a slide which killed 2500 people.
Other problems result from the excessive use of groundwater. Overdrafting occurs when people draw water out of an aquifer faster than nature can replenish it. The most obvious problem created is a shortage of water. Overdrafting, however, can also create significant geotechnical problems. Although not an issue in Canada, at many locations around the world overdrafting has caused land subsidence. This can produce severe engineering difficulties. Parts of Mexico City, for instance, have subsided as much as 10 metres in the past 70 years, resulting in a host of problems in its water supply and sewerage system. Land subsidence may also occur when the water table is lowered by drainage. In the early 1970s, for example, an entire residential subdivision in Ottawa subsided when a collector sewer was constructed nearby. The subsidence seriously damaged the residents' property.
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