Page 115 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
110 Geotechnical Engineering

subside, as the increased weight of the soil compresses soil that is underneath.
A notable example is Mexico City, where the amount of subsidence is measured in
meters or even tens of meters.

Conversely, raising a groundwater table reduces the weight of soil, which is
a reason why abandoned mines should be flooded, to reduce the likelihood that
they will collapse. Pumping out an abandoned mine can be extremely unwise as
it can double the weight supported by the mine.

5.5.3 Locating the Groundwater Table

When infiltrating rain water reaches a groundwater table it moves laterally to the
nearest exit, normally a stream valley. The groundwater table therefore represents
a subdued version of the ground surface, being higher under hills and exiting at or
near the ground surface in valleys.

The groundwater elevation is not constant, but varies seasonally depending on
the rate at which water infiltrates into the ground from rain or snow melt. A low
position of the groundwater table is no guarantee of future immunity from
problems such as basement flooding. The most important clue to transitory
elevations of a natural groundwater table is soil color, discussed below.

5.5.4 Dowsing

Few things can inspire the confidence of a water dowser more than a
$50 bill—unless it is a $100 bill, in which case he will try twice as hard. A
dowser holds a forked stick that mysteriously points down when over water.
Skeptics have tested the system by blindfolding the dowser and walking him
or her over a bridge, but that is not a fair test, as water has to be surrounded by
soil in order to give off the proper aura. A variation for buried water pipes or
electric lines is two steel rods bent in L-shapes and held so that the long ends
are horizontal and point forward. They then swing together over the buried
object.

There is another reason why water witching is successful: groundwater can be
found anywhere, even in a desert, if the hole is drilled deep enough. A boring
eventually is likely to encounter some kind of permeable water-bearing stratum.
If it doesn’t, you get your $50 back.

Some dowsing concepts are simply incorrect, such as groundwater occurring in
‘‘veins’’ or ‘‘rivers’’ instead of as pore fluids. The only underground lakes and
rivers are those in abandoned mines and limestone caverns. For a history and
status of dowsing see Legget (1962). On the other hand, if a good client expresses a
belief in water dowsing it can be unwise to argue with a religion.

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