Page 202 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
Soil Water 197

lagoon is an example of an intentionally perched water table, and a permeable
landfill may create an unintentionally perched condition.

10.2.4 Artesian Conditions
Sometimes when a boring penetrates through an impermeable soil into a more
permeable layer, water will rise up into the borehole and may even emerge or
geyser out at the ground surface. This is an artesian condition, and its occurrence
and the boring depth should be carefully noted in boring logs. The cause is
illustrated in Fig. 10.2, and, as can be seen from the figure, it shows the level of the
groundwater table at some distance uphill from the boring site.

Tapping into an artesian aquifer can go out of control if water emerges rapidly
enough to erode and open the boring. The use of a drilling mud that is heavier
than water can contain moderate artesian pressures, and if high artesian pressures
are anticipated a special dense mud should be used. Artesian conditions can be
expected on floodplains that are capped with clay and are flanked by flat terraces
that gather water. An astute driller may keep a wood fencepost close at hand to
drive down into a boring and prevent a nuisance from becoming a disaster. The
plug can be made more permanent by ramming in dry concrete premix. Auger
borings may even require the sacrifice of the auger if its withdrawal allows
unimpeded erosive flow. Continuing escape of large amounts of artesian water
will draw down the groundwater level and can dry up nearby wells as well as
causing surface erosion and flooding.

A gaping hole, locally known as ‘‘Jumbo,’’ was described as the ‘‘eighth wonder of
the world’’ in 1886 when artesian flow went out of control at Belle Plaine, Iowa,
after the wrong size of casing was inserted into a water well. Flow continued for
over a year and finally was stopped by dumping in 40 railroad cars of rock,
25 tons of Portland cement, a diversity of scrap iron, plus unrecorded amounts of
sand and clay.

10.2.5 Soil Color and the Groundwater Table

The level of a water table changes depending on rainfall, snow melting, and
whether the ground is frozen or saturated when the snow melts. Curiously, rain
can dry frozen soil by thawing and breaking through the frozen barrier that causes
a perched water condition.

Typically the water table is highest during and following spring rains, and lowest
during autumn and winter. Because soil water is continually in motion it seldom
reaches a condition of equilibrium unless there is an impervious cover such as a
pavement that prevents infiltration and evaporation. Prevention of surface
evaporation can create saturated conditions immediately underneath a pavement,
even in a dry climate, and an eventual change in soil color.

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