Environmental Engineering Reference
In-Depth Information
Ground surface
Water table
Sea level
Pumped well
Cone of depression
y
Fresh
groundwater
Fresh
groundwater
Ocean
≈
34
y
Ocean
H
Salt groundwater
Salt groundwater
h
(a)
(b)
FIGURE 8.21
Conditions causing saltwater intrusion into fresh water wells. (a) Natural equilibrium between fresh
groundwater and salt groundwater along a coastline or beneath an ocean island. (b) Pumping causing
saltwater intrusion into a freshwater well.
Springs and Underground Streams
Springs
represent a concentrated flow of groundwater, or effluent seepage, emerging from
the outcrop of an aquifer at the ground surface. The source may be free water moving
under control of the water-table slope (a water-table spring), confined water rising under
hydraulic pressure (an artesian spring), or water forced up from moderate or great depths
by forces other than hydraulic pressures, such as geysers, volcanic, or thermal springs.
Springs provide important information about groundwater conditions when observed in
the field.
Underground streams
truly exist naturally only in limestone or other cavernous rocks
where large openings are continuous and water can flow freely. Excavations into gravel
beds or other free-draining soil or fractured rock below the water table will encounter
large quantities of water, which may pour into an excavation and give the appearance of
a flowing stream. Actually, when confined beneath the surface, movement will be rela-
tively slow even in free-draining materials.
Significance
Geological
To the geologist, the principal significance of groundwater is as a source of water supply.
The primary concern is with the quantities and quality of water available from strata that
are relatively free-draining (aquifers), particularly artesian aquifers, since pumping costs
are reduced.
Engineering
The primary interests of engineers lie with aquifers and aquitards as sources of water. The
water flowing into excavations must be controlled to maintain dry excavations and to
reduce pressures on retaining structures. Water flowing through, around, or beneath dams
or other retaining structures requires control to prevent excessive losses, seepage pres-
sures, and piping. Aquitards, represented by saturated silts, can allow seepage into exca-
vations and may constitute weakness zones in excavation walls and zones susceptible to
“quick” conditions at the bottom of excavations or on slopes. Furthermore, the engineer
must be aware that conditions are transient and must realize that groundwater levels
measured during investigations are not necessarily representative of those that will exist
during construction.
Artesian conditions can result in boiling or piping (see
Section 8.3.2
)
and uplift in exca-
vation bottoms, often with disastrous results if pressures are excessive and not controlled
