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5 GROUND WATER
5.1 Formation of Ground Water
Water commonly gets into the soils and rocks when surface precipitation (rain or snow)
percolates (move downwards) through the interconnecting channels made by the voids in
soils or the cracks in rocks. It is then termed ground water.
Figure 5.1 Formation of ground water
5.2 Aquifer and Aquiclude
A body of soil or rock which holds ground water and allows water to move through itself is
called an aquifer; the opposite of an aquifer is an aquiclude (Figure 2).
Figure 5.2 Aquifer and aquiclude
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5.3 Types and Zones of Ground Water
In soil, there are two types of ground water and they occur in distinct zones separated by the
water table or ground water level or phreatic surface (Figure 3). Ground water level
(symbol g.w.l.) or phreatic surface is the top level of saturation of a body of soil.
Types of ground water
(a) Phreaticor Gravitationalwater, which::
is subject to gravity forces
saturates the pore spaces (voids) in the soil below the water table
has a positive internal pore pressure (i.e., greater the atmospheric pressure)
tends to flow laterally.
(b) Vardosewater, which :
may be transient percolating water moving down to join the phreatic surface (or water
table).
may be capillary water held above the water table by surface tension having a negative
internal pore pressure
Figure 5.3 Zones of ground water
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Vadose
(a)
(b)
ENGINEERING GEOLOGY
AND SOIL MECHANICS
(Figure 5.3).
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Zones of ground water
(a) Saturated Zone: the top surface is at atmospheric pressureand is known as the water
tableor the phreatic surface. Below this surface the soil is saturated with water subject
to positive hydrostatic pressure.
(b) Aeration zone: this zone is subdivided into 3 subzones.
Immediately above the phreatic surface, the soil remains saturated with water due
to capillary actionwhich holds water below atmospheric pressure. Hence, the
pressure in the pore (or pore water pressure) is negative.
Above the capillary saturated one is a partially saturatedsubzone where water is
held by capillary action (surface tension) and absorption.
The top subzone occurs only when there is continuous upward evaporation and/or
downward percolation.
5.4 Unconfined Aquifer and Confined Aquifer
Unconfined aquifer
The surface water usually moves downwards through the void channels of a layer of pervious
(permeable) soil or rock to the top level of saturation or the ground water level (Figure 5.4).An aquifer fed by direct downward percolation which establishes a water table is called an
unconfined aquifer. Water will normally fill a pit or borehole to the level of the ground
water table.
Figure 5.4 Unconfined aquifer
Impermeable layer
Aquifer
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Capillary action: A phenomenon associatedwith surface tension and resulting in theelevation or depression of liquids in capillaries.
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Confined Aquifer
An aquifer, which is sandwiched between two impermeable layers of soil or rock, is fed
with water from a source at some distance awayis termed a confined aquifer. Water in a
confined aquifer is often subject to the pressure (artesian pressure) of a head of water higher
than the ground level immediately above and, if tapped by a cased borehole, a column of
water will rise above the ground surface level The level to which the water will rise in such a
situation is known as the piezometricsurface or piezometric level. (Note that there is no
water table inside a confined aquifer.)
Figure 5.5 Confined aquifer
5.5 Pore Water Pressure
The water in an aquifer, whether confined or unconfined, has pressure at any point due to the
head of water above it. Since the water is in the void spaces in the soil or rock, i.e., in the
pores, this pressure (shown diagrammatically by the manometers in Figure 5.6) is known as
the pore water pressure (symbol u).
Aquifer
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(a) Unconfined aquifer
(b) Confined aquifer
Figure 5.6 Pore Water Pressure in confined and unconfined aquifers
Pore water pressure at A
u = h1w
Pore water pressure at B
u = h2w
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Positive Pore Water Pressure
Normally, water pressure increases positively with depth below the ground water level (g.w.l.)
as shown in Figure 5.7.
Figure 5.7 Normal increase of pore water pressure with depth
In some soils, particularly those with very small void spaces, the top of saturated (g.w.l.) is
modified by the surface tension effect of the very small channels formed by the
interconnecting pores. This is demonstrated by a simple experiment as shown in Figure 5.8.
Aabove the ground water leveling a soil, where it is partially saturated, the narrow channelsformed by interconnecting small voids act like a system of capillary tubes which suck up
water by surface tension. This phenomenon is termed soil suction.
Figure 5.8 capillary rises due to surface tension and resulting pressure distribution
u
Pore water pressure at A
u = h1w
Pore water pressure
u =-h w
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Monitoring (Measurement)of ground Water Level
Where normal positive water pressure exists below the water table it may be measured by the
use of instruments called standpipe and piezometer. (Negative pore water pressure is
measured using a tensiometer.)
Figure 5.9shows a typical standpipe and an open-hydraulic poezometer which are commonly
used in Hong Kong.
piezometer
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5.10
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