4_engineering geology and soil mechanics_chapter 5_groundwater

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SOIL MECHANICS AND GEOLOGY Oct 2009GROUND WATER Dr.Paul Ho

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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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ENGINEERING GEOLOGY AND SOIL MECHANICS


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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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(a)

(b)

ENGINEERING GEOLOGY

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(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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