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Construction Technology: Substructure
DW57 34
Outcome 2
Methods of groundwater control
Water in the ground
These kids probably think there is some kind of magic happening
here ... they pull down a lever and out of the ground below their feet
comes clear, cool freshwater. They (and maybe you) may not realize
that there is an immense amount of water in aquifers below the
earth's surface. In fact, there is a hundred times more water in the
ground than is in all the world's rivers and lakes.
Water in the ground
• Some water underlies the Earth's surface almost everywhere,
beneath hills, mountains, plains, and deserts.
• It is not always accessible, or fresh enough for use without
treatment, and it's sometimes difficult to locate or to measure
and describe.
• This water may occur close to the land surface, as in a marsh,
or it may lie many hundreds of feet below the surface, as in
some arid areas of the West.
• Water at very shallow depths might be just a few hours old; at
moderate depth, it may be 100 years old; and at great depth
or after having flowed long distances from places of entry,
water may be several thousands of years old
Water in the ground
• Groundwater occurs only close to the Earth's surface.
• There must be space between the rock particles for groundwater to
occur, and the Earth's material becomes denser with more depth.
• Essentially, the weight of the rocks above condense the rocks below
and squeeze out the open pore spaces deeper in the Earth.
• That is why groundwater can only be found within a few miles of the
Earth's surface.
• The water within the ground slowly moves towards streams, lakes,
marshes and oceans as shown above. Most rivers receive about half
of their flow from the water in the ground. This is why rivers can
keep flowing for weeks after no rain.
Water in the ground
• The presence of water in soil voids and rock fissures has a
major impact on the engineering behaviour of the ground.
• Difficulties with soils can be due to the water contained in the
voids and not the actual soil itself.
•
A high water-table can present difficulties with foundation
excavations and reduce the bearing resistance of the ground.
• The presence of chemicals in groundwater, such as acids and
sulphates, can cause damage to foundation concrete if it is
not of an appropriate quality.
• Groundwater can cause unacceptable humidity levels around
a building or structure.
So where does all this water come from?
The stages of the hydrological cycle
Rainfall
Ultimate source of all fresh water. When it lands
on the ground it can be dispersed in three ways;
• Evapotranspiration – evaporation from open
water and transpiration by plants
• Runoff – surface water flowing into streams
and rivers
• Infiltration – seepage into ground to become
groundwater
The principal source of
groundwater is
precipitation (rainfall).
While much of this water
may be lost as surface
run-off or by evaporation
and transpiration to the
atmosphere, some of the
water enters the ground
and moves downwards
by infiltration through the
continuous soil voids.
The relationships between the processes of infiltration, run-off,
evaporation and transpiration are influenced by climate,
topography, vegetation and geology.
Groundwater
• all water flowing through or stored within the
ground, in both rocks and soils
• derived from infiltration
• lost by flow to surface springs and seepage
out through seabed
Water Table
• Level in the rocks below which all voids are water filled
• Generally follows the surface topography, but with less relief
• It meets the ground surface at lakes and most rivers
Water Table
• The water table can change depending on a
number of factors
– Seasons (dry summers, wet winters)
– Vegetation such as trees
– How the water occurs, eg liquid, solid or gas
Water Table
• Rain – the more rain the higher the water table. Flooding can
occur where there is excessive rain. Some rain evaporates
soon after it falls and some drains on the surface to join
watercourses
Water Table
• Snow – semi solid format of rain. More snow would increase
the water table.
Water Table
• Sea – the water table moves close to the sea as the tide
comes in or out.
• Watercourses such as rivers or burns – the water table will be
higher the closer it is to a watercourse.
• Marsh land – the soil is retaining water. Water table is either
on the surface or just below it
Water Table
• Soil type – if the soil retains the water, such as clays, this could disturb the
water table. Dependent on how permeable the soil is
• Underlying geological structures – the nature of the bedrock below could
push the water table to peaks if it is folded, faulted or fractured depending
upon pressure.
• Burst water main – this would lead to a high water table
Water Table
• Floodplain – area of land
around a river where the river
encroaches onto when it
exceeds its channel capacity
Groundwater that is in direct contact vertically with the atmosphere through
permeable ground with continuous open spaces, such as pores and voids in
soils and fissures in rock, is termed unconfined.
Confined water is separated from the atmosphere by impermeable ground.
Confined water is sometimes described as artesian when the pressure in the
groundwater corresponds to a head of water which is above ground level.
The first unconfined water to be encountered may be at a perched watertable; the perched water is isolated from groundwater at deeper levels by an
impermeable layer.
Tutorial
• What are the sources of water in the ground?
• What is the water table and how might it be
affected?
• Draw a section indicating where you might
find perched water, a confined aquifer and an
unconfined aquifer.