Construction Technology: Substructure

1
Construction Technology Substructure DW57
34 Outcome 2 Methods of groundwater control
2
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.

3
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

4
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.

5
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.

6
So where does all this water come from?
The stages of the hydrological cycle
7
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

8
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.
9
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

10
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

11
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

12
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

13
Water Table

• Snow semi solid format of rain. More snow would
  increase the water table.

14
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

15
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

16
Water Table

• Floodplain area of land around a river where
  the river encroaches onto when it exceeds its
  channel capacity

17
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 water-table the perched water is
  isolated from groundwater at deeper levels by an
  impermeable layer.

18
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.