2. The impact of groundwater on the construction, maintenance
and long-term performance of highways and highway structures
must be considered on every project. Excessive and
uncontrolled subsurface water has been responsible for large
numbers of pavement and slope failures, and unsatisfactory
projects.
Subsurface drainage systems OR some other suitable technique can
be provided to remove or control groundwater issues and minimize
their impacts on construction projects.
Distress or failure of existing pavement;
Wet spots in pavement, or subgrade material piping through cracks or joints;
Undulating pavement surface;
Unlined irrigation canals on slopes uphill of the highway;
Cattails or wetland vegetation growing in a borrow ditch or on slopes;
Seepage from cut or natural slopes;
3.
4.
5. Subsurface drains are effective in controlling groundwater problems
when properly designed and constructed. Many pavement and slope
failures can be prevented by effective drainage of subsurface water.
Underdrains are used to intercept subsurface seepage before it enters
the structural material supporting the pavement, and are also used to
draw down the water table.
Pipe underdrains consist of a perforated or slotted pipe placed near
the bottom of a narrow trench backfilled with permeable backfill
material. The backfill is typically wrapped in a filter fabric to prevent
clogging of the drain from migration of fines into the permeable
material. A French drain is an underdrain that consists of a trench
backfilled with highly permeable material, but without the perforated or
slotted pipe.
In sloping terrain where slope stability is not a problem, a trench may
be excavated along the uphill side of the roadbed near the toe of the
cut slope.
6.
7. Other solutions may include French drains, sump pits,
culverts, retaining wall with filter material, wick drains,
drop structure/rundown etc. can be an appropriate
option.
8.
9.
10. Impacts of Groundwater on Highway Projects
Groundwater can have a considerable impact on the success of a
highway construction project. If groundwater and seepage are not
identified and adequately addressed, they can significantly impair
constructability, pavement performance, and slope stability.
Groundwater must also be considered in design and construction of
drainage structures, wetlands mitigation sites, foundations, and
retaining structures.
12. Nearby water bodies like river, stream,
swimming pool, dam etc. may cause percolation
of water in to soil leading to lot of geotechnical
problems like subsidence, settlement, sinkhole,
potholes etc.
Stream conspiracy?
28. Ground Water Control
Water can be classified by its relative
position to or within the ground.
29. Ground Water Control
Problems of water in the subsoil:
– A high water table could cause flooding during
wet period.
– Subsoil water can cause problems during
excavation works by its natural tendency to flow
into the voids created by the excavation activities.
– It can cause an unacceptable humidity level
around finished building and structures.
30. Ground Water Control
Control of ground water always referred to
temporary and permanent exclusion.
– Temporary exclusion: lowering of the water table
and within the economic depth range of 1500mm
using subsoil drainage methods, for deeper
treatment a pump or pumps are usually used.
– Permanent exclusion: the insertion of an
impermeable barrier to stop the flow of water
within the ground.
35. Ground Water Control
Jetted Sumps
– Almost the same with simple sump methods of
dewatering.
– A borehole is formed in the subsoil by jetting a
metal tube into the ground by means of
pressurised water to a depth within the maximum
suction lift of the extract pump.
– The metal tube is withdrawn to leave a void for
placing a disposable wellpoint and plastic suction
pipe.
– The area surrounding the pipe is filled with coarse
sand as filtering media.
37. Ground Water Control
Wellpoint systems
– This is a method of lowering the water table to a
position below the formation level to give a dry
working area.
– Jetting into the subsoil a series of wellpoints
which are connected to a common header pipe
which then connected to a vacuum pump.
– Commonly use in trench excavation.
– If the proposed formation level is below the
suction lift capacity of the pump a multi-stage
system can be employed.
42. Ground Water Control
Thin Grouted Membranes
– Work as permanent curtain or cut-off non
structural walls or barriers inserted in the ground
to enclose the proposed excavation area.
– Suitable for silts and sands and can be installed
rapidly but they must be adequately supported by
earth on both sides.
– The only limitation is the depth to which the
formers can be driven and extracted.
44. Ground Water Control
Contiguous Piling
– Forms a permanent structural wall of interlocking
bored piles.
– Alternate piles are bored and cast by traditional
methods after which the interlocking piles are
bored using a special auger or cutter.
– Suitable for most types of subsoil and has the
main advantages of being economical on small
and confined sites; capable of being formed close
to existing foundations and can be installed with
the minimum of vibration and noise.
45. Ground Water Control
Contiguous Piling
– To ensure a complete interlock of all piles over
the entire length may be difficult therefore the
exposed face of the piles is usually covered with a
mesh or similar fabric and face with rendering or
sprayed concrete.
– Suitable for structures such as basements, road
underpasses and underground car parks.
47. Ground Water Control
Diaphragm Wall
– Are structural concrete walls which can be cast in-
situ or using pre-cast concrete methods.
– Suitable for most subsoil and their installation
generates only a small amount of vibration and
noise.
– The high cost of these walls makes them
uneconomic unless they can be incorporated into
the finished structure.
– Normally use for basements, underground
carparks and similar structures.
49. Ground Water Control
Pre-cast Concrete Diaphragm Wall
– Have some applications with in-situ concrete
diaphragm walls.
– Lack in design flexibility.
– The panel or post panel units are installed in a
trench filled with a special mixture of bentonite and
cement with a retarder to control the setting time.
– This mixtures ensures that the joints between the
wall components are effectively sealed.
– To provide stability, the panels of posts are tied to
the retained earth with ground anchors.
51. Ground Water Control
Grouting Methods
– Are used to form a curtain or cut-off wall in high
permeability soils where pumping methods could
be uneconomic.
– The curtain walls formed by grouting methods are
non-structural therefore adequate earth support
will be required and in some cases this will be a
distance of at least 4m from the face of proposed
excavation.
52. Ground Water Control
Grouting Methods
– Grout mixtures are injected into the soil by pumping
the grout at high pressure through special injection
pipes inserted in the ground.
– The pattern and spacing of the injection pipes will
depend on the grout type and soil conditions.
– Grout types:
Cement grouts
Chemical grouts
Resin grouts
54. Ground Water Control
Ground Freezing Techniques
– Suitable for all types of saturated soils and rock
and for soils with a moisture content in excess of
8% of the voids.
– The basic principle is to insert into the ground a
series of freezing tubes to form an ice wall thus
creating an impermeable barrier.
– Takes time to develop and the initial costs are
high.
55. Ground Water Control
Ground Freezing Techniques
– The freezing tubes can be installed vertically for
conventional excavations and horizontally for
tunneling works.
– Normally using magnesium chloride and calcium
chloride with a temperature of -15 to -25 degree
Celsius which takes 10 to 17 days to form an ice
wall 1m thick.
– Liquid nitrogen could be used as the freezing
medium to reduce the initial freezing period if the
extra cost can be justified.
