National Institute of Technology, Hamirpur
Department of Civil Engineering
CE-472(c)
Geosynthetics
Rakesh Kumar Dutta, Ph.D.
Associate Professor
Shallow foundations
The geosynthetic-reinforced foundation
soils are being used to support
footings
of
many
structures
including warehouses, oil drilling
platforms, platforms of heavy
industrial
equipments,
parking
areas, and bridge abutments. In
usual construction practice, one or
more layers of geosynthetic are
placed inside a controlled granular
fill beneath the footings. Such
reinforced foundation soils provide
improved load-bearing capacity and
reduced settlements by distributing
the imposed loads over a wider area
of weak subsoil. In the conventional
construction techniques without any
use of the reinforcement, a thick
granular layer is needed which may
be costly or may not be possible,
especially in the sites of limited
availability of good-quality granular
materials.
Geosynthetic products like
‘Paralink’ as shown in
Figure (a) can be very
effective for use over
soft foundation soils as
well as over voids and
piles (Fig. (b)).
The ideal reinforcing pattern has geosynthetic
layers placed horizontally below the footing,
which becomes progressively steeper farther
from the footing (Fig. (a)). It means that the
reinforcement should be placed in the direction
of the major principal strain. However, for
practical simplicity, geosynthetic sheets are
often laid horizontally as shown in Figure (b).
Filters and Drains
• The role of groundwater flow and good drainage in the stability of
pavements, foundations, retaining walls, slopes, and wastecontainment systems is gaining attention from engineers, practitioners,
and researchers alike. That is why geosynthetics are being
increasingly employed either as filters, in the form of geotextiles
(nonwovens and lightweight wovens), in conjunction with granular
materials and/or pipes (Fig. (a)), or as both filters and drains in the
form of geocomposites (Fig. (b)). Filters also form an essential part of
many types of hydraulic structures. Thus, there are several application
areas for filters and drains including buried drains as pavement edge
drains/underdrains, seepage water transmission systems in pavement
base course layers and railway tracks, abutments and retaining wall
drainage systems, slope drainage, erosion control systems, landfill
leachate collection systems, drains to accelerate consolidation of soft
foundation soils, drainage blanket to dissipate the excess pore pressure
beneath embankments and within the dams and silt fences/barriers.
A filter consists of any porous material that has openings small enough
to prevent movement of soil into the drain and that is sufficiently
pervious to offer little resistance to seepage. When a geosynthetic is
used as a filter in drainage applications, it prevents upstream soils
from entering adjacent granular layers or subsurface drains. When
properly designed, the geosynthetic filter promotes the unimpeded
flow of water by preventing the unacceptable movement of fines into
the drain, which can reduce the performance of the drain.
Geosynthetic filters are being used successfully to replace
conventional graded granular filters in several drainage applications.
In fact, filter structures can be realized by using granular materials
(i.e. crushed stone) or geotextiles or a combination of these materials
(Fig.). The choice between the graded granular filter or geotextile
filter depends on several factors.
When using riprap–geotextile filter, it is recommended that a
layer of aggregate be placed between the geotextile and the
riprap, for the following reasons (Giroud, 1992):
• to prevent damage of the geotextile by the large rocks
• to prevent geotextile degradation by light passing between
large rocks
• to apply a uniform pressure on the geotextile, thereby
ensuring close contact between the geotextile filter and the
sloping ground, which is necessary to ensure proper
filtration
• to prevent geotextile movement between the rocks because
of wave action, thereby ensuring permanent contact
between the geotextile filter and the sloping ground, which
is also necessary to ensure proper filtration.
Vertical strip drains (also called prefabricated vertical band drains (PVD)
or wick drains) are geocomposites used for land reclamation or for
stabilization of soft ground. They accelerate the consolidation process
by reducing the time required for the dissipation of excess pore water
pressure. The efficiency of the drains is partly controlled by the
transmissivity, that is discharge capacity that can be measured, using
the drain tester, to check their short-term and long-term
performance. The discharge capacity of drains is affected by several
factors such as confining pressure, hydraulic gradient, length of
specimen, stiffness of filter and the duration of loading. The
experimental study, conducted in the laboratory by Broms et al.
(1994), suggests that the effect of the length of the drains and the
duration of loading on the discharge capacity of the drain is small,
whereas the stiffness of the filter of drain can have a considerable
effect. The discharge capacity of the drain decreases with decreasing
stiffness of the filter.
In filter applications, the
design
must
be
prepared so as to
avoid, throughout the
design
life,
the
following
three
phenomena
causing
decrease
of
the
permeability of the
geotextile
filter
in
course of time:
1 blocking
2 blinding
3 clogging.
Erosion Control
The problem of soil movement due to
erosive forces by moving water and/or
wind as well as by seeping water is called
soil erosion. Gravity is also one of the
prime agents of soil erosion, particularly
on steep slopes. Soil erosion is associated
with
negative
economic
and
environmental consequences in many
areas such as agriculture, river and coastal
engineering, highway engineering, slope
engineering and some more sections of
civil engineering.
Construction sites with unvegetated steep slopes
are prime targets for soil erosion. Soil erosion by
moving water is caused by two mechanisms: (1)
detachment of particles due to raindrop impact
and (2) movement of particles from surface
water flow. The dislodged particles carry with
them seeds and soil nutrients. Natural growth of
vegetation on the exposed soil slope surface is
thus hindered. High velocity runoff can cause
not only surface soil movement downslope, but
their scouring effects can cause total
undermining of slopes. Rain erosion can act
upon a land surface of any degree of slope;
however, the severity of rain erosion increases
with increasing slope steepness and slope
length.
The exposed denuded slopes become increasingly
vulnerable to erosion agents and are ultimately
destabilized. To control erosion is to curb or
restrain the gradual or sudden wearing away of
soils by wind and moving water. The goal of any
erosion control project should be to stabilize
soils and manage erosion in an economical
manner. Since surface water flow cannot be
eliminated, the most feasible solution to erosion
problems is slope protection. The slope
protection serves two functions: (1) it slows
down the surface water flow and (2) it holds soil
particles, grass or seedlings in place. If an
element is incorporated into the soil to prevent
the detachment and transportation of soil
particles, then the slope would be able to
withstand greater forces.
The solutions of soil erosion problems typically involve the
use of basic erosion control techniques such as soil cover
and soil retention. The use of revetments is very common
in civil engineering practice for erosion control (Fig.). A
cover layer (called armour) of a revetment can be
permeable or impermeable. An open cover layer
substantially reduces the uplift pressures, which can be
induced in the sublayers and provides protection against
the external loads. Riprap, blocks and block mats,
grouted stones, gabions and mattresses, and concrete
and asphalt slabs are most commonly used as revetment
armours.
Three-dimensional erosion control geosynthetic mats and geocells that
are nowadays commercially available with various dimensions can be
used in permanent erosion control systems. Geocells are threedimensional honeycomb structures that have a unique cellular
confinement system formed by a series of self-containing cells up to
20 cm deep. They have the ability to physically confine the soil placed
inside the cells (Fig.). They retain soil, moisture and seed, and thus
create situations for the growth of vegetative mats on slopes where
vegetation may be difficult to establish. The vegetative mats provide
reinforcement and the system’s cells increase the natural resistance of
these mats to erosive forces and protect the root zone from soil loss.
At the same time, the cellular confinement system facilitates slope
drainage.
The most common and natural element used for erosion control is vegetation.
Roots of the grasses protect the slope surface from erosion. The deeper
roots of plants, shrubs and trees tend to reinforce and stabilize the deeper
soils. The application of vegetation as bank protection is preferred rather
than the application of conventional materials such as riprap, concrete
blocks, etc. If necessary, vegetation and appropriate geosynthetics can be
applied in combination (Fig.). The selection of vegetation must be done on
the basis of soil and climatic conditions of the specific area of application.
The vegetation will on the one hand stabilize the body of the channel,
consolidate the soil mass of the slope and bed and reduce erosion. On the
other hand, the presence of vegetation will result in extra turbulence and
retardation of flow. Geotextiles and other perforated geosynthetics and open
blocks provide additional strength to the root mat and can reduce much of
the direct mechanical disturbance to plants and soil.
Geotextiles are also used in toe and bed
protection, which consists of the armouring of
the beach or bottom surface in front of a
structure to prevent scouring and undercutting
by water waves and currents (Fig.). The
stability of toe is essential, because its failure
will generally lead to failure of the entire
structure.
In many cases, geotextile is used to wrap a fill
material (sand, gravel, asphalt or mortar),
creating geobags, geotubes or geomats, known
collectively as geocontainers, which are used in
hydraulic and coastal engineering (Fig. ).
Ponds, Reservoirs, and Canals
Liquid containment and conveyance facilities, such as
ponds, reservoirs and canals, are required in several
areas including hydraulic, irrigation and environmental
engineering. Unlined ponds, reservoirs, and canals can
lose 20–50% of their water to seepage. Traditionally,
soil, cement, concrete, masonry or other stiff materials
have been used for lining ponds, reservoirs and
canals. The effectiveness and longevity of such
materials are generally limited due to cracking,
settlement and erosion. Sometimes the traditional
materials may be unavailable or unsuitable due to
construction site limitations, and they may also be
costly.
Flexible geosynthetic lining materials, such as
geomembranes, have been gaining popularity as
the most cost-effective lining solution alone or in
combination with conventional lining material for
a number of applications, including irrigation and
potable water. Figure below shows typical
schematics
of
liquid
containment
and
conveyance facilities (ponds, reservoirs, and
canals) involving application of geosynthetics in
addition to conventional materials.
Geosynthetic liner/barrier materials can be classified as
GMBs, GCLs, thin-film geotextiles composites or
asphalt cement-impregnated geotextiles. The selection
of lining material is governed by the location and
environmental factors. Placement, handling and soil
covering operations can also affect geosynthetic
selection. When GMBs are used as lining material,
geotextiles can be used with GMBs for their protection
against puncture by the granular protective layer,
which may also be required to prevent UV- and
infrared-induced ageing of geosynthetics, as well as
any effects of vandalism and burrowing animals. A
geotextile, if used below the GMB liner, can function
as a protection layer as well as a drainage medium for
the rapid removal of leaked water, if any. For
economical reasons, the GMB liner may be left
uncovered.
Earth Dams
Earth dams are water impounding massive
structures and are normally constructed using
locally available soils and rocks. One of the
principal advantages of earth dams is that their
construction is very economical compared to the
construction costs of concrete dams. Apart from
the conventional materials used in the earth
dam, geosynthetics are being employed in
recent times for new dam constructions and for
the rehabilitation of the older dams. Properly
designed and correctly installed geosynthetics, in
an earth dam, contribute to increase in its safety
which corresponds to a positive environmental
impact on dam structures (Singh and Shukla,
2002).
The reasons for which geosynthetics are used
extensively in earth dam construction and
rehabilitation are the following:
• The use of geosynthetics in earth dams may
serve several functions: water barrier, drainage,
filtration, protection and reinforcement.
• The geosynthetics are soft and flexible –
therefore, they can endure some elasto-plastic
deformations resulting from the subsidence,
expansion, landslide and seepage of soil.
• The geosynthetics (geotextiles and geogrids)
possess certain mechanical strength, which is
favourable as dam-filling materials.
• The permeability of geomembranes is much
lower than that of clay or concrete.
The long-term performance of various components of an earth
dam is critical to the performance of the dam as a whole. If a
geotextile is to be used as a filter, careful assessment of the
properties, extensive testing and monitoring are required to
ensure its suitability. The locations in earth dams where
geotextile filters may be used are in the downstream chimney
drain and in the downstream drainage blanket (see Fig.). If the
dam is subjected to rapid drawdown, then drainage systems
using geosynthetics may also be installed on the upstream side
of the core. In the past, geotextile filters, mostly nonwovens,
have been used for the construction or the rehabilitation of
numerous embankment dams (i.e. earth or earth and rockfill
dams) in various parts of the world.
Tunnels
Tunnels are used for various purposes in
civil
engineering,
including
traffic
movement and fluid flow. Waterproof
tunnels are required at some sections of
the highway and railway alignments. A
crack-free concrete lining is needed for a
waterproof
tunnel.
Geotextiles
and
geomembranes are commonly used in
modern-day
tunnel
technology
to
construct waterproof tunnels.
Figure below shows the cross-section of a tunnel vault with the
general arrangement of the lining system. The shotcrete lining
placed over the excavated surface provides a smooth surface
for the geosynthetics. In addition the rock surface is supported
by the shotcrete immediately after excavation so that the
radially acting forces can be accepted adhesively (Wagner and
Hinkel, 1987). The nonwoven geotextile (generally needlepunched) acts as a drainage layer and as protection for a
waterproofing geomembrane. It also acts as a cushion (stressrelieving layer) to significantly reduce the formation of cracks in
the inner concrete lining by allowing free shrinkage deformation
of the concrete during the setting process.
It should be noted that geomembrane sheet
sealing with a protective nonwoven
geotextiles
drainage
layer
has
predominated over the conventional
sealing methods such as asphalt
membranes or spray applied glass fibrereinforced plastic or bitumen-latex based
products. The geosynthetic system not
only meets the demands of the rapid
tunnelling rates but also the demands for
rough construction treatment.
Stabilization
Slopes can be natural or man-made. Several
natural and man-made factors, which have
been identified as the cause of instability to
slopes, are well known to the civil engineering
community. Many of the problems of the
stability of natural slopes are radically different
from those of man-made slopes mainly in
terms of
1. The nature of soil materials involved
2. The environmental conditions, location of
groundwater level
3. The stress history
In man-made slopes, there are also essential
differences between cuts and embankments.
The latter are structures which are built with
relatively well-controlled materials.
In cuts, however, this possibility does not exist. The
failures of slopes, called landslides, may result in loss
of property and lives and create inconvenience in
several forms to our normal activities. Several slope
stabilization methods are available to improve the
stability of unstable slopes. The slope stabilization
methods generally reduce driving forces, increase
resisting forces, or both. The advent of geosynthetic
reinforcement materials has brought a new dimension
of efficiency to stabilize the unstable and failed slopes
by constructing various forms of structures such as
reinforced slopes, retaining walls, etc. mainly due to
their corrosive resistance and long-term stability. In
recent years geosynthetic-reinforced slopes have
provided innovative and cost-effective solutions to
slope stabilization problems, particularly after a slope
failure has occurred or if a steeper than safe
unreinforced slope is desirable. They provide a wide
array of design advantages as mentioned below:
•
•
•
•
•
•
•
•
•
Reduce land requirement to facilitate a change in grade
Provide additional usable area at toe or crest of slope
Use available on-site soil to balance earthwork quantities
Eliminate import costs of select fill or export costs of unsuitable
fill
Meet steep changes in grade, without the expense of retaining
walls
Eliminate concrete face treatments, when not required for
surficial stability or erosion control
Provide a natural vegetated face treatment for environmentally
sensitive areas
Provide noise abatement for high traffic areas and minimize
vandalism
Offer a design that is easily adjustable for surcharge loadings
from buildings and vehicles.
Thank You