Use of Nonwovens as
Geotextiles
Introduction
Nonwoven geotextiles are
widely used for the functions of separation, reinforcement, drainage, erosion
control, and moisture barrier. Geotextiles are an very important part of the
whole nonwoven industry. The total North American geotextile market was
estimated at over 420 million square yards in 1994. Approximately 115 million
square yards are used as separation / stabilization geotextiles, about 105
million square yards are used as paving fabrics [1]. China is a big market for
geotextiles. According to Wang [2], 60-80 million square meter geotextiles will
be required for 5000 projects before year 2000. These projects include highway
construction, airport construction, water conservation and soil erosion protection
[2]. Some important applications are described as follows:
- Separation: Fabrics are used to separate
two dissimilar materials.
- Reinforcement: Fabrics reduce the level of
stress in the foundation soil due to horizontal shear stresses mobilized
by vertical loads. This application spreads the load over a large area and
thereby decreases its intensity. That is the unit vertical stress is
reduced and means less likelihood of failure.
- Drainage: The second largest area of
current fabric utilization since the permeability of the fabric can be
controlled.
- Erosion
control:
Fabric acts as both a separator and as a drainage layer, but when its
primary function is to aid in erosion control it is placed in this special
category.
- Moisture
barrier: The
hydrophobic nature of some of the olefin fibers can be exploited to
prevent the moisture in seeping through and disturbing fine soil
structure.
Apart from these fabric
materials can act as forms to be filled with other materials and thereby
conform to the shapes and topography of any surface on which they are
constructed. The impermeable fabrics coated with vinyl, polyurethane etc. act
like air- supported structures (inflated floodgates in tidal zones).
Fiber/ Fabric Types From the raw material viewpoint, approximately
¾ of geotextiles are made from polypropylene (PP); the remainder are from
polyester , other polymers and natural fibers [1]. Low cost, low specific
gravity and strength properties make PP the predominant raw material in the
field of geotextiles.
Separation Function and
Bearing Capacity of Nonwovens Pile Foundation In peats and silts of a soft plastic to liquid
consistency, constriction or uncontrolled flow out to the sides of the fresh
concrete into the soil has to be avoided. This can be achieved by wrapping a
stretchable nonwoven fabric around the reinforcement cage. The hydraulic
pressure of the new concrete presses the nonwoven fabric outward, when the pipe
is removed, thus guaranteeing sufficient cover over the steel.
Vertical Drains Vertical drains accelerate primary
consolidation of soft, highly saturated soils. As they do not improve secondary
creeping, they are hardly suitable for peats but work well for weak clays,
silts and organically contaminated soils. Among the various techniques the
nonwoven fabrics drains developed in Austria represent an innovation. They look
like long hoses being filled with sand and lowered into vertical holes. The
usual diameter is 10 cm; the width of the grid is approximately 2-4 m,
according to the kind of soil. As filter sand, a grain size of 2-7 mm is
preferred.
Sewer Pipes Pipes bedded into the subsoil can
withstand stresses of static and dynamic load better, if geotextiles are
incorporated. Most unfavorable conditions occur (maximum bending points of the
pipe) if the pipe is not supported on the sides but only rests on a linear
line. In soft and inhomogeneous subsoil, there is the additional danger of the
pipe resting on uneven and yielding ground, which can cause localized excessive
stresses, especially in the connection sockets.
If a nonwoven fabric is
laid onto the subsoil, the base on the pipe trench can be covered with sandy
gravel that can be sufficiently compacted. Thus the bearing capacity of the
pipe is increased considerably and differential settlements reduced.
Metal Culverts These flexible structures are able
to absorb very high radial pressures without any significant deformation, if
the all surrounding-embedding is carefully compacted (Fig. 2). If the subsoil
conditions are poor, nonwoven fabrics can be used instead. These geotextiles
enable a sufficient compaction of the base bedding and good drainage of the
water saturated subsoil during consolidation. The advantages of using flexible
culverts are lower cost, shorter construction period and reduced differential
settlements between embankments and culvert.
The great deformability of
flexible pipes and culverts is of advantage for heterogeneous soil conditions
and especially for embankments on soft subsoil. In such a case the culvert is
placed upon a thin replaced layer (on geotextiles) ahead of filling the dam.
Railway construction Due to heavy dynamic load
resulting from train traffic, the stony ballast is very often contaminated by
rising fine particles from the underlying soil. This "pumping up" not
only leads to a deformation of the ground, but also to a greater susceptibility
to freezing and thawing (in the ballast). This eventually causes a gradual
decrease in the bearing capacity of the upper structure. Graduated sandy-gravel
filters and synthetic nonwovens can be used as separating layers. The use of
separation and stabilization geotextiles keeps the subgrade soil from
pumping-up into the aggregate that support the pavement or unpaved aggregate
surface. Rigid soil stabilizers (cement, lime etc.) can not provide long-term
stabilization. There is also uncertainty about the long-term effectiveness of
bituminous sealing of the track bed.
Nonwoven fabrics with a
low stretch property should always be covered with a thin layer of coarse sands
in order to prevent perforation by the sharp stones of the ballast. This danger
exists during lying (particularly with tamping machines), when the subsoil is
inhomogeneous. It also exists later on through dynamic loading. Finally, the
coarse sand should protect the nonwoven fabric against damage caused by the
chains of the machines, which clean the ballast track bed.
Long-term observations
have shown that neither staple fiber nor thin rigid nonwovens are suitable, for
railway construction. Staple fiber nonwoven shows material fatigue (chemical
bonding) and thin rigid nonwovens because of mechanical damage. It is found
that the geotextiles made of endless long fiber that are mechanically bonded
generally fulfill the expectations.
Environmental
Applications
Nonwoven geotextiles play
a major role in engineered systems designed to achieve the stricter waste
disposal and emissions control regulations that improved over the past two
decades. Both liner and cover systems are layered systems that include hydraulic
barriers and liquid/ gas collection systems. Geotextiles can be used in
landfill systems to provide the following functions:
- Filtration:
the liquid collection systems must be protected against clogging caused by
soil or waste particles carried by the liquids.
- Protection:
The barrier systems include a thin geomembrane that must be protected from
puncture.
- Bio-barrier:
Impregnated with a biocide agent, geotextiles are being used to limit root
penetration into the landfill system.
- Barrier:
Geosynthetic clay liners (GCLs) use geotextiles to confirm bentonite
granules into sheets that can be used to form liners.
Filtration Collector/drain systems in the
covers beneath the waste must be designed to handle waters that may have high
total dissolved solids (TDS). Additionally, it may be a very open system that
must perform its role adjacent to layers of waste or soil. The geotextile must
then provide an opening size that will allow the liquid to enter the collector
system while retaining waste or soil particles. This filter layer can be
constructed using a geotextile or a graded soil filter. Both the filter and the
geotextile must be designed to provide adequate flow capacity and soil
retention to resist clogging. Graded filters are commonly used of several
layers of sands of different grades (fine to coarse) and are several feet
thick. Geotextiles serve the same function as them at reduced cost and space.
In general, most
geotextiles have permittivity values adequate for landfill applications. The
geotextile filter design determines the opening size required to retain the
soil of waste particles upstream of the geotextile. The 95% opening size (O95)
of the geotextile (ASTM D4751), is related to the particles retained through
the following type of relationship
O95 = |(d50,
CU, DR)
Where d50 is the 50% size
of the upstream particles, CU is the uniformity of the particle size; DR is the
relative density of upstream particles. An increase in the hydraulic flow
gradient across the geotextile or a significant reduction in flow rate affects
degree of clogging the geotextile.
Nonwoven materials are
vulnerable to clogging and biological growth, which can reduce the effective
permittivity of the geotextile 20 fold. Because of this, some European
countries and US designers have stopped using geotextile filters between the
waste and leachate collection system. All geotextiles and nonwovens in
particular resist efforts to back wash particles out of he fabric.
Protection Both covers and liners of waste
containment systems rely on a barrier system composed of several feet of
compacted soil covered by a thin geomembrane. The geomembrane is commonly a
60-mil HDPE or 40-mil PVC field seamed system. This thin membrane must be
protected from puncture both during construction and after the facility is
placed in service. The ability of a geotextile to protect the geomembrane from
puncture is more a function of its basis weight than its physical strength and
thickness.
Bio-barrier The geotextile in the cover
sections exposed to root growth from plants require prevention of cover soil
erosion. Such penetration remains the most critical challenge to the long-term
integrity of cover systems. The product has a limited life expectancy. It is a
function of moisture in the contact soil and also it is quite expensive.
Barriers Geosynthetic clay liners (GCLs)
were developed to waterproof building basements and for lining of landscaping
ponds. GCLs are made by bonding bentonite granules between layers of
geotextiles. The bonding can be performed by sandwiching the bentonite between
two nonwovens and then needling the layers or by the use of water-based
adhesive to bond bentonite between the two geotextiles.
Two problems associated
with these kinds of fabrics limit the application of these materials to
landfill slopes:
- The
geotextile surface has a low coefficient of friction when placed against
the geomembranes
- As the
bentonite hydrates, the strength of bentonite decreases and the GCL may
act as a plane of weakness
Hence, improved methods of
needling or sewing the two geotextiles that form the GCL together must be
developed.
The
Educational information in this section has been graciously donated to the
Apparel Search Company by Professor Kermit Duckett.
Submitted by: Terezie
Zapletalova and Rongguo Zaho
References:
1.
Mark L.
Marienfeld " Geotextiletrends and developments" Tappi Journal 78 (9)
: 143-146 (1995)
2.
Wang Xiang-nong
" Annual Textile Meeting Held In China" Nonwoven Industry, March : 41
(1997)
3.
Koerner, Welsh:
Construction and Geotechnical Engineering Using Synthetic Fabrics, John Wiley
& Sons Ltd., 1980
4.
Second
International Conference on Geotextiles, vol.II, August 1-6th, Las Vegas, p441
5.
IDEA'92, Book
of papers
