Geotextile: Soil Improvement Techniques

SOIL IMPROVEMENT TECHNIQUE
Kamble Zunjarrao B.
D.K.T.E.S. Textile & Engineering Institute
Ichalkaranji, India
Email: kamblezunjar@gmail.com
 




ABSTRACT:
Soil alone is strong enough in compression but comparatively weak in tension. Reinforcing soil is the technique where tensile elements are placed in the soil to improve stability and control deformation. The geotextiles are used as reinforcement, their prime role is to provide tensile strength to soil at strain level which is compatible with the performance of the soil structure. Textiles are used as reinforcement in the form of fibers, fabric form like woven, knitted, non wovens. Geosynthetics are used as reinforcement in paved roads, in railway tracks, embankment of shallow weak soils, earth retaining walls, mining subsidence protection etc. This papers deals with the different types of geosynthetics which are majorly used in reinforecement of soil, so that the soil gets stabilized and the problems like erosion can be controlled.

KEYWORDS: Geosynthetics, tensile reinforcement, non-woven geotextiles, surface reinforcement.

Introduction
The first auther to advocate the concept of reinforced soil was probably C .W. Pasley in 1822. The technique of reinforcing soil in its modern form was first developed by M. Henri Vidal & who named his system as ‘Reinforced Earth’. The soil is strong in compression & weak in tension, and therefore to provide tensile strength to soil, the reinforcement of soil is necessary. The three main areas where soil reinforcement may be applied are as follow: slope & embankments, foundations, retaining walls. The introduction of geomaterials with higher strength & higher tensile stiffness make novel solution for geotechnical problems.

Now, we should first understand what are the geotextiles, their types & then their actual practical application & implementation. The geotextiles are defined as permeable textiles used in conjunction with soil or rock, as an integral part of manmade projects. Every textile applied under the soil is a geotextile. Geotextiles are functioning as separator, filter, drainage material, reinforcement, sealing & protection. Depending upon the required function, they are used in open mesh form, woven form, non-woven form & knitted form. The geotextiles are broadly classified into two categories as biodegardable & non-biodegradable. The geotextiles which are non- biodegradable are also named as geosynthetics as their basic raw material is manufactured from petroleum products.

Different Categories of Geosynthetics:
  1. Geotextiles- These are flexible textile fabrics of controlled permeability used to provide filtration, separation or reinforcement in soil, rock and waste material.
  2. Geomembranes- These are impermeable polymeric sheets used as carrier for liquid or solid waste containment.
  3. Geogrids- Stiff or flexible polymer grid like sheets with large aperture used primarily as reinforcement of unusable soil and waste masses.
  4. Geonets- stiff polymer net like sheets with in plane opening used primarily as a drainage materials within landfills or soil and rock masses.
  5. Geosynthetic clay liners- prefabricated bentonite clay layers incorporated between geotextile and geomembrane and used as a barrier for liquid or solid waste containment.
  6. Geopipes- Perforated or solid wall polymeric pipes used for the drainage of various liquids.
  7. Geocomposites- Hybrid systems of anyor all the above geosynthetics types which can function as specifically designed for use in soil,rock, waste and liquid related problems.
  8. Geofoam- A newer category of product is geofoam. Which is the generic name for any foam material utilized for geotechnical application. Geofoam is manufactured into large blocks which are stacked to form a light weight thermally insulating mass buried within a soil or pavement structure.
Fig. Geocell
Different Types of Geotextiles:
Woven geotextiles – Woven geotextiles are manufactured from by adopting technique similar to clothing textiles. This type has characteristic appearance of two stes of parallel threads or yarns.They have a surprisingly wide range of applications and they are used in lighter weight form as soil separators, filters and erosion control textiles. In heavy weights, they are used for soil reinforcement in steep embankments and vertical soil walls; the heavier weight products also tend to be used for the support of embankments built over soft soils. The beneficial property of the woven structure in terms of reinforcement, is that stress can be absorbed by the warp and weft yarns and hence by fibres, without much mechanical elongation. This gives them a relatively high modulus or stiffness.

Non-woven geotextiles – Non-woven geotextiles can be manufactured from either short staple fibres or continuos filaments. The fibers can be bonded together by adopting thermal, chemical or mechanical techniques or a combination of techniques. The type of fibre (staple or continuous) used has very little effect on the properties of the non – woven geo synthetics. Non-woven geotextiles are manufactured through a process of mechanical interlocking or chemical or thermal bonding of fibres/filaments.

Knitted geotextiles – knitted geosynthetics are manufactured using another process which is adopted from clothing textiles industry. In this process, interlocking a series of loops of yarn together is made. The majority of knitted geosynthetics made from polypropylene & polyester fibres. Knitted fabrics, as used in the field of geotextiles, are restricted to warp-knitted textiles, generally specially produced for the purpose.Warp-knitting machines can produce fine filter fabrics,medium meshes and large diameter soil reinforcing grids. However, it is generally found that only the high strength end of the product range is cost effective, usually for soil reinforcement and embankment support functions.
Fig.woven geotextile structure    Fig.non woven geotextile structure   Fig. Knitted geotextile structure
How geotextile functions as reinforcement in soil:
Load on the soil produces expansion. Thus, under load at the interface between the soil and reinforcement (assuming no slippage occurs, i.e. there is sufficient shear strength at the soil/fabric interface). These two materials must experience the same extension, producing a tensile load in each of the reinforcing elements that in turn is redistributed in the soil as an internal confining stress. Thus the reinforcement acts to prevent lateral movement because of the lateral shear stress developed. Hence, there is an inbuilt additional lateral confining stress that prevents displacement. This method of reinforcing the soil can be extended to slopes and embankment stabilisation.

Strength created by the introduction of geotextile into the soil & developed primarily through the following three mechanisms-
  1. Lateral restraint through interfacial friction between geotextile and soil/aggregate.
  2. Forcing the potential bearing surface failure plane to develop at alternate higher shear strength surface.
  3. Membrane type of support of the wheel load
The structural stability of the soil is greatly improved by the tensile strength of the geosynthetic material. This concept is similar to that of reinforcing steel to the concrete. Since concrete is weak in strength & tension, reinforcing steel is used to strengthen it. Geotextile materials function in a similar manner as the reinforcing steel by providing strength that helps to hold the soil in place. Reinforcement provided by the geotextiles and geogrids allow embankment & roads to be built over very weak soils & allows for steeper embankments to be built.

Steep faced embankment reinforcement using geosynthetics:
To construct a very steep slope at an angle at an inclination of 75 0 or more to the horizontal, then the structure would be more akin to an inclined retaining wall. The stress concentration beneath the steep faced embankment usually precludes their use over soft deposite. With reinforced embankment slope above or near to the natural angle of repose of the fill careful thought must be given to the surface finish. If the geotextile grid with an aperture size greater than the diameter of the fill particles is used, then the geptextile filer sheet should also be palced behind the reinforcemment grid at slope face. Usually the face will be covered with top soil & seeded. The wrap-around method envolve folding the geotextile over the exposed slope edge upto the underside of the next reinforcement layer as per the required angle & then anchoring the free end by burial within the fill.

The Tabing-Duku project near the town of Padang of the Indonesian is-lands, Sumatra, required an existing road from the airport to the city center to be widened at the most cost-efficient price possible. In spite of extremely problematic ground conditions on the site with low load-bearing capacities and a high ground-water level. The solution involved an embankment reinforced with uniaxial Secugrid® R geogrid. The reinforced slope had an inclination of more than 50° and was constructed with the wrap-around method. The slope surface was finally covered by natural vegetation.
Fig. Reinforced slope during construction                    Fig. Finished Project
Geotextile are also used for better compaction of the fill. This application is particularly well established for railway embankment in japan. As railway embankments are relatively narrow incomparison with highway embankment, it follows that greater proportion of the embankment will suffer from impaired compaction in the case of railway embankment. The climatic & seismic conditions in the japan make poorly compacted embankments faces susceptible to surface movement & erosion. On the uestu railway in japan, it was found that the presence of geogrid within the embankment enhanced both the degree of compaction & stiffness of the soil.
Fig. Collapsed railway embankment after flooding      Fig. Geogrid-reinforced railway embankment
Column supported embankments(CSEs) reinforcement with geosynthetics:
The problem associated with constructing highway embankments over soft compressible soil have led to the development and use of many ground improvement techniques. CSEs consists of vertical column that are designed to transfer the load of the embankment through the soft compressible soil layer to a firm foundation. The selection and type of column used for the CSEs will depends on the design loads, constructability of the column, cost, etc. the load from the embankment must be effectively treansferred to the column to prevent punching of the column through the embankment fill which causes differential settlement at the surface if the embankment. If the columns are placed relatively close together, soil arching will occur and the load will be transferred to the columns. Some battered columns are required at the sides of the embankment to prevent lateral spreading. In order to minimize the number of columns required to support the embankment and to increase the efficiency of the design, a geosynthetically reinforced load transfer platform(LTP) may be used. The LTP consists of one or more layers of geosynthetic reinforcement placed between the top of the columns and the bottom of the embankment. The first application of a CSE with geosynthetic reinforcement in the USA was in 1994 for the westway terminal in philadelphia, pennsylvania.

Subgrade stabilization & base reinforcement using geotextiles in roads:
A large variety of detrimental factors affect the service life of roads and pavements including environmental factors, subgrade conditions, traffic loading, utility cuts, road widenings, and aging. The four main applications for geosynthetics in roads are subgrade separation and stabilization, base reinforcement, overlay stress absorption and overlay reinforcement. Subgrade stabilization and base reinforcement involve improving the road structure as it is constructed by inserting an appropriate geosynthetic layer. Subgrade separation and stabilization applies geosynthetics to both unpaved and paved roads. Base reinforcement is the use of geosynthetics to improve the structure of a paved road.

Permanent roads carry larger traffic volumes and typically have asphalt or portland cement concrete surfacing over a base layer of aggregate. The combined surface and base layers act together to support and distribute traffic loading to the subgrade. Problems are usually encountered when the subgrade consists of soft clays, silts and organic soils. This type of subgrade is often water sensitive and, when wet, unable to adequately support traffic loads. If unimproved, the subgrade will mix with the road base aggregate – degrading the road structure - whenever the subgrade gets wet. The geotextiles used for reinforcement of road can be natural or synthetic. The natural geotextile e.g. jute geotxtiles, whereas the synthetic includes synthetic geotextiles geogrids, geonet, Geosynthetic clay liners etc.

In paved roads, lateral restraint called confinement is considered to be the primary function of the geosynthetic. With the addition of an appropriate geosynthetic, the Soil-Geosynthetic- Aggregate (SGA) system gains stiffness. The stiffened SGA system is better able to provide the following structural benefits:
  1. Preventing lateral spreading of the base.
  2. Increasing confinement and thus stiffness of the base.
  3. Improving vertical stress distribution on the subgrade.
  4. Reducing shear stress in the subgrade.
Fig. Load Spreading Phenomenon of Sub-base on Sub Grade
Application of geotextiles for roads in Ichalkaranji:
This was the first project in 1990, when geotextiles are used in india. The problem was deterioration of road frequently. Woven geotextiles were used in road & the problem was reduced to a great extent. To check the efficacy of geotextile, the geotextile was excavated after 10 years. Test results showed that there is no significant change in strength inmachine direction, proving that the geotextiles can be a long time solution for problem of the road.

(Table - The properties of geotextile used)
Particulars
Before burial
After burial
Mass per unit area, GSM
225
440
Thickness, mm
2.38
1.20
Breaking strength(KN/m)      MD
4.12
4.94
                                                CD
14.88
7.74
 
Application of geosynthetics in rail track stabilization:
Geosynthetics have been used in various ways in new rail tracks and track rehabilitation for almost three decades. When appropriately designed and installed, geosynthetics provide a cost-effective alternative to more traditional techniques. There are several problems required to be corrected in railway tracks, increasing the bearing capacity of the subgrade soil, preventing contamination of the ballast by subgrade fines, and dissipating the high pore water pressures built up by cyclic train loading. The woven fabrics or non-wovens are used to separate the soil from the sub-soil without impeding the ground water circulation where ground is unstable. Enveloping individual layers with fabric prevents the material wandering off sideways due to shocks and vibrations from running trains.

Maintaining track bed geometry is critical for efficient railroad operation. Subgrade pumping into the overlying ballast can create an uneven track bed, resulting in delayed arrivals and even derailments. Geotextiles perform multiple functions in railroad applications. Nonwoven fabrics are used to stabilize both new and rehabilitated tracks. They prevent contamination of new ballast with underlying fine-grained soils and provide a mechanism for lateral water drainage. Using nonwoven geotextiles beneath track beds ensures that the ballast can sustain the loads for which it was designed. These geotextiles are used in all track applications, including switches, turnouts and grade crossings. High-strength woven geotextiles can also be used to reinforce weak subgrade soils and reduce required embankment fill materials.
Fig. Placement of gextextile under railway track.
Discussed the physical and mechanical properties of ballast that affect the performance of railtracks. The results of cyclic tests on ballast, based on large-scale cylindrical triaxial testing, indicate that the ballast particle size distribution has a significant influence on ballast degradation,with the uniformly graded distribution being the most prone to breakage. The findings of this study suggest that the deformations of fresh and recycled ballast vary non-linearly with the number of load cycles. Irrespective of the type of ballast, reinforcement and saturation,the settlement of ballast stabilizes within about100000 loadcycles. The experimental results of this study clearly showed that with the insertion of any type of selected geosynthetics the extent of degradation and settlement in fresh and recycled ballast were reduced. It is also recommended that a bonded geosynthetics be employed because of the need to prevent the ingress of liquefied mud into ballast voids under cyclic loads, and to maintain an efficient pore pressure dissipation layer. The effectiveness of geosynthetics in improving fresh ballast behavior (deformation and degradation) was marginal, whereas it was more evident when used with recycled ballast in wet or dry conditions. According to the results, the inclusion of geocomposites in recycled ballast reduces the breakage index almost to that of fresh ballast(without geosynthetics). Hence the use of recycled ballast stabilized with geosynthetics would be a cost-effective and environmentally attractive option. The ballast and its engineering behavior have a key role in governing the stability and performance of railway tracks. The deformation and degradation behavior of ballast under static and dynamic loads was studied based on large-scale triaxial testing. The possible use of different types of geosynthetics to improve the performance of fresh and recycled ballast was also investigated .

Soil reinforcement for Rainfall erosion control:
On steep ground with little or no covering of vegetation, rainfall erosion can be a major problem. Erosion-susceptible slopes may occur naturally, for instance where vegetation is unable to become established because of poor or very thin topsoil, or where vegetation is suddenly removed by a forest fire.
Vegetative cover
Annual soil loss (tonnes/ hectare)
Forest
0.01
Grass
0.04
Crops
40
None
240
 
Table showing Relationship between soil erosion & ground cover.

The geotextile based methods of limiting rainfall erosion by stabilizing the soil can generally be devide into two categories- A. Surface cover geotextiles, B. Surface reinforcement geotextiles.

A. Surface cover geotextiles:
The surface cover geotextiles are providing temporary cover over the soil surface which dissipates the raindrop impact energy in a similar manner to foliage. Erosion control geotextiles currently available are in two different forms:

1. Paper strips held together by a knitted polymer yarn .They are placed over the ground surface after seeding and should ideally decompose sufficiently for the seedlings to push through shortly after germination. The paper strip geotextile sheet is normally anchored at the top of the slope by burial in a trench.

2. Woodwool sandwiched between two layers of polymer net.The woodwool geotextile are consists of shredded pine wood and have a weight of about 0.5 kg/m2. The outer netting is often made from 0.2mm diameter polypropylene yarn with a typical aperture size of about 35*25 mm. The greater selfweight of the woodwool geotextile and the interlocking action of plant shoots growing into the tangled woodwool also make it less vulnerable to being pushed up by seedlings. As well as protection against raindrop impact, woodwool geotextiles act as a thick blanket with many of the attributes of a conventional mulch, namely:
  • Limiting the speed of any rainfall run-off.
  • Reducing the evaporation from the soil.
  • Protecting germinating seeds from extremes in temperature. There geotextiles also provide protection against wind erosion, making then suitable for coastal same dunes.
B. Surface reinforcement geotextiles-
Surface reinforcement geotextilesfunctions in a similar manner to plant roots by reinforcing the soil surface & holding the soil particles together. Unlike surface cover geotextiles, geotextile mats are seeded after the geotextile has been laid. Another difference is that the sheet of geotextile mats are usually unrolled shallow slow, rather than laid parallel to the ground contours. After the geotextile mat has been secured on the ground surface, seed(usually grass) is sown through the mat & mixture of topsoil & seed then brushed over the mat to completely feel it. The celluler geotextiles are used for reinforcing the topsoil layer. The cellular geotextile is formed from a mechanically bonded nonwoven products which has been partially impregnated with resin in order to give it slight rigidity.

The four main sub-divisions of surface reinforcement geotextiles are:
  1. Thick three dimensional mats
  2. Cellular geotextiles
  3. Geotextileswoven from thick, widely spaced yarns
  4. High profile geotextiles nets.
Geo textiles for reinforcement of retaining walls:
Retaining walls help to maximize their land use. However, building a concrete gravity or crib wall is often impractical because of their high construction cost. Geotextiles are used for a wide assortment of reinforcement applications, including embankments over soft soils, levees and retaining walls. Geotextiles are well-suited to construction of walls with timber, precast panel and segmental block facing. In fact a geotextile retaining wall can be built for less than half the cost of a conventional wall. Woven geotextiles offer other significant advantages over conventional methods, such as simplified installation and construction, and the ability to use on-site backfill material. Polypropylene geotextiles cost approximately half the amount of polyester and polyethylene geogrids, and they require considerably less labor to install.

Geotextiles for Sports field construction & reinfocrement:
Geotextiles are widely used in the construction of Caselon playing fields and Astro turf. Caselon playing fields are synthetic grass surfaces constructed of light resistance polypropylene material with porous or nonporous carboxylated latex backing pile as high as 2.0 to 2.5 cm. Astro Turf is a synthetic turf sport surface made of nylon 6,6 pile fibre knitted into a backing of polyester yarn which provides high strength and dimensional stability. The nylon ribbon used for this is of 55 Tex. It is claimed that the surface can be used for 10 hr/day for about 10 years or more. Modern Astro Turf contains polypropylene as the base material.

Hockey field in Hamilton, New Zealand was stabilized using geotextiles. Nonwoven geotextile was used as a solution to separate weak pumping subgrade and clean gravel base. The actual subgrade CBR achieved on site was seven. Nonwoven geotextile was laid on the subgrade which enabled the subbase and base course to remain clean and maintain its strength, with the primary function to allow water to pass quickly and lower pore pressure build up in the subgrade.
Fig. Reinforced retaining wall              Fig. Hockey field, Hamilton, New Zealand
Conclusion:
Textiles are not only clothing the human body but also our mother land in order to protect her. The structural stability of the soil is greatly improved by the tensile strength of the geosynthetic material. Geotextiles like geonets, geogrids, geocomposites are functioning as separator, filter, drainage material, reinforcement, sealing & protection. The reinforcement acts to prevent lateral movement because of the lateral shear stress developed. Reinforcement provided by the geotextiles and geogrids allow embankment & roads to be built over very weak soils & allows for steeper embankments to be built. Therefore the geotextiles are applied in paved roads, rail road embankment stabilization, erosion control, sport field construction.

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  17. www.fiber2fashion.com
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  21. www.gmanow.com 
 

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