Geosynthetics for Ground Improvement

Long ago, when difficult sites for construction purposes were to be dealt, the conventional practice was limited to either the replacement of unsuitable soils or adopting suitable foundation which sometimes increases the cost of foundations. Innovative soil modification approaches are evolved to solve soil related problems. One among them is the usage of geo-synthetics. When used to enhance the soil strength they have following advantages.

• They are space savings,
• Better material quality control,
• Better construction quality control,
• Cost savings,
• Technical superiority,
• Construction time saving,
• Material deployment,
• Material availability,
• Environmental sensitivity.

Different Types of Geosynthetics

10 different types of geosynthetics are given below.

1. Geotextiles

Geo-textiles are indeed textiles in a traditional sense, but consist of synthetic fibres rather than natural ones like cotton, wool and silk. Thus bio-degradation is not a problem. The major point is that they are porous to water flow across their manufactured plane and also within their plane, but to a widely varying degree. Geo-textile Polymer is manufactured from polyester or polypropylene. Polypropylene is a material lighter than water (it has a specific gravity of 0.9). It is considered to be strong and very durable. Polyesters used are heavier than water and it gives excellent strength and creep properties. There are two types of geotextiles. They are woven and non-woven geotextiles. The woven yarns and non-woven geotextiles are manufactured using polypropylene filaments and staple fibers. Non-woven types are manufactured from Staple fibers. They usually are 1 to 4 inches in length or a continuous filament randomly distributed in layers onto a moving belt to form a felt like “Web”. It is mainly used as a surface drainage. Woven geotextiles are made from weaving monofilament, multifilament or slit film yarns. Slit film yarn is further classified into Flat tapes and Fibrillated yarns. There are two steps in the process of making woven textiles. They are manufacturing of the filaments and weaving. Slit films are used in sediment control and road stabilization works but are poor choice for sub surface drainage and erosion control works as they have low permeability. Alternatively fabrics made with fibrillated tape yarns have better more uniform openings and permeability than the flat tape.

2. Geogrids

Geo-grids are plastics formed into a very open netlike configuration. Single or Multi-layer materials are usually made from extruding and stretching high density polyethylene or by weaving or knitting the polypropylene. The resulting grid structure possesses large openings called apertures. These apertures enhance the interaction with the soil and aggregate. It is a good soil and aggregate reinforcement due to its good tensile strength and stiffness.

3. Geonets

Geonets are stacked criss-crossing polymer strands that provide in-plane drainage. The geonets are all made of polyethylene. The molted polymer is extruded through slits in counter rotating-dies which forms a matrix or a net of closely spaced “stacked” strands. When the layers of strand are two then it is called as “biplanar” and three layers of strand are called “triplanar”.

4. Geocomposites

Geocomposites are geotextile filters surrounding a geonet. Some of the functions of the geocomposites are as blanket drains, panel drains, edge drains and wick drains. Blanket drains are generally used as Leachate, Infiltration collection, removal layers within landfill. Panel drains are placed adjacent to the structure to reduce the hydrostatic pressure. Edge drains are used adjacent to pavement structures which helps collect and remove lateral seepage from the road base.

5. Geomembranes

Geomembranes are impervious thin sheets of rubber or plastic material primarily used for linings and covers of liquid- or solid-storage impoundments. Thus the primary function is always as a liquid or vapor barrier. They are relatively impermeable when compared to soils or geotextiles. They are divided into two general categories, they are, Calendered and Extruded. For Calendered type, materials used are polyvinylchloride, chlorosulphonated polyethylene, chlorinated polyethylene and polypropylene. For Extruded type, material used is high dense polyethylene.

6. Geosynthetic Clay Liners

Geosynthetic clay liners (GCLs) include a thin layer of finely-ground bentonite clay. The clay swells and becomes a very effective hydraulic barrier when wetted. GCLs are manufactured by sandwiching the bentonite within or layering it on geotextiles and/or geomembranes. The bondings of the layers are done with stitching, needling and/or chemical adhesives.

7. Geofoam

Geo-foam is a newer category of the geosynthetic product. It is a generic name for any foam material utilized for geotechnical applications. Geofoam is manufactured in large blocks which are stacked to form a lightweight and thermally insulating mass buried within the soil or pavement structure. The most common type of polymer used in the manufacturing of geofoam material is polystyrene. The applications of geofoams are

• It is used within soil embankments built over soft, weak soils
• Used under roads, airfield pavements and railway track systems which are subjected to excessive freeze-thaw conditions
• Used beneath on-grade storage tanks containing cold liquids.

8. Geopipe

Another significant product which has been adopted as a geosynthetic is the plastic pipe. The specific polymer resins that are used in the manufacturing of plastic pipes are high-density polyethylene (HDPE), polyvinyl chloride (PVC), polypropylene (PP), polybutylene (PB), acrylonitrile butadiene styrene (ABS), and cellulose acetate buytrate (CAB). There is a wide variety of civil engineering applications for these products. These include leachate removal systems, interceptor drains, and highway and railway edge drains.

9. Turf Reinforcement Mats

Turf reinforcement mats (TRMs) are 3-dimensional structures composed of fused polymer nettings, randomly laid monofilaments, or yarns woven or tufted into an open and dimensionally stable mat. Erosion protection can be increased by applying these Mats, which can provide more protection compared to that of plants grown normally. Proven performance has resulted in the broad use and ensured the acceptance of TRMs as a permanent, cost effective and environmentally friendly alternative to hard armor erosion protection solutions such as concrete and riprap.

10. Geocell

3-D honey comb like structures filled with soil, rock and concrete. They are made of strips of polymer sheets/ geotextiles, connected at staggered points in order to form a large honey comb mat when its strips are pulled apart. Geo-cells were manufactured from a novel polymeric alloy called Neoloy. The geocell with a higher elastic modulus has stiffness of the reinforced base and a higher bearing capacity. Geo-cells made from NPA are found to be significantly better in stiffness, ultimate bearing capacity and reinforcement relative to geocells made from HDPE. NPA geocells show better creep resistance and better retention of creep resistance and stiffness particularly at elevated temperatures, as verified by plate load testing and numerical modeling. A full scale research demonstrated that NPA geocells have a lower thermal expansion coefficient and creep reduction factor. It showed a higher tensile stiffness and strength than HDPE geocells and NPA geocells increased the bearing capacity and reduced settlement of compacted sand base courses significantly more than geocells fabricated from HDPE.