Extruded Geo-Grid
1. General

Geogrid is a planar, polymeric structure consisting of a regular open network of integrally connected tensile elements, which may be linked by extrusion, bonding or interlacing. They have open grid like configuration with large apertures between individual ribs. The key feature of all geogrids is that the apertures are large enough to allow soil communication, or strike through, from side of geogrid to the other. Geogrids have relatively high strength, high modulus and low-creep-sensitive polymers.

Geogrids provide uniform distribution of loads over a larger area by increasing stiffness of base, as the same resists flexural deformation. Geogrids is a synthetic planar structure formed by a regular network of tensile elements with apertures of sufficient size to allow interlocking with surrounding soil, rock, earth or any other geotechnical materials.

2. Types of Geo-grids

Depending on the strength of the geogrid in various directions, the following types of geogrids are available in the market:

(A) Uniaxial Geogrids:

Also known as mono-oriented geogrids, it is a planar grid, which possesses a much higher strength in one direction than in the other direction.

Fig. 01 : Uniaxial Geo-grid
(B) Biaxial Geogrids:

Also known as bi-oriented geogrids, it is a planar grid, which possesses similar strength in both ortho-directions i.e. longitudinal & transversal.

Fig. 02 : Biaxial Geo-grid
(C) Triaxial Geogrids:

A recent addition to the geogrid family, this product is having ribs in triangular pattern or other shapes so that it gives tensile strength in other than two ortho-directions also. These geogrids are manufactured from a punched polypropylene sheet oriented in multiple, equilateral directions to form triangular apertures, resulting in high radial stiffness throughout the full 360 degrees.

Fig. 03 : Triaxial Geogrid
3. Extruded Geogrids :

Extruded types of geogrids are produced by extruding polymers and by stretching uniaxially or biaxially extruded integral structure. These are generally rigid in nature compared to the more flexible types of bonded or woven geogrids.

Fig. 04 : Extruded Geogrid
4. How Geogrid Works

The desired reinforcement effect by the geogrid is provided by a combination of following mechanisms:

4.1 Lateral Confinement

  • It limits horizontal movements of aggregate and provides lateral confinement by interlocking(Fig. 05).
  • Increases stiffness, at equal deformations it is possible to apply a higher load.
  • Decrease in vertical stresses applied to subgrade and an increase of compressive horizontal stresses.

Fig. 05 : Lateral Confinement
4.2 Increase of load distribution angle

The inclusion of geogrid allows to increase the load distribution angle below rails (Fig. 06); in this way the pressure applied to subgrade is decreased, hence settlements and deformations are reduced.

Fig. 06 : Load Distribution angle below rails
4.3 Tension Member Effect

A loading capacity is generated, i.e. an upward vertical force that contributes to support the applied load, thus decreasing the stress applied to subgrade (Fig. 07).

Fig. 07 Tension Member Effect
5. Application of Bonded Geo-grid in Railway
5.1. Reduction in thickness of Blanket and/or Prepared Subgrade

Geogrid can be used to reduce thickness of Blanket Layer and/or Prepared Subgrade Layer (which are costly granular material) in case of new constructions, on techno-economic considerations and/or to reduce the adverse impact on environment due to quarrying/mining (Fig.08).

Design is to be submitted to RDSO for approval. [Clause 7.3 & 7.4 of Specifications RDSO/2018/GE:IRS-0004(D) Part-IV, July'2019].

Fig. 08
5.2 Rehabilitation/Strengthening of weak/unstable formation

Weak/unstable formations mostly have subsoil and/or subgrade of expansive clays (e.g. Black Cotton Soil). These soils swell when mixed with water, thereby losing shear strength, and shrink on drying. Swelling (due to ingress of water in rainy season) and shrinkage (in dry season) causes disturbance to track parameters and ballast penetration.

This problems can be addressed by reducing ingress of water in the formation, by providing a non-woven geotextile separator layer, which reduces water entering into the subgrade and also prevents upward migration of fine particles from fine grained expansive clays into the top coarse layers.

In addition, provision of a blanket layer acts as separator as well as reinforcement layer reducing the pressure on the formation below. In case providing blanket layer of larger thickness in running traffic condition is not possible, its' thickness can be reduced by providing a reinforcement layer of geogrid in the middle of the blanket layer.

The preferred arrangement is as shown in Fig.09 below:

Fig. 09

But in cases where it is not possible to adopt this method, then lesser preferred alternative is laying a separator layer of non-woven geotextile with a reinforcement layer of bi-axial geogrid over it, just below the ballast (Fig.10), during BCM working, by adding suitable attachments with BCM.

Fig. 10
Video of Laying Geo-synthetics with BCM

During subsequent deep screening cycles, 100-150mm thickness at bottom of ballast should not be disturbed, which will avoid entanglement of geo-synthetics with BCM and act as a confining layer for geo-synthetics, improving their efficiency.

In addition to above:

  • Provide proper cross slope, to prevent ingress of moisture from cess.
  • Provide proper erosion control measures on the side slopes, to prevent ingress of moisture from sides.

6. RDSO Specifications for Geogrid