Geogrids is a geosynthetic materials used as reinforcement in construction works. Types of geogrids, its functions and applications in construction works are discussed.
Geogrids can be categorized as geosynthetic materials that are used in the construction industry in the form of a reinforcing material. It can be used in the soil reinforcement or used in the reinforcement of retaining walls and even many applications of the material are on its way to being flourished.
The high demand and application of Geogrids in construction are due to the fact that it is good in tension and has a higher ability to distribute load across a large area.
Origin of Geogrids and its Manufacture
The geosynthetic material, geogrids, are polymeric products which are formed by means of intersecting grids. The polymeric materials like polyester, high-density polyethylene and polypropylene are the main composition of geogrids.
These grids are formed by material ribs that are intersected by their manufacture in two directions: one in the machine direction (md), which is conducted in the direction of the manufacturing process. The other direction will be perpendicular to the machine direction ribs, which are called as the cross-machine direction (CMD).
Fig. Geogrid’s rib formation in machine and cross machine directions of manufacturing process
These materials form matrix structured materials. The open space, as shown in the above figure, due to the intersection of perpendicular ribs are called as the apertures. This aperture varies from 2.5 to 15cm based on the longitudinal and transverse arrangement of the ribs.
Among different types of geotextiles, geogrids are considered stiffer. In the case of geogrids, the strength at the junction is considered more important because the loads are transmitted from adjacent ribs through these junctions.
Many manufacturing choices are available for ribs. Here we are going to discuss three most used method of manufacturing of geogrids:
Method-1: By Extruding
This manufacturing method of geogrids involves extrusion of a flat sheet of plastic into the desired configuration. The plastic material used may be high-density polypropylene or high-density polyethylene. Already set punching pattern is placed over the sheet to make holes for the formation of desired grids.
The punching of a pattern of holes would result in the formation of so-called apertures. Next step involves the development of tensile strength, by stretching the material to both longitudinal and transverse direction. A figure representing an extruded Geogrid is shown below.
Fig. A Geogrid manufactured from the method of extrusion
Method-2: By Knitting or Weaving
In this method of Geogrid manufacture, single yarns of polyester or polypropylene material undergo either knitting or weaving to form flexible junctions forming apertures. These materials are recommended to have high tenacity, to give the Geogrid the final desired property.
The product is bought into the market by giving them an additional coating of either bituminous material or a polyvinyl chloride or a latex. This choice varies with the manufacturer of geogrids.
Fig. A Geogrid sample manufactured by knitting
Method-3: By Welding and Extrusion
This is recently developed a method by Secugrid manufactures. The method involves the extrusion of flat polyester or polypropylene ribs by passing them through rollers as shown in the figure below. These are done in automated machines that run at different speeds, enabling stretching of ribs and increasing their tenacity.
Fig. The stretching of ribs through extrusion process
As shown in the figure below, the ribs obtained are sent to the welding section through either side. One in the machine direction and other in the perpendicular direction. Forming a high-quality Geogrid.
Fig. The welding of the ribs forming apertures
Functions and Working of Geogrids
The geogrids serve the function of holding or capturing the aggregates together. This method of interlocking the aggregates would help in an earthwork that is stabilized mechanically. The apertures in geogrids help in interlocking the aggregates or the soil that are placed over them. A representation of this concept is shown below.
Fig. A representation of Geogrid Confining the aggregates
The geogrids as mentioned above helps in redistribution of load over a wider area. This function has made the pavement construction more stabilized and strong. It has the following functional mechanisms when applied for pavement construction:
Tension Membrane Effect
This mechanism is based on the concept of vertical stress distribution. This vertical stress is from the deformed shape of the membrane as shown in the figure below. This mechanism was initially considered as the primary mechanism. But later studies proved the lateral restraining mechanism is the major criteria that must be taken into consideration.
Improvement of Bearing Capacity
Fig. Mechanism for Improved Bearing Capacity
One of the main mechanism happening after Geogrid installation in pavement is the reduction in lateral movement of the aggregate. This would result in the elimination of stresses; that if exists would have moved to the subgrade.
The Geogrid layer possesses sufficient frictional resistance that opposes subgrade lateral movement. This mechanism hence improves the bearing capacity of the layer. Reduction of outward stresses means inward stresses are formed, which is the reason behind the increase in bearing capacity.
Lateral Restraining Capability
The stresses produced by means of the wheel loadings coming over the pavement results in the lateral movement of the aggregates. Which in turn affects the stability of the whole pavement arrangement. The Geogrid act a restraint against this lateral movement.
Types of Geogrids
Based on the manufacturing process involved in geogrids it can be of
- Extruded Geogrid
- Woven Geogrid
- Bonded Geogrid
Based on which direction the stretching is done during manufacture, geogrids are classified as
- Uniaxial geogrids
- Biaxial Geogrids
These geogrids are formed by the stretching of ribs in the longitudinal direction. So, in this case, the material possesses high tensile strength in the longitudinal direction than on the transverse direction.
Here during the punching of polymer sheets, the stretching is done in both directions. Hence the function of tensile strength is equally given to both transverse and longitudinal direction.
Fig. Uniaxial and Biaxial Geogrids manufactured by the method of extrusion
Applications of Geogrids in Construction
Applications of Geogrids in Construction of Retaining Walls
The usage of geogrids in retaining wall construction is in the area of soil backfills. Holding the soil together will help in a stable retaining wall construction. The soil structural integrity can be increased by reinforcing it by geogrids. This help in confining backfill as well as help in distributing the loads. The geogrids solve the problems with soft backfill or sloping ground.
Fig. A Typical Arrangement of Geogrids in Retaining Walls
The increase in the length of geogrids will help in increasing the mass of the structure. This helps in building taller walls. The concept means the geogrids will make the whole unit to behave as a single mass. The minimum height from which the geogrid laying has to start depends upon the soil type, the extent of pressure the wall is subjected to from the backfill and other factors.
Characteristics of Geogrid Retaining Wall System
The geogrid retaining wall system has certain unique characteristics that differ when compared with the traditional retaining wall construction, like a concrete retaining wall and gravity retaining walls.
Fig. A completed Geogrid Retaining Wall
The geogrid reinforced retaining wall construction gain the following characteristics:
- The geogrid system is more flexible in nature. The retaining wall with the geogrid system has the higher adapting ability with the deformation of the foundation, when compared with traditional construction, which is very stiffer in nature.
- More flexibility implies they behave well as earthquake resistant
- This construction can be made more economical, compared to traditional method. The landfill can be made steeper, which shows a cost reduction. More height in wall and steepness are created with the help of reinforced soil system.
- The onboard Geogrid arrangement has afforestation protection. This brings environmental benefits, which is an important parameter in sustainable construction.
- The Geogrid retaining wall construction guaranty quality and reduced cost of construction. This helps in rapid and convenient construction.
- With time the geogrid reinforcing retaining wall construction and its advantages have earned appreciation, which had made its demand to increase in the construction in highways, railways, dams, ports, planning city and projects focusing on the environment.
Applications of Geogrids in Foundation Soil
Geogrids can be used to stabilize the soil below the foundation mainly in shallow foundation. To know that the modes of failure under the reinforced soil foundation has to be understood.
Four failures are observed to occur:
- Failure 1: Bearing Capacity Failure
- Failure 2: Pullout Failure of the Geogrid layer
- Failure 3: Breaking of the geosynthetic layer
- Failure 4: Creep Failure of the geosynthetic layer (Geogrid)
The figure below shows an arrangement of geogrid layer below a rectangular foundation. Let us consider the dimension of foundation be B x L and that of geogrid layer be b x l (in the width x length form as shown in the figure below).
As shown in figure, ‘h’ is the distance between each geogrid layer. The first geogrid layer is placed at a height ‘u’ below the ground level.
If there are ‘N’ layers of geogrid reinforcement, the total depth of the geogrid can be given by the equation
d = u + (N – 1) h –> Equation-1
Fig.1: Cross section and top plan of a rectangular foundation supported over a Geogrid reinforced soil
The figure-2, below show a general relation between the load and the settlement caused for a foundation under two cases:
- Reinforced soil, and
- Unreinforced soil.
The effect of reinforcement can be measured in terms of bearing capacity ratio (BCR). The bearing capacity ratio is formed with the help of the ultimate bearing load capacity at a given maximum settlement.
Say BCRU be the bearing capacity ratio when measured in terms of ultimate load. Then from the figure below
BCRU = qu(R)/qu –> Equation-2
If BCRS is the bearing capacity ratio at a given settlement . Let it be Se, then
BCRS = qR/q –> Equation-3
Fig.2: Load-settlement Curve for a foundation that is supported with reinforced and unreinforced soil
Fig.3: The variation of ultimate bearing capacity with the u/B ratio
The figure 3, above shows the variation of bearing capacity with the variation of u/B ratio. It is seen that BCRu is maximum at a value of u/B > (u/B)cr. At a value say (u/B)max, the BCRu value is lower.
The first range is said as zone-1, the range between (u/B)cr and (u/B)max is called as zone-2, the range for u/B > (u/B)max is zone 3.
Figure 4 and 5 shows the respective failure surface for zone 1,2 and zone 3 respectively.
Fig.4: Zone-1 and Zone-2 conditions
Fig.5: Failure surface in Zone 3
It can be generalized that under the zone 1, the increase in the bearing capacity ratio is due to the most confining pressure of the geogrid layers. The zone 3 have a lower bearing capacity, as it acts as semi-rigid in nature.
Applications of Geogrid in Pavement Construction
The Geogrid construction in pavement construction have following features:
- Improvement of subgrade: The subgrade, which is the most important load bearing strata, is made solid and strong by the geogrids. The problem of soft subgrade can be solved by this method.
- Reinforcement of pavement base: The thickness of base if increased would increase the stiffness of base. But increasing thickness enormously is not economical. The reinforcement to a given base layer would give adequate stiffening that helps in reduction of thickness and time of construction. This also helps in increasing the life of the pavement.
The procedure involved in geogrid construction for subgrade preparation are shown in figure6,7 and 8.
Fig.6: Laying the Geogrids over leveled soil sub-base as reinforcement
Fig.7: Placing Aggregates over the Geogrid layer
Fig.8: Final Compacted and rolled
Advantages of Geogrids in Construction
- Ease of Construction: he Geogrid can be installed in any weather conditions. This makes it more demanding.
- Land Optimization: This method of Geogrid installation in soils makes an unsuitable area suitable for preparing it to meet desired properties for construction. Geogrid thus helps in proper land utilization.
- Geogrid promotes soil stabilization
- A higher strength soil mass is obtained
- Higher load bearing capacity
- It is a good remedy to retain soil from erosion
- No requirement of mortar. The material is implemented dry.
- No difficulty in material availability
- Geogrids are flexible in nature. They are known for their versatility.
- Geogrids have high durability reducing maintenance cost. They are highly resistant against environmental influences.
- Materials are tested based on standard codes and regulations.