Geogrid Reinforced or Stabilized flexible pavement

The estimated total length of road in India as per the IBEF report, June 2020 is around 5.89 million kilometers (km) making it the second-largest road network in the world after the United States. This road network transports 64.5 percent of all goods in the country and 90 percent of India's total passenger traffic uses the road networks to commute. Road transportation has gradually increased over the years with improvements in connectivity between cities, towns, and villages in the country. The sale of automobiles and the movement of freight by roads is growing at a rapid rate.

But these roads are not giving the desired result due to the poor CBR value and increased loading.  They are mostly subjected to problems like the formation of potholes, ruts, cracks, localized depression, and settlement, especially during the rainy season. These are mainly due to the insufficient bearing capacity of the subgrade in water-saturated conditions. The subgrade soil mostly yields low CBR value than required. Pavement design as per IRC:37-2018 presents the thickness of pavement which is increasing with a decrease in the CBR value of subgrade soil which increases the cost of construction. So the incorporation of geogrid in pavement layers not only helps to alter the geotechnical properties of existing pavement material but also helps to reduce the layer thickness which leads to a sustainable cost-effective solution MoRT& H clause 703.2.2 mentions the use of geogrids in case of flexible pavement.

As per IRC SP 59 clause 1.9, Geogrids helps in stabilizing and reinforcing different layers of the pavement to provide subgrade restraint, to stabilize the subbase or and base course. It reinforces the bound layer i.e. the surface course to increase the service life of the pavement by preventing fatigue and reflective cracking.

So, the use of geogrid helps to stabilize, minimize, and reinforced the pavement section.  Numerous experimental and numerical study has been done on this material and results were positive which not only helps in reducing the pavement construction cost but also the maintenance cost of the project. 

Typical flexible pavement consists of a bituminous surface course over base course and sub-base course. The surface course may consist of one or more bituminous or Hot Mix Asphalt (HMA) layers.

These pavements have negligible flexure strength and hence undergo deformation under the action of loads. The structural capacity of flexible pavements is attained by the combined action of the different layers of the pavement. The load from trucks is directly applied on the wearing course, and it gets dispersed (in the form of a truncated cone) with depth in the base, subbase, and subgrade courses, and then ultimately to the ground. Since the stress induced by traffic loading is highest at the top, the surface layer has a maximum stiffness (measured by resilient modulus) and contributes the most to pavement strength. The layers below have lesser stiffness but are equally important in the pavement composition. The subgrade layer is responsible for transferring the load from the above layers to the ground. Flexible pavements are designed in such a way that the load that reaches the subgrade does not exceed the bearing capacity of the subgrade soil. Consequently, the thicknesses of the layers above the subgrade vary depending upon the strength of soil affecting the cost of a pavement to be constructed.

MoRT & H clause 700-3 and IRC SP-59 specifies the minimum junction strength efficiency of the geogrid.

It is an important criterion for selecting the geogrid in case of pavement stabilization.

In base and subbase stabilization geogrids are placed within or at the bottom of bound layers. Extruded biaxial geogrids are polypropylene (PP) produced by an extrusion process characterized by a tensile resistance both in the longitudinal and cross direction that exhibits high modulus and high strength at low elongations providing tensile reinforcement to soil and aggregate structures. All soils, whether cohesive or granular, have poor resistance to tensile stresses, making them prone to movement and potential failures. MacGrid EG (Geogrid) has high junction strength hence it distributes applied loads over a greater area to reduce vertical pressure on the subgrade. This reinforces and stabilizes the base course materials (unbound layers) and reduces the thickness of the granular structure layer required. Geogrid shall be resistant to installation damage, long-term degradation, and chemicals found in most soil environments. The high tensile strength and junction efficiency of biaxial geogrids confines and restrains aggregate from lateral movement, ensuring proper distribution of imposed stresses and ultimately extending the service life. Extruded geogrids exhibit better interlocking properties, junction stiffness, and very high 

modulus which means this will pick up the stresses quickly with little or no movement in the overlying base materials compared to other geogrids.

MacGrid EG is used for reinforcing and stabilizing the granular layer. The provision of reinforcement with biaxial extruded PP geogrid increases the elastic modulus of the granular layer and increases the load-carrying capacity. MacGrid EG shall reinforce the granular layer and the basic mechanism of reinforcing can be identified as (a) lateral restraint, (b) improved bearing capacity, and (c) tensioned membrane effect.

Lateral restraint refers to the confinement of the aggregate material during loading, which restricts the lateral flow of the material from beneath the load. Since most aggregates used in pavement systems are stress-dependent materials, improved lateral confinement results in an increase in the modulus of the base course material. The effect of increasing the modulus of the base course is an improved vertical stress distribution applied to the subgrade and a corresponding reduction in the vertical strain on the top of the subgrade. Figure 1-a illustrates the lateral restraint reinforcement mechanism.

The second mechanism, improved bearing capacity, is achieved by shifting the failure envelope of the pavement system from the relatively weak subgrade to the relatively strong base course material. Figure 1-b shows the improved bearing capacity concept.

The third fundamental reinforcement mechanism has been termed the “tensioned membrane effect.” The tensioned membrane effect is based upon the concept of an improved vertical stress distribution resulting from tensile stress in a deformed membrane. Figure 1-c illustrates the tensioned membrane effect.

The thickness of the base course can be reduced by 20 to 30% and the subbase can be reduced by 10 to 30% respectively by reinforcing with extruded geogrid, thereby reducing the quantity of material and material laying cost. This shall reduce the overall project cost.

This percentage reduction in cost and thickness of pavement depends upon several factors like CBR value, properties, cost, and availability of the granular material, (base course and subbase course) and a number of layers.

The reinforced sections can be designed by introducing the geogrid at the base course or can be designed considering

 geogrid at the base and sub-base course within the pavement. Fig. 2 depicts the thickness reduction with geogrid at base layer/ base and subbase layer respectively. In the case of fig.3, the same thickness of granular layer for unstabilized and for stabilized sections is provided. This kind of solution shall help in reducing the maintenance cost and shall increase the life of the pavement.

Based on the CBR value of the existing soil the elastic modulus for base and subbase material is determined. As per figure 5, horizontal and vertical strain are calculated for the unreinforced section. The modified resilient modulus is determined by incorporating the LCR value which is pertaining to geogrid in the base or subbase layer. These values are given in the IRC recommended IITPAVE software and strain values are evaluated as shown in figure 5. These strain values are checked with permissible strain values. If it is not within the permissible limit, then by increasing thickness or by changing the grade of geogrid the design of the reinforced section is finalized.

NHIDCL has decided to take up the development of Two laning with a hard shoulder, Rehabilitation, and up-gradation of various roads. The project corridor is a section of National Highway-223 (New NH-4) in Andaman. The road surface drainage of NH 223 is in poor condition and subgrade CBR in the majority of the length is poor, either less than 2% or ranging between 2-3%. The available soil is highly plastic in most of the reaches with PI up to 42%. The availability of quality aggregates for road works is limited to only a few areas of the island. The poor quality of subgrade soil will lead to differential settlement forming progressive ruts and eventually failure of the pavement. The conventional 

the solution, treatment of the subgrade soil with lime up to a depth of 500 mm would be expensive. The replacement of the subgrade with granular soil (at least 500 mm) will not be feasible due to the high cost and unavailability of good quality soil.

The solution involves, reinforcing the soft subsoil to improves the load-bearing capacity and subgrade properties 

in terms of CBR, by using biaxial extruded geogrid (MacGrid EG). It was proposed to excavate subgrade up to a depth of 440 mm and compact the same in two layers to 95% of MDD. On the finished compacted layer, a non-woven geotextile (MacTex N) was laid for separation and filtration.

On top of the MacTex, the MacGrid EG layer was laid. It was followed by filling of 60 mm of crushed aggregate (Minimum CBR 80) and compacting it to 95% of MDD.

Table: Represents the Reduction in thickness of Pavement layers using IIT Pave software