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TechFab India Industries Ltd.

Manish Barot, Deeraj Kumar Reddy, Rahul Kumar

Project Brief & Problem Description:

The construction and maintenance of highways in regions characterized by weak subgrade soils present persistent challenges to civil engineers, infrastructure planners, and policymakers. In India, particularly in the northeastern state of Assam, these challenges are magnified by the prevalence of expansive clayey soils, high rainfall, and fluctuating groundwater tables. National Highway 137 (NH-137), which forms a vital link between Tamenglong and Mahur, exemplifies the difficulties inherent in constructing durable pavements over subgrades with inherently low strength and high compressibility. This highway, stretching from kilometer 156.489 near P. Leikul to kilometer 176.410 near Borowapu, is an essential corridor for the movement of goods and people, connecting remote districts to larger economic centers and facilitating regional development.

The geotechnical investigations conducted along the NH-137 alignment revealed that the existing subgrade soils possess a California Bearing Ratio (CBR) of merely 3%. Such a low CBR shall significantly increase the required pavement crust wherein very high volumes of aggregates would be required and same are scarce in the project location. The subgrade’s low shear strength, high plasticity, and pronounced moisture sensitivity make it susceptible to bearing capacity failures, excessive settlement, and differential deformation under repeated traffic loading. These soil characteristics, combined with Assam’s intense monsoonal rainfall - often exceeding 3,000 mm annually—result in frequent subgrade saturation, capillary rise, and subsequent loss of support to the overlying pavement layers. Historical data from similar highway projects in the region indicate that such conditions have led to premature pavement distress, including rutting, cracking, and surface undulations, often necessitating costly and frequent maintenance interventions.

The imperative to stabilize the subgrade of NH-137 thus arose not only from the need to meet technical specifications but also from the broader objective of ensuring the long-term serviceability and safety of the highway. The consequences of inadequate subgrade stabilization are multifaceted, encompassing increased maintenance costs, reduced pavement lifespan, heightened risk of traffic disruptions, and diminished economic returns on infrastructure investments. In the specific context of NH-137, the anticipated volume of heavy vehicular movement - comprising commercial trucks, passenger buses, and emergency vehicles - further underscored the necessity for a robust and resilient pavement foundation. The challenge was to devise a solution that would elevate the subgrade CBR from its existing value of 3% to the design requirement of 12%, thereby providing a stable platform for the construction of the pavement structure.

Traditional approaches to subgrade improvement in India have often relied on the excavation and replacement of poor soils with imported granular materials or the chemical stabilization of in-situ soils using lime, cement, or fly ash. While these methods can be effective under certain conditions, they are frequently associated with high material and transportation costs, increased construction time, and environmental concerns related to resource depletion and carbon emissions. Moreover, in remote and ecologically sensitive regions such as Assam, the logistical and environmental challenges of sourcing and transporting large quantities of granular material or stabilizing agents can be prohibitive. These constraints necessitated the exploration of alternative, sustainable, and cost-effective ground improvement techniques tailored to the local context.

In response to these challenges, the NH-137 project adopted an innovative approach centered on the use of a certified TechGrid biaxial geogrid for subgrade stabilization. Geogrids are high-strength polymeric materials engineered to reinforce soils by providing tensile resistance and facilitating load distribution. The strategic placement of the TechGrid biaxial geogrid within the subgrade was designed to enhance the structural integrity of the pavement foundation by interlocking with the surrounding soil, reducing stress concentrations, and minimizing potential deformations. This reinforcement mechanism not only improves the load-bearing capacity of the subgrade but also mitigates the risk of differential settlement and surface irregularities, which are critical for maintaining ride quality and structural performance over the design life of the pavement.

Solution:

Given the existing soil condition with a California Bearing Ratio (CBR) of 3% and the target CBR of 12%, it is proposed to incorporate a certified TechGrid biaxial geogrid to achieve the necessary stabilization. The TechGrid TGB100 biaxial geogrid will be strategically installed between two layers of subgrade soil to enhance the structural integrity of the system. By reinforcing the soil, the geogrid helps to evenly distribute loads, reducing stress concentrations and minimizingpotential deformations. This placement significantly improves the stability of the subgrade, making it more resistant to shifting or settling under applied loads. The reinforcement provided by the biaxial geogrid ensures a durable foundation, suitable for road construction. 

The selection of the TechGrid TGB100 biaxial geogrid was informed by a rigorous evaluation of its mechanical properties, installation feasibility, and compatibility with the local soil conditions. The geogrid’s bidirectional tensile strength and optimized aperture geometry enable effective soil interlock and stress transfer, thereby transforming the behavior of the subgrade from that of a weak, deformable medium to a composite system with enhanced stiffness and reduced susceptibility to rutting. The design methodology for the geogrid-reinforced subgrade adhered to the guidelines of IRC:37-2018, incorporating a Modulus Improvement Factor (MIF) of 2, as validated by third-party testing at the Indian Institute of Technology Hyderabad. This approach ensured that the improved subgrade would achieve a resilient modulus commensurate with the target CBR of 12%, thereby satisfying the structural requirements for pavement design.

As mentioned, the CBR of existing soil was 3% and hence the objective of proposing a non-woven geotextile at the interface of existing soil and prepared subgrade of 500mm was to provide a separation and filtration system in order to prevent the intermixing of poor soil (CBR 3%) with a relatively good quality soil (8%). The non-woven geotextile shall prevent the migration of soil particles and allow the free movement of sub-surface water between the pavement layers. This arrangement shall ensure the long-term sustainability of the design parameters considered in the design.

Execution on Site:

Site preparation : The construction area was first cleared of all vegetation, debris, and unsuitable material. The subgrade surface was then leveled and graded to the required profile, ensuring a smooth and stable base for further construction. to achieve the necessary stabilization.

Geotextile installation: Immediately after site preparation, a non-woven geotextile was unrolled directly over the prepared subgrade surface. The geotextile sheets were overlapped by at least 300 mm and anchored at the edges to prevent displacement, providing separation and filtration between the subgrade and the overlying soil.

First layer of subgrade: A layer of approved subgrade soil, 300 mm thick (loose), was spread over the installed geotextile. The soil was moisture-conditioned as necessary and compacted using vibratory rollers to achieve at least 95% of Modified Proctor Density. Field density tests were conducted to ensure uniform compaction.

Geogrid installation: The certified TechGrid TGB100 biaxial geogrid was then unrolled directly on top of the compacted 300 mm subgrade layer. The geogrid was aligned parallel to the road centerline, with adjacent rolls overlapped by a minimum of 300 mm. The overlaps were secured to prevent movement during subsequent soil placement.

Second layer of subgrade: A second layer of subgrade soil, 200 mm thick (loose), was carefully placed over the geogrid. The soil was spread gently to avoid disturbing the underlying geogrid and then compacted to the specified density, completing the total stabilized subgrade thickness of 500 mm.

Quality Control at Site. At each stage, compaction was verified through field density tests. The placement, overlap, and condition of both the geotextile and geogrid were inspected before covering. Upon completion, in-situ CBR tests were conducted to confirm that the target CBR (≥12%) had been achieved.

Conclusion:

The NH-137 Subgrade Stabilization Project in Assam is a successful example of how advanced geosynthetic solutions can overcome challenging ground conditions. Faced with weak, moisture-sensitive subgrade soils having an initial CBR of only 3%, the project utilized TechFab India's certified TechGrid TGB100 Biaxial Geogrid in combination with a nonwoven geotextile to achieve a stabilized subgrade with a CBR exceeding 12%, fully complying with IRC:37-2018 guidelines. The design was validated through independent third-party testing and IITPAVE analysis, confirming enhanced pavement stiffness, reduced deformation, and long-term structural performance. The solution enabled the effective use of in-situ soils, significantly reducing aggregate requirements, material transportation, construction time, and overall project costs while lowering the carbon footprint. Through meticulous installation practices and stringent quality control, the project delivered a durable, sustainable, and cost-effective pavement solution, establishing a benchmark for highway infrastructure development in regions with weak subgrade conditions.

For further details kindly contact :

TechFab India Industries Ltd.

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Tel: + 91- 22 - 2287 6224 / 6225

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