Research Projects

Soil nailing is a well-established in-situ reinforcement technique used for stabilising roadway excavations, embankments, landslide-prone slopes, and existing retaining structures. Its popularity is due to proven effectiveness, ease of construction, and lower cost compared to other slope protection methods. In Kerala, the National Highways Authority of India (NHAI) has adopted drilled and grouted soil nails to stabilise steep cuts in residual lateritic formations as part of NH-66 widening works. However, failures of soil nail walls have been reported during recent monsoons, raising concerns about long-term performance under rainfall infiltration. Current design practices adopt bond strength values from literature (e.g., FHWA, AASHTO), later verified through field pullout tests. Lateritic residual soils, being heterogeneous due to varying degrees of weathering, exhibit low true cohesion and suction-dependent apparent cohesion, which diminishes upon saturation during rainfall. Lateritic soils in the Western Ghats, with high fines, are prone to time-dependent deformation (creep), which may be accelerated by seasonal moisture variation. Creep affects long-term wall stability in two ways: (1) creep in the soil behind the facing (yielding zone) can reduce shear strength and increase nail forces, affecting stability; (2) creep at the grout–soil interface in the stable zone can reduce bond resistance over time, leading to wall deformation and potential pullout failure. While some studies have addressed creep in sand and clay, the behaviour of lateritic soils under moisture variation remains inadequately explored. In the absence of clear guidelines, thumb rules such as high liquid limit, high plasticity index, low undrained shear strength, and organics are commonly used to identify creep-prone soils (FHWA). Helical soil nails, which resist pullout mainly through the bearing action of their helices, are gaining popularity due to their ease of installation using torque and crowd. Their creep behaviour is expected to differ from grouted nails due to different load transfer mechanisms. Recently, a newer system -grouted helical soil nails - has been proposed to overcome installation-related disturbances caused during nail penetration. In this method, grout is injected through the hollow shaft of the helical nail during installation, forming an enhanced geomaterial matrix around the helices upon hydration, thereby improving interface resistance (Rajagopal, 2025). Model-scale tests in sand have shown improved pullout resistance, but their performance in lateritic soils under moisture variation remains unstudied. This study aims to investigate the long-term response of soil nails in lateritic soils under rainwater infiltration, with focus on creep deformation influenced by soil properties and moisture variation. The approach includes: • Creep behaviour under seasonal wetting and drying cycles • Time-dependent interface response of drilled and grouted, helical, and grouted-helical nails • Element-scale testing and monitoring of an instrumented trial wall • Coupled hydro-mechanical numerical modelling calibrated with test data Unlike earlier studies on sand or clay, this research provides a complete creep model for residual lateritic soils, which behave differently due to weathering, suction, and natural cementation. Expected outcomes include: • Understanding rainfall effects on pullout resistance and creep • A moisture-coupled creep model for residual soils • Comparative pullout behaviour of different nail types • Quantification of the influence of fine content, stress level, viscosity, and suction on the creep response of residual soils and the resulting increase in nail force over the service life of soil-nailed walls. • Recommendations for modifying design methods to include long-term creep effects These outcomes aim to support safer and more reliable design of soil-nailed walls in rainfall-prone tropical regions.

Soil nails, Creep, Lateritic Soils, Rainfall Infiltration, Hydro-mechanical modelling, Long-term stability