Effective CBR and Elastic Modulus of Geogrid-Stabilized Prepared Subgrades Overlying Existing Soft Subgrades

It is a common practice to stabilize soft subgrades by overlying a granular material stabilized with a layer of suitable geosynthetic. The present study proposes an approach to obtain the effective modulus of soft subgrades stabilized with locally available soil material in conjunction with geogrids. Relatively soft subgrades with California bearing ratio (CBR) equal to 2, 5, and 7% are simulated in the laboratory. Locally available soil material with CBR equal to 8, 14, and 20% stabilized with polypropylene (PP) and polyethylene terephthalate (PET) geogrids are prepared over soft subgrades. Geogrids considered in the study (PP30 and PET100) covers the low to high tensile stiffness range of available geosynthetics. In total, 30 large-scale model pavement experiments were carried out in large-size test chamber having inside dimensions equal to 1.5 m × 1.5 m × 1.0 m (length, width and height, respectively) under monotonic loading. The benefit is expressed in terms of modulus improvement factor (MIF) and improved subgrade CBRs, in accordance with mechanistic–empirical equations proposed in Indian Road Congress guidelines. The overall MIF values ranged between 1.2 and 2.8 for the test combinations considered in the study. Geogrid of high stiffness has excellent capability to improve the effective subgrade CBR to as high as 13% compared to unstabilized subgrade CBR equal to 5%. Results showed that improvement is significant for low CBR conditions and high stiffness of geogrid and vice versa. The cost-to-benefit analysis conducted within the study scope indicated that about a 48% reduction in the cost is observed for PP30 geogrid-stabilized subgrade compared to cement-stabilized subgrade.