Strength Improvement of Poor Subgrade Soil Reinforced with Polyester Biaxial Geogrid

This paper presents a laboratory study on the effect of polyester biaxial geogrid on the strength behaviour of poor subgrade soil. The soil used in the present study is classified as clay of intermediate compressibility in accordance with IS 1498 (1970) having a substantial amount of fines; i.e., a high percentage of clay and silt and losing its strength significantly upon coming in contact with water. Geogrid sheets are placed in single and double layers at various depths of soil subgrade and heavy compaction. California bearing ratio (CBR) and unconfined compressive strength (UCS) tests are conducted. The test results indicate significant improvements in CBR, UCS and axial strain at failure as well as a reduction in post peak strength loss. Maximum improvements of 36% and 41%, respectively, in CBR and 62% and 70%, respectively, in UCS values are observed when the geogrid is placed in a single layer (i.e., 0.2H) and in double layers (i.e., 0.2H and 0.4H) from the top of the specimen. Scanning electron microscopy (SEM) is used to study the micromechanical interaction between soil and geogrid surface. It is observed that interlocking and surface friction between soil particles and fibers of the geogrid are responsible for strength enhancement of weak subgrade soil. Multiple linear regression models were developed for predicting soaked CBR and UCS.