Evaluating the Shear Strength of Subbase-subgrade Interface Using Large Scale Direct Shear Test

The inclusion of geogrid in road pavements can improve pavement performance through increasing the lateral confinement, bearing capacity, and overall rigidity of the pavement, as well as reducing the vertical and lateral pavement deformations. The materials used in the present study are: subbase granular materials Type B, two types of subgrade soil; clay and sandy soil, and two nonwoven biaxial geogrids (G۱ and G۲) used as reinforcing materials. Direct shear testing was adopted by manufacturing a large-scale direct shear apparatus consisted of an upper, square box of size ۲۰ cm × ۲۰ cm × ۱۰ cm, and a lower, rectangular box of size ۲۰۰ mm × ۲۵۰ mm × ۱۰۰ mm is used in the present study. The results show that, for the four normal stresses equal to ۲۵, ۵۰, ۷۵ and ۱۰۰ kPa, the interface shear stress curves increased and followed similar trend. For clay-subbase interface, installation of geogrid decreases the apparent cohesion of the material from ۱۶.۵ kPa (without reinforcement) to be ۸ kPa and ۱۳.۵ kPa for G۱ and G۲, respectively. At sand-subbase interface, using geogrid leads to increase the cohesion of the material from ۳.۵ kPa to be ۱۵.۵ and ۱۶ kPa for G۱ and G۲, respectively. The friction angle increases slightly from ۳۰o (without reinforcement) to be ۳۵o for G۱ and G۲ when the interface is subbase over clay. While, it decreased from ۳۵.۸o to be ۳۲.۱o for G۱ and G۲ at sand-subbase interface. The interaction coefficient for G۱ and G۲ increased when the normal strength increased at the clay-subbase interface. Otherwise, the behavior of interaction coefficient of the sand-subbase interface appears deferent trend, where increasing normal stress leads to decrease the interaction.

Keywords:

Shear Strength , Interface , Direct Shear Test , Geogrid , Subbase