Small-scale pullout test of a geogrid-reinforced unsaturated soil with suction monitoring

Abstract

The use of local fine-grained soils in geosynthetic-reinforced structures can significantly reduce costs when granular materials are difficult to access. Another beneficial aspect of these soils is that enhanced mechanical behavior is expected with fine-grained soils under unsaturated conditions. With structures built in unsaturated conditions, stability analyses based on unsaturated soil mechanics would properly characterize and accurately predict the structural behavior with fine-grained soils. Accordingly, the interface shear strength between fine-grained soils and geosynthetics should also be assessed by unsaturated pullout or direct shear testing. In this article, the effect of matric suction on the pullout behavior of a polyester geogrid embedded in a fine-grained soil was evaluated using a small-scale testing apparatus. A miniature tensiometer located in the proximity of the soil-geogrid interface enabled monitoring of the matric suction of soil during pullout to evaluate its effect on the interface shear strength. A design approach for prediction of unsaturated interface strength is also assessed in this study. Three typical values of matric suction were identified during pullout: initial, peak, and residual. In the case of interfaces under water inundation, the pore water pressures developed and increased with the increase of overburden pressures. For drier interfaces, matric suction increases with increases of overburden pressure. Significant reductions (60 %) in peak pullout forces were observed for minor increases in soil moisture contents. Analyses using the moisture reduction factor indicated up to a 70 % loss of soil-geogrid interface shear strength as a result of wetting. The increase in unsaturated soil-geogrid interface strength due to the reduction of moisture content was attributed more to adhesion than to friction for the fine-grained soil used in this research. The analytical approach proposed in this study to predict the pullout strength of unsaturated interfaces has been demonstrated to be quite consistent with the actual strength values assessed from the small-scale pullout tests, primarily for higher values of overburden pressures. Matric suction values obtained from the soil-water retention curve were shown to be reliable parameters for use in the proposed analytical approach for prediction of the pullout strength of unsaturated soil-geosynthetic interfaces.