Counterface Soil Type and Loading Condition Effects on Granular/Cohesive Soil – Geofoam Interface Shear Behavior

The primary function of the geofoams consists of providing; i) lightweight fill for construction on soft ground (i.e. embankment), ii) relatively stiff base for subgrade installation below a highway (i.e. roadway, runway foundation), bridge approach (i.e. abutment backfill), and iii) slope stabilization for retaining structures. In those applications, the geofoams are in direct contact with soils and this interaction results in development of an interface where likelihood of a failure to initiate is higher. For this reason, the frictional resistance and the type of shear response mobilizing at these soil – geofoam interfaces control the stability of composite system, and hence, govern the integrity of the infrastructure. Soil – geofoam interfaces have been studied through an extensive experimental program by performing multiple series of interface shear tests using two different granular soils (i.e. beach sand and construction material sand) and one cohesive soil (i.e. bentonite clay) as well as a soil mixture containing 75% sand and 25% clay by dry weight at distinct loading conditions (i.e. normal stresses: 25, 100, 250; low, moderate, high loading conditions, respectively). Using the shear stress versus horizontal displacement curves obtained, some important engineering design parameters including peak shear stress, residual shear stress, interface sensitivity (i.e. peak/residual ratio) and displacement required to reach peak stress have been determined and the variations in those interface mechanical properties as a function of loading condition and counterface soil type have been investigated. It was seen that the peak as well as residual shear stresses increased with an increase in normal stress. Further, granular soil (sand) interfaces demonstrated relatively larger frictional strengths (both peak and residual) as compared to that of not only cohesive soil (clay) interface but also soil mixture (sand and clay) interface. Additionally, the higher the angularity of granular soil particles became, the larger the interface shear strengths (peak and residual), when sheared against geofoams, developed in light of experimental results attained as a result of interface shear tests on different material combinations.