Shear and Hydraulic Properties of Compost-Amended Topsoils for Use on Highway Slopes

The amount of municipal solid waste (MSW) generated in the US increases every year, and about 30% of the MSW generated is either recyclable or compostable. Utilization of compost-amended topsoils as a vegetative layer on highway slopes contributes to large-volume beneficial reuse of these materials. This study examines the shear and hydraulic properties of two types of composts, biosolids and leaf compost, and their blends with a topsoil for their potential use on highway slopes. Direct shear and consolidated-undrained triaxial shear tests were performed to obtain the shear strength parameters. Flexible-wall hydraulic conductivity tests and unsaturated hydraulic tests were performed to evaluate the saturated and unsaturated hydraulic behavior of the materials, respectively. Compost addition resulted in an increase in effective friction angle, whereas modest changes were observed in effective cohesion, total cohesion, and total friction angle of the topsoil. Two shape parameters determined via digital image analysis, angularity and relative form of 2-dimensional images, correlated well with the measured effective friction angles of the materials tested. Compost treatment resulted in an increase in saturated hydraulic conductivity and the plant-available water content. Unsaturated hydraulic conductivities of all materials were comparable at the matric potential of field capacity (10 kPa), and the compost-amended topsoils experienced 1-3 orders of decrease in their unsaturated hydraulic conductivities during the drying process.