Laboratory evaluation of the geotechnical characteristics of wastewater biosolids in road embankments

An extensive suite of geotechnical laboratory tests were undertaken on wastewater biosolids to evaluate their sustainable usage as a fill material in road embankments. Geotechnical tests undertaken include particle size distribution, specific gravity, Atterberg limits, compaction, consolidation, hydraulic conductivity, California Bearing Ratio (CBR), field vane shear, direct shear and triaxial shear tests. The geotechnical tests indicated that biosolids are equivalent to organic fine-grained soils of medium to high plasticity with high moisture content and liquid limit values. Consolidation tests indicate that biosolids have similar consolidation characteristics to that of organic soils. Shear strength tests on compacted biosolids samples indicated relatively high internal friction angles, comparable to that of inorganic silts. Compacted biosolids samples exhibit a modest cohesion comparable to organic clays. CBR tests results indicate high deformation potential of biosolids. Chemical and environmental assessment tests indicated that heavy metals, Dichloro Diphenyl Trichloroethane (DDT) and Organochlorine Pesticides concentration along with pathogens (bacteria, viruses or parasites) results were within acceptable limits for usage in geotechnical applications. With regards to contaminants containing nitrogen, phosphorus and total organic carbon, the biosolids were found to require special protection, in the event there is potential leaching/flow to adjoining water bodies. The geotechnical testing results indicate that untreated biosolids have insufficient bearing capacity to enable its usage as a fill material. The biosolids will have to be stabilized with an additive or blended with a high quality material to enhance its geotechnical properties to enable it to be considered as an engineering fill material.