Shear strength and liquefaction resistance of sand mixed with biochar

Abstract

Biochar is a carbonaceous material produced by heating any organic biomass in oxygen-free environment through pyrolysis process. Given its aromatic nature, biochar is highly recalcitrant, so as soil additive, it has been considered to have great potential to sequester carbon and reduce greenhouse gas emission. Since this material has been produced to be buried in the ground, it is important to assess its effect on soil properties from geotechnical point of view. However, to date, little work has been done in this field. A few studies have shown that biochar could increase the shear strength of clays and desaturate soil particles; therefore, it may be used to increase the liquefaction resistance of loose sand. In this study, the effect of adding biochar to sand (3% and 5% by weight) was investigated. Specimens were tested using simple shear test apparatus. Results of drained monotonic tests indicated that biochar provided higher shear resistance at lower confinement pressure (൑ 100 kPa). These specimens also developed less volumetric deformation. Undrained cyclic test results showed that the addition of biochar increased the liquefaction resistance of sand by delaying the generation of excess pore water pressure and restraining the development of shear strain. To provide an insight on the mechanism of improvement, Fourier Transform Infra-Red (FTIR) spectrum of biochar surface was measured, and the effect of time on the interaction between water and biochar was evaluated through rheological measurements. It is believed that the mechanism of improvement is related to the active functional groups on the surface and inside the pores of the biochar particles. These volatiles interact with water by repelling it (hydrophobic) and at times by attracting it, creating a complex network between water and biochar particles that helps delay the increase in excess pore water pressure and provides some shear resistance.