Consolidation and Hydraulic Properties of Sand–Biochar and Lightly Cemented Sand–Biochar Mixtures

Soil–biochar mixtures have been increasingly investigated for geotechnical applications (e.g., landfill cover system and roadway stormwater filter). However, biochar particles have lower stiffness and strength than soil particles, which can affect the behaviors of soil–biochar mixtures. This study investigated the one-dimensional consolidation properties [e.g., strain, void ratio (Cc, and Cr)], shear-wave velocities (S-wave velocity), and hydraulic conductivities of sand–biochar mixtures (SBM) with biochar contents (BC) ranging from 0% to 100%. Also, SBM were treated with 5% cement [i.e., lightly cemented sand–biochar mixtures (LCSBM)] to assess its effect on the consolidation properties, S-wave velocities, and hydraulic conductivities of SBM. Scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy were performed to investigate the morphology and elemental compositions of biochar and LCSBM. The results showed the increase of BC from 0% to 100% in SBM specimens increased the maximum strains (i.e., the strains at the applied stress of 768 kPa) from 1.2% to 6.8% and reduced the S-wave velocities at the applied stress of 12 kPa from 261 to 106 m/s and the average hydraulic conductivity from 8.7 × 10−4 to 9.2 × 10−6 m/s. LCSBM specimens had lower maximum strains, lower Cc, and higher S-wave velocities than the corresponding SBM specimens at BC ≤ 4.8%, while higher maximum strains and Cc at BC ≥ 16.7%. These variations of the consolidation properties were attributed to the different levels of cementation in the LCSBM specimens. It was found that 5% cement treatment was only effective when BC < 5% to reduce the compressibility and increase the hydraulic conductivity of SBM.