Simple Model on Water Retention and Permeability in Soil Mixed with Lignocellulose Fibres

Natural or lignocellulose fibres have been widely used for reinforcing soils in geotechnical infrastructures by using their mechanical reinforcement. However, less attention has been taken to the hydraulic properties of soil-lignocellulose fibre composites, namely Soil Water Retention Curve (SWRC) and soil water permeability. These hydraulic properties are the key parameters when conducting transient seepage analysis in reinforced slope stability calculation. Till now, there is no model yet that can capture SWRC and water permeability of soil-lignocellulose fibre composite. This technical note aims to develop a new and simple model for predicting the SWRC and water permeability of soils mixed with lignocellulose fibres. The model considers the void ratio change by incorporating the air void from fibres. The void ratio function is then fed into a void-ratio-dependent SWRC model. SWRCs and water permeability of soils mixed with two lignocellulose fibres (jute and coir) were measured systematically to provide high quality data to validate the proposed model. There were three replicates for each case. It shows that the presence of pore structures in natural fibres reduced air entry value of soils from 8 kPa to 2–3 kPa, while it had no effects on desorption rates. Moreover, those pores in lignocellulose fibres increased the water flow path, resulting in increased water permeability. As demonstrated by dye tracer experiments, the increased water flow was along the cellulose, hemicellulose fibrils inside the fibre and soil-fibre interface. The comparisons between experimental measurements and model predictions indicate that the proposed simple model can capture the effects of natural fibres on soil hydraulic properties quite well, with the maximum discrepancy less than 15% and 28% for SWRC and water permeability, respectively.