Influence of biopolymer gel-coated fibres on sand reinforcement as a model of plant root behaviour

Aims The contribution of plant mucilage and microbial biofilms in the rhizosphere to the physical behaviour of roots, and therefore on geotechnical performance, is not fully understood. We explore the impact of biopolymers on the ability of fibrous inclusions in soil to resist shear loading, to test the hypothesis that biopolymer-enhanced cohesion will be most significant at shallow depths where frictional effects are less, whilst exploring the response of biopolymer to changes in the moisture regime. Methods Artificial root/biopolymer systems comprising 3D-printed fibres and xanthan gum biopolymer in sand have been tested under direct shear at low vertical normal stress (1–30 kPa). The impact of drying and wetting on the ability of the reinforced sand to resist shear was assessed. Results Fibres combined with fresh biopolymer caused an increase in mobilisable shear stress, which is proportionally more significant at lower normal stress and so shallower depth (up to 30% increase at 1 kPa). Increased shear resistance and sand aggregation were observed with progressive drying. A cyclic shear response was observed over wetting and drying cycles with considerable strengthening after drying, which was enhanced by preceding wetting increasing the biopolymer zone of influence around the fibre. Conclusions The behaviour of this idealised system attests that root-associated biopolymers contribute significantly to the stabilisation of shallow soil by creating bonds between the root and soil grains, but the response is dependent on the soil moisture regime