Exploring the effect of biopolymers in near-surface soils using xanthan gum-modified sand under shear

Biopolymers produced in near-surface soils by living organisms, including microbial extracellular polymeric substances and plant mucilage, offer enhanced moisture retention and protection from dry environments, lubricate roots to allow penetration through soil, and link soil grains together physically to form soil aggregates. At the aggregate scale their effects and behaviour are known and significant, but their impact on geotechnical behaviour of shallow soil bodies at the mesoscale and beyond is largely unexplored, including their response to the moisture cycling typical in vadose zone soils. In this work we explore the effects of moisture conditions, including multiple dry-wet cycles, on the shear behaviour of sand amended with xanthan gum as a model biopolymer. Drying causes a significant improvement on shear strength, even at low concentrations of biopolymer, but this is largely lost upon wetting. The extent of shear strength improvement is dependent on the moisture path taken (i.e., the wetting-drying history) and deteriorates over a number of moisture cycles. We present a conceptual model that poses redistribution of the biopolymer around the sand grains as the cause of the observed behaviour and demonstrate that biopolymers can provide a significant although transient enhancement of shear strength of sand in near-surface conditions.