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.

The utilizing of traditional chemical stabilizers could improve soil engineering properties but also results in brittle behavior and causes environmental problems. This study investigates the feasibility of the combined utilization of an ecofriendly biopolymer and fiber inclusions as an alternative to traditional cement for reinforcing soft soil. A series of unconfined compression tests were conducted to examine the combined effect of the biopolymer and fibers on the stress–strain characteristics, strength improvement, failure pattern, and reinforcement mechanism of soft soil. The results show that the biopolymer associated with fibers has an unconfined compressive strength similar to that of fiber-reinforced soil. However, it then increases with different curing times and conditions, which can be up to 1.5 MPa–2.5 MPa. The combined effect of fibers and the biopolymer is not simply equivalent to the sum of the effects of each individual material. The fiber shows its role instantly after being mixed into soil, whereas the effect of biopolymer gradually appears with sample curing time. The biopolymer plays a dominant role in increasing the peak unconfined compressive strength and brittleness of soil, while the amount of fiber is crucial for reducing soil brittleness and increasing ductility. It is shown that the biopolymer not only contributes to the particle bonding force but also facilitates the reinforcement efficiency of fibers in the soil. The fibers in return assist in reducing the soil brittleness arising from biopolymer cementation and provide residual resistance after post-peak failure.

Particle surface morphology is an important factor influencing sand structure and mechanical properties. In this study, the effect of sand particle sphericity on sand direct shear performance is investigated by using the discrete element method (DEM). Two ways are adapted to simulate different approaching methods from round particles to irregular sand. The macroresponse shows that irregular sand has a higher shear strength at lower normal stress than round particles. The shape of the particle has less influence on shear strength at higher normal stress. The irregular shape of sand leads to an increase in the shear band proportion. However, the shear band proportion is not related to the sphericity. Under all conditions, particles within the shear band have a larger average rotation angle than those outside the shear band. When the particle shape approaches round (regardless of the round particle proportion and particle shape), the average rotation angle of particles within and without shear bands increase, while the coordinate number and contact anisotropy decrease.

The study of the high-performance of biopolymers and current eco-friendly have recently emerged. However, the micro-behavior and underlying mechanisms during the test are still unclear. In this study, we conducted experimental and numerical tests in parallel to investigate the impact of different xanthan gum biopolymer contents sand. Then, a numerical simulation of the direct tensile test under different tensile positions was carried out. The micro-characteristics of the biopolymer-treated sand were captured and analyzed by numerical simulations. The results indicate that the biopolymer can substantially increase the uniaxial compressive strength and tensile strength of the soil. The analysis of the microparameters demonstrates the increase in the contact bond parameter values with different biopolymer contents, and stronger bonding strength is provided with a higher biopolymer content from the microscale. The contact force and crack development during the test were visualized in the paper. In addition, a regression model for predicting the direct tensile strength under different tensile positions was established. The numerical simulation results explained the mechanical and fracture behavior of xanthan gum biopolymer stabilized sand under uniaxial compression, which provides a better understanding of the biopolymer strengthening effect.

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.