Monotonic and cyclic shear behaviour of xanthan gum-stabilised sand considering curing conditions

Abstract

Biopolymer stabilisation has emerged as a promising technique in geotechnical engineering due to its effectiveness and environmental sustainability. However, research on the effects of curing conditions and sand gradation on the monotonic and cyclic shear behaviour of xanthan gum-stabilised sand remains limited. This knowledge gap hinders the broader implementation of such materials in pavement engineering applications.

In this thesis, a comprehensive approach integrating laboratory experiments, image-based microstructural analysis, and regression-based modelling is employed to develop a quantitative understanding of the relationships between macroscopic mechanical behaviour and underlying microstructural characteristics, with a specific focus on the influence of curing conditions and sand gradations. Unconfined compression tests, conventional triaxial tests, and cyclic triaxial tests are conducted to evaluate unconfined compressive strength (UCS) and shear behaviour under both monotonic and cyclic loading conditions, respectively. Scanning electron microscopy (SEM) and X-ray computed tomography (CT) are utilised to characterise hydrogel morphology and the three-dimensional sand-hydrogel interaction network. Furthermore, numerical and regression analyses are applied to establish predictive models that link microstructural descriptors to macroscopic deformation responses, including shear strength and permanent strain.

The results indicate that, curing relative humidity (RH) exhibits an optimum value of approximately 40% for xanthan gum-stabilised sand with densely packed gradations (i.e., continuously graded sand or gap-graded sand with intermediate fines content). At this optimal RH, the material demonstrates peak UCS and shear strength, along with the lowest permanent strain, indicating enhanced mechanical performance and deformation resistance. These improvements are attributed to the combined contributions of network and hydrogel effects. Moreover, within the investigated temperature range, an increase in curing temperature leads to a nearly linear increase in UCS, since an elevated kinetic energy and conformational transitions alter the network and the hydrogel effects.

For xanthan gum-stabilised sand with gap gradation, two key factors influencing its behaviour are fines content and size ratio. In fine-dominated and transitional packing states, a higher size ratio results in increased shear strength and resilient modulus. However, this trend reverses in coarse dominated packing states. These observations are attributed to the effects of xanthan gum hydrogels. Fines content exhibits a critical threshold between 30% and 70%. Below this range, an increase in fines content generally leads to enhanced shear strength and minimal changes or slight increases in accumulated permanent strain. Above this threshold, shear strength decreases while accumulated permanent strain increases. At intermediate fines content levels, fine particles contribute to the soil skeleton, with both fine particles and hydrogels filling most pores, thereby enhancing structural integrity and ductility.

Additionally, the normalised size ratio (NSR) significantly influences the cyclic behaviour of xanthan gum-stabilised sand. At low NSR values, the permanent strain rate decreases rapidly, and the effects of various factors are less pronounced. High NSR values lead to continuous accumulation of internal microstructural damage, resulting in sustained permanent strain and increased dissipated energy.

Moreover, the hydrogel volume per unit volume, sand hydrogel effective contact ratio and average effective coordination number between sand particles have been incorporated into predictive models as an improved structural term for estimating shear strength under monotonic loading conditions. The improved structural term has been integrated into predictive models for estimating accumulated permanent strain under cyclic loading conditions.

The findings of the thesis contribute to a deeper understanding of the monotonic and cyclic behaviour of xanthan gum-stabilised sand. The proposed mechanisms and predictive models provide both theoretical insights and practical guidance for designing sustainable soil stabilisation strategies using xanthan gum biopolymers.

Date of Award	18 Jul 2026
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Juan Wang (Supervisor), Shu Liu (Supervisor) & Yong Ren (Supervisor)