Stabilized soil composites incorporating Cr3+-crosslinked xanthan gum (CrXG), a self-stiffening cation-cross- linked biopolymer, have recently emerged as sustainable construction materials for earthen structures. However, the influence of curing conditions and soil composition in altering the mechanical properties of CrXG-soil composites has so far received limited attention. This study investigates the effects of fine contents and curing conditions on the time-dependent strength development and durability of CrXG-soil composites. CrXG-soil composites, ranging from poorly graded sand to clayey silty sand, are subjected to unconfined compressive strength (UCS) and durability tests under various curing conditions, including wet, submerged, and dry conditions. Microscopic structural changes are characterized using scanning electron microscopy (SEM) and Fourier- transform infrared spectroscopy (FTIR). The results showed that the UCS of CrXG-soil composite increases nonlinearly, reaching up to 4.8 times the initial wet UCS after 28 days of curing, closely aligning with predictions from a hyperbolic model. Notably, CrXG-soil compositions with a clay-sand mixture (CSM) containing 15 % fine content (CSM15) demonstrated consistent strength parameters across all curing conditions in UCS tests. CSM15 also maintains a 90 % of durability index after eight dry-wet cycles and a dry UCS of 300 kPa after 130 days of atmospheric weathering. Microscopic-scale analysis confirms the stable agglomeration of CrXG-clay matrices between sand grains, with the peak wavelength of the major functional group remaining constant, even under multiple cycles. These findings contribute to a deeper understanding of CrXG-soil composites, offering valuable insights into optimizing soil compositions and enhancing the technical feasibility of applying these composites as a sustainable surface protection strategy for earthen structures, such as levees and road slopes.

As urbanization accelerates and surface space becomes increasingly scarce, the development and utilization of urban underground space have become more critical. The sand-fine mixture soils commonly found in river-adjacent and coastal areas pose significant challenges to the design and construction of underground structures due to their unique mechanical properties. In soil mechanics, the minimum and maximum void ratios are crucial indicators for assessing soil compressibility, permeability, and shear strength. This study employed the discrete element method (DEM) to simulate the minimum and maximum void ratios of sand-fine mixtures under various conditions by setting six fine contents and three mean diameter ratios. The results indicate that as the fine content increases, these void ratios exhibit a trend of initially decreasing and then increasing, which can be effectively modelled using a single-parameter quadratic function. Additionally, the initial shear modulus was closely related to the uniformity of contact distribution at the microscopic level within the specimens. This study also introduced a dimensionless parameter that simultaneously described changes in contact distribution and initial shear modulus.

In order to understand the influence of sand content on the secondary consolidation behavior of sand-fine mixtures, a series of one-dimensional creep tests were conducted. These tests used mixtures with sand contents of 0%, 16.67%, 28.57%, 50%, and 60% and were run for 3,000 min. As the sand content increases, the structure of the mixtures transitions from being fine-supported to sand-supported. This results in changes in the time at the end of primary consolidation (TEOP), the proportion of secondary consolidation deformation in the total deformation (PCT), and the coefficient of secondary consolidation. These parameters decrease before the sand content reaches 28.57% and increases after this point. The sand-fine mixtures with a sand content of 28.57% exhibit the minimum TEOP, PCT, and coefficient of secondary consolidation. When the sand content is less than 28.57%, bound water (especially weakly bound water) significantly impacts the secondary consolidation behavior of the sand-fine mixtures. However, when the sand content exceeds 28.57%, the secondary consolidation deformation of the mixtures is primarily governed by particle crushing in the sand grains.