Trivalent cation-crosslinked xanthan gum-soil improvement : geotechnical engineering performance and applications

Abstract

thus, it is recommended to respond to the pH effect by adjusting the Cr/XG ratio. The field-scale implementation of XG–Cr$^{3+}$-treated soil was conducted on a levee slope using the pressurized-spraying method with a watering system. We confirmed that the levee whose surface is reinforced by XG–Cr$^{3+}$-treated soil has much higher resistance against flood overtopping failure compared to the natural levee and XG-compound reinforced levee. In this dissertation, the wet strength and water-related stability in XG-soil treatment are remarkably improved by the Cr$^{3+}$-crosslinking method without additional heat and pH control. The enhanced geotechnical engineering performance can widen the field applicability of XG-based soil treatment. The XG–Cr$^{3+}$ treated soil can be effectively applied to ground improvement near the shallow foundation, hydraulic barrier construction in water-retaining structures, and backfill grouting around underground structures.

Soil improvement is one of the most important issues in geotechnical engineering practices to increase soil strength and stiffness and control hydraulic conductivity and seepage. Cement and chemicals have been widely used for various soil improvement strategies owing to their performance, workability, and affordability. However, demand for an alternative to cement and chemicals is rising due to concerns with carbon dioxide (CO$_2$) footprint and environmental impacts on the ground ecosystem. In response, several bio-inspired approaches have been introduced in the geotechnical engineering field. Biopolymer is an environmentally friendly compound produced from natural resources, and recently it has been intensively investigated to modify various engineering properties in soil. In particular, xanthan gum (XG), a commercialized polysaccharide biopolymer, has shown soil strengthening, permeability reduction, increased erosion resistance, vegetation promotion, and promising economic feasibility. Although XG-soil treatment has positively affected geotechnical engineering performance even with a small dosage to soil mass, several challenges now need to be solved to improve practical sustainability. XG-soil treatment concerns low strengthening efficiency at a hydrated state and durability under water-exposure conditions. These water-related vulnerabilities in XG-treated soil raise concerns about practical sustainability and narrow down its feasibility in field application. In response, to overcome the issues, this dissertation studied on utilization of the trivalent cation-induced crosslinking method to enhance XG-soil treatment from a geotechnical engineering perspective. The main objectives of this study are to (1) identify the XG–Cr$^{3+}$ crosslinking characteristics in rheology, (2) multifariously assess geotechnical engineering performance and applicability of XG–Cr$^{3+}$ soil treatment, and (3) suggest feasible application strategies to widen the potentials of XG-based soil treatment technique. Rheological analysis showed that the Cr$^{3+}$-crosslinking method achieves XG gel, forming soft to stiff gel without any additional heat and pH control. As gelation proceeded, the yield stress of XG–Cr$^{3+}$ gel significantly increased, and the final gel strength and gelation time can be adjusted depending on the recipe. Cr$^{3+}$-crosslinking effectively enhances the wet unconfined compressive strength (UCS) of XG-treated soil, which is peculiarly remarkable in sandy soil. Strengthening in the hydrated state is induced by that rigid XG–Cr$^{3+}$ gel structure filling the pore spaces provides interparticle connections, which increase the cohesion of sand particles more effectively at low confinement. In addition, the strengthening effect was more evident in around 10% of clay fraction, while higher clay fraction rather interferes with the gelling effect of XG–Cr$^{3+}$ gel. XG–Cr$^{3+}$-treatment can effectively reduce the hydraulic conductivity of sand by approximately four orders of magnitude compared to clean sand. As the hydraulic gradient increases, the wash-out occurs easily in pure XG-treated sand, while Cr$^{3+}$-crosslinked XG treated soil has a higher resistance to the breakdown of permeability and residual pore filling effect. The increased yield stress of XG gel by Cr$^{3+}$ crosslinking was attributed to a stability increase in hydraulic performance. Meanwhile,the increase of yield stress is concerned with the injectability of XG–Cr$^{3+}$ gel into soil media. Since the flow stoppage of XG and XG–Cr$^{3+}$ gel has been dominantly affected by their yield stress, thus the injectability reduction occurred by Cr$^{3+}$ crosslinking, but sufficient injection performance is still available in coarse sand. Contrary to pure-XG-treated sand that is wholly disturbed in water, the XG–Cr$^{3+}$-treated sand samples remain stable up to 100 d without any swelling and failure because the water reactivity is reduced effectively via consumption of carboxyl group in crosslinkage. Moreover, the acute fish toxicity test showed that the XG–Cr$^{3+}$ gel has less toxicity than cement-based material in terms of mortality. However, XG–Cr$^{3+}$ gel induces acidification of the water due to acidic hydrolysis of Cr$^{3+}$