Microscale and Macroscale Characterization of Biopolymer-Stabilized Sulfate-Rich Expansive Soils

Abstract

Problematic soils like expansive soils cause significant damages to civil infrastructure. The use of calcium-based stabilizers in the treatment of sulfate-rich expansive soils is not suggested due to the formation of ettringite. Infrastructure such as pavements and embankments built on expansive soil are often exposed to the damaging impacts of freeze–thaw cycles in areas prone to seasonal freezing, making them vulnerable to cracking and spalling. A native expansive soil from South Dakota with a sulfate content of more than 10,000 ppm was stabilized using biopolymer (BP) and cement in this study. A comparison of the geotechnical properties of the untreated and treated soil such as Atterberg limits, one-dimensional (1D) swell, linear shrinkage, unconfined compressive strength (UCS), and resilient modulus (MR) for curing periods of 7 and 28 days were presented in the study. The swelling in cement-stabilized soil specimens was observed to increase after a long period due to the formation of ettringite. The study investigated the effectiveness of cement and biopolymers as co-additives to treat the sulfate-rich expansive soil. The experimental study investigated the strength and stiffness properties of the control and treated soil after the various freeze–thaw (F–T) cycles. The reduction of strength and stiffness properties of soil for 6% cement and the co-addition of 3% cement and 1.5% biopolymer after the F–T cycles were found to be comparatively less. Soil morphology provided insights into the configuration of biopolymer networks and the development of ettringite within treated soils. Biopolymers were used as an environmentally friendly substitute for traditional energy-intensive stabilizers in expansive soil stabilization, and potentially reducing carbon footprints. The study found that the incorporation of biopolymer as a co-additive with cement can be a viable alternative for stabilizing sulfate-rich expansive soil subgrade. Chemical stabilizers such as cement are typically used to enhance the load-bearing capacity of weaker soils. However, for sulfate-rich soils, this approach may be counterproductive and result in sulfate-induced heave. This study attempted to find alternative techniques where biopolymers such as guar gum are used to sustainably stabilize sulfate-rich soils in the presence of a smaller concentration of cement. Potential applications include mitigating the distress in pavements in the colder regions due to freezing and thawing. As a source of calcium, cement might have triggered the cross-linking of the biopolymer network that bound the soil together and the treated soil samples showed higher strength and stiffness with minimal volume changes even after application of freezing and thawing cycles as compared to the traditional approaches. This technique increased the ability of soil to deform more before failure. Apart from the increased strength, in case of impending failure, there might be greater warning signs in the form of soil movements, which may allow appropriate mitigation measures to be applied.