Swelling Research of Expansive Soil Under Drying-Wetting Cycles: A NMR Method

Abstract

The main purpose of this study is to examine the influence of drying-wetting cycles on the swelling of disturbed expansive soil at the micro-scale. Swelling curve -the relationship between swelling displacement and water-absorbing time -is measured based on specimens that experienced 0-4 drying-wetting cycles. Assisted by Nuclear Magnetic Resonance (NMR), the stage characteristic and mechanism influencing swelling curve are analyzed from a pore-change perspective. Specimens that experienced 1-4 cycles show a higher swelling rate when compared with the specimens without drying-wetting cycles; the swelling rates of each swelling curve are found to decrease with an increasing water-absorbing time. These swelling curves are divided into rapid-swelling stage, slow-swelling stage, and slow-stable stage, where three straight lines are used to fit these stages to each curve. The swelling displacement of the specimen increases after the first cycle, whereas it decreases gradually during the following cycles. Changes of pore structure at each stage are considered to be the main factor affecting the swelling rate during the saturating process. Some fine particles and water-soluble cements are lost after multiple cycles, and some smaller pores are converted into larger pores, resulting in an increase in volume and a decrease of swelling displacement. Moreover, the increase of larger pores and the suction difference after various cycles are important factors leading to the difference of the swelling curve and the swelling displacement.

Microstructural Transformations and Prediction Models of an Expansive Soil Subjected to Simulated Rainfall Conditions

“…Over decades, several traditional methods for testing pore structure, such as mercury intrusion porosimetry, gas adsorption method, and imaging techniques, have been developed. However, it is apparent that while these traditional pore testing methods are theoretically robust and widely used, they have stringent requirements for specimens and are prone to disturbances during soil sample preparation, limiting their ability to continuously monitor the evolution of the internal structure [18][19][20][21]. In recent years, advanced nuclear magnetic resonance (NMR) relaxation measurement techniques have enabled non-invasive and continuous monitoring of changes in soil pore size distribution.…”

“…He discovered that as the number of wet-dry cycles increased, the soil's shear strength decreased, the stress-strain curve became more ductile, and the pore structure shifted from microporous to macroporous. Additionally, G.F. Wei [19] applied NMR techniques to analyze the stage characteristics and mechanisms influencing the swelling curve from the perspective of changes in pore space, noting a decrease in swelling rate with prolonged water absorption. Fugang Shi [24], meanwhile, investigated the alterations in the pore size distribution of expansive soil during dewatering and shrinkage using the NMR relaxation method.…”