Shear strength and damage characteristics of compacted expansive soil subjected to wet–dry cycles: a multi-scale study

Abstract

In expansive soil regions, engineering geological disasters frequently occur in wet-dry (WD) environments, which are inseparable from the degradation of soil shear strength and structural damage. This study attempts to assess the underlying mechanisms of shear strength degradation and micro-and mesoscale damage to expansive soil under WD cycles. Recompacted specimens were subjected to several WD cycles, then triaxial shear, nuclear magnetic resonance, and scanning electron microscopy tests were performed. Moreover, the influence of the WD cycles on the shear strength of expansive soil was evaluated. The state-of-the-art image processing and analysis techniques were combined to quantitatively investigate the evolution of the soil micro-and mesostructural characteristics. The results reveal that the deterioration of soil shear strength is primarily embodied in the initial 3 WD cycles, and is mainly due to the severe loss of cohesion (about 51.69%) during this period. Soil meso-cracks were formed under the combined effects of swelling potential and tensile stress, which mainly occurred in three stages: initiation stage (1-2 cycles); propagation stage (3-4 cycles); equilibrium stable stage (5-6 cycles). Indeed, the strength and average width of cracks were increased to varying degrees, which degraded the structural integrity of the soil. In addition, the microstructure of the soil was deeply affected by WD cycles; the total volume of pores increased significantly, the particles roundness decreased marginally, and an isotropic particles orientation was achieved as a whole with local preferential orientation (depolarization). The deterioration of shear strength of the expansive soil may be attributed to alterations in humidity, which causes the clay minerals in the soil to swell and shrink repeatedly, thereby yielding the irreversible fatigue damage of microstructure and propagation of meso-cracks. This work sheds light on the properties of shear strength evolution and mechanisms of structural damage in expansive soils in semi-arid regions.

Effect of Saturation Degree on Mechanical Behaviors of Shallow Unsaturated Expansive Soils

“…(III) Sample ( 6) was stretched (or compressed) at a constant speed of 0.4 mm/min by setting the parameters of (10). (IV) The tensile (or compressive) force was measured by (4) during the tensile (or compressive) process and transmitted to the data acquisition equipment (11) in real time. The tensile strain can be obtained according to the mathematical relationship between the stretching (or compression) speed and time.…”

“…The shear strength of expansive soils has become a popular research topic in recent years. Many researchers have systematically carried out experimental studies on the effects of many factors on the shear strength of expansive soils by changing test conditions and methods, such as the effects of water content and dry density [5][6][7][8][9][10]; cyclic wetting and drying [6,[11][12][13]; temperature [14]; freeze-thaw cycle [15]; sodium chloride solution [16] on the shear strength of expansive soils; and the relationship between crack rate and shear strength [17][18][19]. Meanwhile, Zhai et al (2019Zhai et al ( , 2020a studied the relationship between the soil-water characteristic curve and the shear strength of unsaturated soil, and they predicted shear strength based on their relationship [20,21].…”

Effect of Wetting and Drying Cycle on the Behavior of Teff Straw Ash-Stabilized Expansive Soil

“…Additionally, higher water content in soil correlates with a lower coefficient of air permeability [6]. The ratio of cracks and the number of wetting and drying cycles share a direct relationship [7]. Pores in the soil are categorized into four types based on their size: small pores, mesopores, macropores, and cracks [8].…”