Swelling and Shrinkage Performance of Expansive Soils under Repeated Moisture Variations

Expansive soils increase in volume when they absorb water and contract as they dry out. This continuous variation in volume can cause uneven settlement, leading to cracks in light structures. The expansive soils under lightly loaded constructions, such as light buildings, pavements, are subjected to almost two cyclic shrink-swell per annum in the tropical zone due to the seasonal climatic variation. Expansive clay soils, characterized by high concentrations of montmorillonite, smectite, or other clay minerals, tend to shrink and swell with changes in moisture content. These volume changes can lead to notable deterioration in lightly loaded constructions if not properly addressed. This study aims to analyse the behaviour of expansive clays under shrink-swell cycles by means of direct shear and oedometer tests. The soil samples were collected in the region of Pobè (Issaba, Ahoyèyè) of Zogbodomey (Koto, Zoukou) and of Sèhouè Massi in the La Lama depression in the Republic of Benin. Soil characterization was carried out by means of geotechnical physical and mechanical tests. It was found that soil samples significantly swell for the first two cycles of alternated drying-wetting, after which the magnitude of swelling decreased. However, the cracking magnitude increased with increasing cyclic number of alternated swell-shrink, that result in the increasing of the mesopore of the samples.

Direct shear and oedometer tests were performed on samples of expansive soils subjected to alternated drying-wetting cycles. The results of direct shear tests under unconsolidated undrained conditions showed a notable decline in shear strength, indicated by a decrease in internal friction angle and cohesion, and an increase in void ratio with the increasing number of cycles. Furthermore, the paths of the undrained internal friction angle and the cohesion in function of the number of cyclic alternated wetting- drying converged to an L–shaped curve. Hereby, equilibrium condition was reached beyond 3 to 4 cycles. Most of the degradation of the shear strength occurred almost for the first three cycles of drying-wetting. Undrained internal friction angle decreases to 30% of the initial undrained internal friction angle, while undrained cohesion decreases to 35% of the initial undrained cohesion.

The results of oedometer tests showed an increase in void ratio e, in compressibility index Cc und in constrained modulus Eoed with increasing number of cyclic alternated drying-wetting. However, the swelling index Cs decreased with increasing number of cyclic alternated drying-wetting. Therefore, this study improves the comprehension of these complex processes underlying the behaviour of expansive clays under cyclic drying-wetting conditions and provides a basis for developing effective strategies to manage and mitigate their impact on infrastructure.