Geotechnical investigation of a fissured highly expansive clay profile

Abstract

This study details some observations and challenges in interpreting site investigation data for highly expansive soils. Expansive soils exhibit large volume changes in response to changes in water content. This leads to a high degree of reworking and often a strongly fissured macrofabric in situ. A site underlain by a ~7.0 m thick active clay deposit, containing highly plastic and highly expansive smectites, was chosen for large-scale pile tests. Two identical areas were identified for the tests. The first was kept at natural water content conditions. The other was kept submerged under water through infiltration wells for six months, aiming to facilitate maximum swell of the active layers. As part of the site investigation, continuous surface wave (CSW) tests and standard penetration tests (SPT) were conducted at various points in time. These results have been compared for the natural water content and the submerged profiles. Small-strain moduli interpreted from the CSW tests showed no significant difference between the two profiles, contrary to the intuitive expectation that inundation of the highly expansive clay layers and dissipation of suctions in the order of several megapascals might significantly reduce the stiffness. Due to the small strains imposed during the test, the characteristics of the fissures and joint infill material are measured, rather than that of the intact masses. The large-strain SPT results showed a softer response for the inundated profile, as expected. Challenges in obtaining intact tube samples from the stiff and highly fissured unsaturated profile have also been discussed.

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