Mechanical behaviour of undisturbed diatomaceous soil

Abstract

The mechanical behaviour of sedimentary marine soils containing diatoms does not follow classical trends usually found in Soil Mechanics, and evidence of geotechnical singularities of undisturbed samples are still scarce. This article presents new insights on the behaviour of natural diatomaceous soil in compression, shearing and cyclic loading, on undisturbed highly plastic diatomaceous silt from Mejillones Bay in northern Chile. Microscopic observations show that the soil contains significant amount of siliceous diatom. Isotropic compression results in high overconsolidation (OCR > 20) and high compressibility after yielding. Similarly, at low confining stress the undrained shear strength does not depend on pressure and is virtually equal to the UCS = 730 kPa. At higher stresses, a transition to frictional behaviour is observed, characterized by high variability in excess pore pressure generation during undrained triaxial shearing tests, presumably due to heterogeneous diatom layering in undisturbed soil samples. Microscopic observations indicate that high compressibility could be attributed to massive diatom breakage and disturbance of the soil microstructure. Compared to conventional data on fine soils, the cyclic behaviour of the diatomaceous soil is more brittle at low strains, with higher normalized shear modulus and lower damping at shear strains lower than 2 × 10−1%; however, it presents higher degradation at larger strains.