Diatoms microfossils and their microstructural effects on the sensitivity of metastable silty-clayey soils

Abstract

In 2013, an accident occurred in Port of Santana, located at the mouth of the Amazon River, in the State of Amapa, Brazil, comprising a large volume of soft silty-clay soil flowing very fast, for a great distance, towards the bottom of the river. Field tests performed afterwards identified the occurrence of sensitive soil layers at the site. Subsequent laboratory tests verified the presence of diatom microfossils in layers with highest liquidity indices. Owing to peculiarities of the accident, a large research programme started at PUC-Rio, aiming to investigate the role of these biomineral grains on soil behaviour. Thus, based on physical-chemical-mineralogical tests performed on samples from the site, higher sensitivity natural soils from Port of Santana were reconstituted. Chemical, mineralogical and grain size reconstitution was carried out using manufactured soils containing equal proportions of minerals as in the natural soils, but with a difference in the portion of non-plastic siliceous grains: a reconstituted soil used quartz, in silt size, and the other used diatomaceous earth. This article presents and discuss results of undrained shear strength data obtained through laboratory fall cone and vane tests, as well as microstructure analyses - including scanning electron microscopy, mercury intrusion porosimetry and X-ray microtomography - in high sensitivity natural diatomaceous soils and reconstituted soils, in the liquidity index range of 1.0 ≤ LI ≤ 1.6. It is shown the fundamental role of the diatom microfossils in microstructural metastability, impacting the high sensitivity measured in soil reconstituted with diatoms and in natural diatomaceous soils.