Importance of sedimentation process for formation of microfabric in clay deposit

When a clay sample is reconstituted from a slurry state to a normally consolidated state, a higher initial water content generally derives a higher void ratio in e–logp relationship. However, in undisturbed Holocene clay samples collected from a soil layer under the seabed, void ratio is much higher than that for the reconstituted, even though the reconstituted sample was consolidated from a sample slurry state with a very high initial water content. Therefore, the higher void ratio for undisturbed Holocene clay cannot be explained by the initial water content. There must be another key factor to explain the higher void ratio for undisturbed Holocene clay. In this study, the difference between the microfabric of deposited clay from suspension state and reconstituted clay from a slurry state is investigated to clarify the key factors contributing to the higher void ratio. The clay in this study is Osaka Bay Holocene clay sampled from GL −15 to −19 m below the seabed. Sedimentation tests from the suspension state were carried out in various conditions of initial water content, salt concentration, and pH. A series of sedimentation tests was carried out with two types of hollow cylinders: an observation glass cylinder with an inner diameter of 60 mm and a height of 400 mm (specimen height of 350 mm), and a cylindrical acrylic container with an inner diameter of 100 mm and a height of 1800 mm (specimen height of 1500 mm). In the 100-mm diameter cylinder, the deposited sample was consolidated by a pressure of 19.6 kPa, then mounted in an oedometer ring with an inner diameter of 60 mm and a height of 20 mm. An incremental loading oedometer test was then carried out up to 39.2 kPa. The microfabric of the consolidated specimen was observed by scanning electron microscope (SEM) and quantitatively investigated by mercury intrusion porosimeter (MIP). It was shown that the high void ratio of natural clay deposit is not only due to the high initial water content but also to the sedimentation process with flocculation. This plays an important role in the formation of the microfabric of natural clay deposits characterized by a significantly higher void ratio than reconstituted samples.