Geotechnical properties of surface sediments in the INDEX area

As a part of the environmental impact assessment studies, geotechnical prop erties of sediments were determined in the Central Indian Basin. The undrai ned shear strength and index properties of the siliceous sediments were det ermined on 20 box cores of uniform dimension collected from various locatio ns in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian o r diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0-10 cm) have a shear strength of 0-1 kPa; this value increases,vith depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varie s in the wide range of 312-577% and decreases with depth. The clay minerals such as smectite and illite are dominiant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any n otable variation with depth, thereby indicating a uniform, slow rare of sed imentation. The average porosity of sediments is 90.2%, specific gravity 2. 18, and wet bulk density 1.12 g/cm(3). Sediments exhibit medium to high pla sticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an inc rease in natural water content and a decrease in undrained shear strength o f sediments in the top 10- to 15-cm layer.