Experimental Investigation of Microventing in Kaolinite

The present research focuses on the causes of microventing, a phenomenon generally occurring in cohesive sediments during their deposition and consolidation. Microvents are characterized by volcano--like formations, remnants of the expulsion of fluid and sediment along a vertical channel through the sediment layer. The size of these vent formations ranges from the order of millimeters to several centimeters, hence the term ``microvent.'' Several causes proposed for microvents may be generally grouped as biological processes and mechanical processes. An example of a biological process is the production of subsurface gas due to organic decomposition; rising bubbles can produce microvents. The present study is concerned only with mechanical processes, which are hypothesized to be: \begin{itemize}

Pressure fluctuations in pore fluid

Solids concentration perturbations near sediment--suspension interface

Gas expulsion by sediment compression The objectives of the study are to examine microventing through direct and indirect means, in order to examine these hypotheses. Sedimentation tests have been performed in the laboratory using kaolin clay in water; a natural microventing has been observed to occur at sediment concentrations similar to that of a lutocline. A series of controlled experiments are presently underway to force microventing by means of water or gas injection into the sedimentation column, in order to approximate the natural microventing. It is believed that analysis of pore water pressure and total (solid and water) pressure time series will lead to better understanding of the causes of microventing, and allow discrimination between those causes. Pressure gradients and solids concentration gradients will also be examined in these tests, and image analysis will be used to compare the visible characteristics of the microvents which are formed.