INVASION OF PRESSURIZED CLAY SUSPENSIONS INTO GRANULAR SOIL

Soil plastering is important in trenchless tunneling to ensure the stability and the integrity of the wall of a borehole. Two key processes in soil plastering with clay suspensions are viscous invasion (mud spurt) and filter cake formation. This study describes the conditions under which these two different processes occur. The results of experimental and theoretical developments are compared with some data from tunneling projects. Filter cake formation can only commence when the invasion velocity falls below the Peclet criterion for undrained behavior of the suspension. Whether a deep layer of invaded clay suspension or a filter cake develops (and situations in between) depends on the ratio between the time scale of viscous invasion and the time scale of filter cake formation. In clay suspensions with quartz flour filler or suspended sand, filter cake formation commenced earlier, in agreement with classical filter rules for filter cake formation. The analysis shows how the permeable compressible fabric of colloids influences invasion depth. The presented concept, in which consolidation and rheology are considered, can be utilized to develop drilling fluids designed for specific tasks in tunneling and drilling for hydrocarbon reservoirs. The test results can be used as verification material for porous media models containing a fluid with a permeable compressive fabric of colloids.