Evaluation of the sticking potential of clays to a tunnel boring machine cutterhead

In recent years there has been a trend in the tunneling industry towards underground projects within increasingly complex geologic settings. Advancing a TBM through a potentially adhering or "sticky" clay formation may present issues for the construction schedule and therefore budget. Several factors influence the potential for a clay to adhere to the excavating face of a TBM. The factor that most contributes to clay sticking potential is the swelling capacity which is most commonly estimated from the Atterberg limits of the clay. The other important factors is the moisture content of the sample which, assuming the formation is below the groundwater table, typically depends on the consolidation state and overburden history of the clay. Some other important considerations include the material type and surficial micro-roughness of the excavation face and tooling of the TBM.;Historical studies which include interfacial testing provide a lot of information on the mechanisms and representative constitutive models for the interfacial shear strength between soil and construction grade steel materials. More recent studies have focused specifically on the applications to soft ground tunneling. Multiple adhesion tests including this study and ones performed previously indicate that thresholds for sticking potential include a surficial roughness of greater than 2microm for stainless steel surfaces and a clay plasticity index greater than 20 with a consistency index between 0.3 and 0.7.;An independent testing program utilizing a ring shear device with modified top and bottom rings is presented in detail. Results indicate good repeatability and compare well with previous adhesion tests using modified direct shear apparatus. The relative benefit of the ring shear device over the direct shear device is the more uniform distribution of shear strains and the ability to test a clay to residual strengths without stopping to reverse shear direction. The drawback is the time and attention to detail required to mold the sample into the cylindrical chamber. Results from 19 ring shear adhesion tests indicate that maximum previous confining pressure has a significant effect on adhesive strength and a bi-linear Mohr Columb type strength is a representative model for the interfacial shear strength.