Abstract

Tunnel boring machines excavating through soft soils face different challenges, one of which occurs when the soil sticks to the cutter face or the conveyor band and obstructs the machine. This phenomenon, commonly referred to as clogging, leads to wearing of the cutting wheel and transportation system, delays in the time schedule and economic loss. Although several laboratory devices can evaluate the adhesion mechanism of soil to metal, the method to measure adhesion has not been standardised. As clogging is also directly related to the construction phase, engineers are more concerned with methods to avoid this problem during construction. In this regard, the application of soil conditioners has become useful, the benefits of which include torque reduction, easier soil manageability and clogging reduction. However, the effectiveness of the soil conditioners is difficult to evaluate. To provide some insight into the topic, this paper describes the basic mechanism of clogging, the key parameters for its evaluation, the laboratory tests conducted up to date, the classification diagrams developed to assess clogging risk and mitigation of this risk in underground tunnelling. This paper also describes some of the additives and their functions, the ratios used for measurement and application, and the current tests to evaluate their performance. Finally, the conclusions summarise the current findings in the issue of clogging, pointing to the advantages and shortcomings of previous research, as well as some lines of investigation to improve identification and mitigation of this problem.