Experimental assessment of clayey layers for clogging potential in TBM tunnel driving and the influence of compositional factors.

• Adhesion behavior of clayey soils are investigated through experimental tests. • Adhesion increases firstly and decreases thereafter with increasing water content. • Clay fraction and mineralogical composition significantly influenced the adhesion. • The updated semi-empirical diagram considering compositional factors is proposed. Clogging is common for tunnel boring machines, especially in clayey soil, and when it happens, the cutter heads or even the entire tunnelling device can become completely clogged, causing low excavation efficiency and high project costs. Because clogging stems from the adhesion of the excavated geomaterials, a better understanding of soil adhesion is required if clogging is to be avoided. Reported here are piston pull-out tests conducted to investigate the adhesion of natural clayey soils and artificial soils prepared using pure kaolin, montmorillonite, and silica sand, thereby enabling consideration of the effects of compositional factors (e.g., clay fraction, and mineralogy). The results show that with increasing water content, the adhesion stresses of the studied samples increased initially and then decreased. With increasing clay fraction, the adhesion stresses increased linearly because of the larger real contact area of the interface between the soil and a foreign object. With the addition of montmorillonite, the soil adhesion also increased significantly because of its microstructural properties. An updated semi-empirical diagram is proposed that allows the effects of compositional factors on clogging potential to be evaluated qualitatively. The measured maximum adhesion stresses of the studied samples occurred in the region of strong clogging in the semi-empirical diagram, showing the feasibility of using adhesion stress as an index for assessing the clogging of tunnel boring machines. This study provides valuable experimental data on soil adhesion and a better understanding of the adhesion behavior of clayey soils. [ABSTRACT FROM AUTHOR]