# Criteria for Cutting Head Clogging Occurrence during Slurry Shield Tunneling
## 1 Introduction  
In recent years, closed-face tunneling machines, namely earth pressure balance shield machines and slurry shield machines, are being widely used to excavate tunnels in soft ground because of their many merits, such as safety, high advance rate, and reduced disturbance to the surrounding ground. When tunneling in fine-grained soils, excavated materials sometimes adhere to the cutting head and/or the conveyor system of the tunneling machines, which is referred to as clogging [1, 2] or adhesion [3, 4]. These issues, if not properly dealt with, will cause excavation difficulties, the extension of the construction duration, and consequentially result in budget overrun. The problem of clogging in soft ground shield tunneling has aroused widespread concern, and numerous efforts have been devoted to it. Because clogging is concerned with both cohesion of soil particles and the adhesion of the soil to a metal surface, existing studies have attempted to evaluate it in two ways. The first approach of estimating the clogging potential is dependent on the indices of plasticity and consistency of the involved soils, where the plasticity index is defined as the difference in moisture content of soils between the liquid and plastic limits expressed in percentage, and the consistency index \(l_{\mathrm{c}}\) is defined as:  
\[l_{\mathrm{c}}=(w_{L}-w)/I_{p} \tag{1}\]  
where the parameter \(w\) represents actual water content, \(w_{L}\) is the liquid limit, and \(I_{p}\) is the plasticity index. This approach is particularly beneficial for practical purposes,and many researches focused on it. [PERSON] and [PERSON] (2004) [1] presented a clogging diagram only by judging the properties of an in situ clay regarding the particular operating conditions of a slurry-supported shield. Based on a combined evaluation of water content and plasticity, [PERSON] and [PERSON] (2013) [5] developed a new diagram to evaluate the clogging potential of clay soils for all types of tunneling machines, and this new diagram enabled the assessment of the consequences when the water content of a clayey soil is changed during excavation. The second approach is based on assessing soil adhesion to the metal surface using direct shear tests, vane shear tests, and other laboratory tests. For example, [PERSON] et al. (2000) [6] investigated the adherence behavior of clay by measuring the shear stress required to shear the clay over a steel surface in a direct shear box, and results showed a great influence of the steel roughness, contact time and mineral type. Using kaolin clay, [PERSON] and [PERSON] (2009) [7] did tests to simulate clay sticking to the cutting wheel of the machine to evaluate the likelihood of adhesion potential, and the results of tests suggested that the adhesion changed with the compression force. Other factors, including clay mineralogy and roughness of the face of the cutting wheel, were also considered in the tests. [PERSON] et al. (2011) [8] developed a new classification scheme for clogging potential based on the cone pull-out test to detect the adhesion/clogging propensity of a soil or rock. [PERSON] and [PERSON] (2012) [9] proposed new methods for stickiness and adhesion quantification, and a newly developed device was used to measure the tangential adhesion and sliding resistance between a steel plate and soft soil pastes at different applied pressures. [PERSON] et al. (2016) [10] performed normal piston pull out and modified direct shear tests to determine the normal or tangential adhesion stress of the clayey soil samples. The results indicated that normal adhesion values showed meaningful variation with consistency index and were compatible with the base of field clogging assessment criteria. Based on discussing the laboratory tests for assessing clogging potential, [PERSON] and [PERSON] (2016) [11] explained a diagram for assessing clogging risks for all types of tunneling machines, and introduced a new testing scheme for the evaluation of sedimentary rocks regarding clogging. [PERSON] et al.