In the largest cities of China, the subway/metro systems have been increased dramatically due to the large population and limited surface spaces in recent years. So far, most of the subway tunnels in the cities have been constructed using the Earth Pressure Balanced (EPB) shield tunneling methods because of the many advantages that they offer over the conventional methods. Due to alignment constraints, many subway tunnels in the cities such as Beijing, Shenyang and Chengdu, are buried in whole or part in the coarse grain ground mainly consisting of fine/medium coarse sand and sandy gravel/cobble stratum with boulders. Various difficulties are encountered in the shield tunneling work undertaken in such ground conditions. Particularly, when tunneling in the cobble and boulder laden ground, the cutting tools suffer excessive wears and tears, and are liable to damage, fail and pull out. And even worse the cutterhead is worn through, which will adversely affect the rapid construction of tunnels. Consequently, it is of great significance to study the reasonable configurations of the cutterhead and cutting tools that is appropriate for the shield tunneling in coarse grain soils.

The cutterhead selection and the cutting tools configuration are rather difficult in grasping the complex interaction between the pressurized shield driving and the ground. The complex interaction has been studied mainly by approaches of numerical simulations, laboratory tests and field observations. Manuel and Luis (2005) using discrete numerical models analyzed the thrust and torque needed for the shields to excavate the tunnels on two Madrid Metro extension projects, and studied soil stability problem at the tunnel face [1]. Shen et al. (2009) provided a 3D finite element model for dynamic simulation of the cutterhead and soil interaction in slurry shield tunneling by coupling the ALE approach in commercial package LS-DYNA with submodeling method in ANSYS [2]. Glenn and Mustafa (2011) analyzed critical operational characteristics during the design phase of shield machines with the use of Caterpillar proprietary discrete element method software, and examined how discrete element analysis could be applied to cutterhead performance optimization [3]. Hunt and Del Nero (2011) provided an overview of tunneling in the cobbly-bouldery ground, cited key references for more detailed study and gave emphasis to the key risk issues and approaches to mitigate them [4]. Lambrughi et al., (2012) developed a 3D numerical model using the software FLAC3D to simulate the overall process of excavation and construction of an EPB shield tunnel with the emphasis on the reasonable constitutive equation of soil [5]. Wu et al. (2013) presented a 3D model for cutterhead excavation system of an EPB machine with the discrete element method software PFC3D, and the measured system performance parameters indicating stability of excavation face, soil discharging rate, cutterhead system torque and cutter wear were in accordance with the in-situ data [6]. Katushin et al. (2013) briefly explained the importance and gave the insight on laboratory wear testing of the drag bit materials for tunneling machines [7]. Mosleh et al. (2013) presented an experimental study of tool wear using a laboratory tester which simulated the operating conditions of cutting tools in soil excavation operations, and analyzed effects of the relative hardness and moisture on tool wear in soil excavation operations [8]. Jakobsen et al. (2013) summarized the development of the Soft Ground Abrasion Tester (SGAT) apparatus, and showed its capabilities to evaluate, quantify and compare how the soil mineralogy, water content, pressure, compaction, and the use of soil conditioning additives influenced the wear rate on the SGAT excavation tool [9]. Nishitake (1987) described the EPB shields designed by Mitsubishi Heavy Industries, Ltd. to cope with large size boulders scattered in rather soft alluvial soil, which were equipped with both drag bits and rolling disc cutters on the cutterheads [10]. Dowden and Cass (1991) summarized the designs of shielded type machines and their application, and discussed the new developments which blend hard and soft ground tunneling technology [11]. Burger (2007) recalled evolution of the design of both slurry and EPB cutterheads, reviewed the state of the art, and highlighted both common as well as geological specific design features [12]. Herrenknecht and Bӓpler (2007) focused on the design and technical challenges of the three largest soft ground tunnel boring machines: two slurry shields (15.4 m diameter) for the Shanghai River Crossing project and one EPB shield (15.2 m diameter) for a road tunnel project in Madrid in Spain [13]. Burger and Wehrmeyer (2008a, 2008b) examined the development of the modern mixshield including the design of cutterhead, and looked at two particularly influential projects and their impact on the future mixshield technology [14, 15]. Metzger et al. (2009) focused on design considerations of the largest mixshield machine ever implemented in North America to excavate the 8534 linear m × 6.7 m ID tunnel for the City of Portland Oregon’s, Bureau of Environmental Services East Side CSO Tunnel Project, which included machine configuration, extended-wear protection, hyperbaric interventions, boulder digestion and so on [16]. Maidl and Comulada (2011) pointed out the key performance indicators for estimating performance of the EPB shields based on the evaluation of data from a great of projects [17]. Bilgin et al. (2012) performed some theoretical estimations to estimate the effect of replacing disc cutters with chisel tools on performance of a tunneling machine in difficult ground conditions based on field measurements [18]. Willis (2012) showed the advances of quality and configuration for the cutting tools developed by Robbins engineers to reduce cutterhead wear at the EPB shield machines [19]. Havekost et al. (2013) introduced a plan to remove the close face shield machine and replace it with a digger shield when constructing the 1829 m long Portsmouth Force Main Tunnel in Portland and the machine was unable to advance through running soils containing cobbles and boulders [20]. Köppl and Thuro (2013) set up a prognosis model for cutter life in soft ground for mixshield machines based on empirical correlations with soil parameters and machine design and advance parameters after the detailed data analysis of the 18 mixshield drives [21]. Duhme and Edelmann (2014) presented the general principles of inspection and monitoring technology developed by Herrenknecht of the shield machine cutting tools as well as discussed examples of their applications [22]. Roby and Willis (2014) analyzed the record-breaking and high-performing projects to identify factors that contribute to fast advance of the EPB shield machines, such as adequate cutting tools and cutterhead design, and an effort was made to form simple, high level guidelines for the optimal shield machine excavation in mixed ground conditions [23]. In-door tests can only be used to deal with limited interaction features and sometimes are not realistic because of the ignorance of some basic factors. Numerical simulation seems to be a promising way to capture the interaction between tunneling shields and soils, but its application is greatly limited in practical engineering on account of difficulty in determining constitutive relations of the coarse grain soils. Field observations remain the commonly recognized approach for cutterhead performance prediction. The soft ground shield machine cutterhead design has historically been driven by the iterative designs based on empirical data, observations over various projects and industry rules of thumb [3]. Realistic performance predication of the EPB shield is only possible on the basis of the specific project data [17]. Any in-situ study conducted in the lately finished shield tunnel projects in coarse grain soils is valuable to promote the work of the cutterhead and cutting tools configurations.

This issue of the cutterhead and cutting tools configurations for the tunneling shields in coarse grain soils has caused widespread concern in the last decade in China with the completion of many subway shield tunnels, and a growing attention and interest is focused on it. Methodologies for shield tunneling in coarse grain soils progress steadily and some experiences of the cutterhead selection and cutting tools configurations have been published. Among others, Le (2003) recommended the key technical parameters concerning configurations of the shield cutterhead and cutting tools based on analyzing typical ground conditions encountered in Beijing area [24]; Wang (2005) introduced the first application of the EPB tunneling method in Beijing subway construction to dig Line 5 tunnels and described the main design parameters of the shield machine [25]; Liu and Song (2007) dealt with the problem of cutting tools wear in building Chengdu metro Line 1 tunnels in coarse grain soils and presented some recommendations after investigating failure causes of the disc cutters and scrapers [26]; Huang (2008) discussed influences of the oversized grains in the water-rich sandy gravel soils of Lot 13 of Shenyang metro Line 2 on configurations of the cutterhead and cutting tools as well as layout of the screw conveyor [27]; Zhang et cl. (2008) presented the monitored abrasions of cutting tools used in building some shield tunnel of Beijing subway line 4, and established the relation between abrasion coefficients of tools and shield tunneling parameters [28]; Wang (2008) analyzed the crucial technical parameters, such as opening ratio of the cutterhead and combination of the cutting tools fit for Chengdu geological conditions relying on the experiences and lessons from the construction of Phase 1 projects of Chengdu metro Line 1 [29]; Wang and Zhang (2011) studied the arrangement and structure of the cutterhead and cutting tools suitable for boring subway tunnels in Shenyang medium-coarse sand stratum in virtue of the experiences from construction of the tunnels on Shenyang metro Line 1 [30]; Zhu (2011) briefly illuminated the use of the spoke type cutterhead with opening ratio of 60% and the combination of precut bits and face scrapers in the Phase 2 Lot 7 project of Beijing subway Line 10 [31]. Nearly all known publications in China concentrated on only a single case study mainly because of the tribal knowledge of the involved tunnel project. Generalized studies from the cases study in combination with some undocumented recordings are necessary to improve the shield tunneling work, resulting in a more economic process in the coarse grain soils and further providing references for the succeeding similar projects. Due to the theoretical difficulties in mastering the interactions between coarse grain soils and cutting tools, the common understanding and recognition reached from the in-situ performances of the shield cutterheads remain the well accepted approach for configurations of the cutterhead and cutting tools suitable to dig tunnels in the coarse grain soils. On the basis of briefly summarizing main characteristics of the coarse grain soils disclosed in China, the conclusions on which consensus can be reached are generalized from the typical construction cases, and the rule-of-thumb suggestions on the configurations of the cutterhead and cutting tools are put forward.

It is of paramount importance to grasp mechanical and physical properties of the coarse grain soils to be encountered when the EPB shield tunneling method will be utilized. Presented in Fig. (1) is the typical sandy gravel/cobble soil with boulders found in building Beijing subway tunnels. Main characteristics of the coarse grain soils disclosed in China subway tunnel construction can be briefly summarized as follows:

1. Typical physical and mechanical properties are the large content of 30-70% by volume of coarse grains (gravel, cobble, and boulder), the general grain size of 30-200 mm with the maximum size of over 500 mm and the uniaxial compressive strength of grains as high as 180 MPa, taking the coarse soils encountered in Beijing for instance. Particularly, in the tunnel project of Lot 3 of Beijing subway Line 9, the content of the coarse grains is as high as 80%, and the maximum particle size is more than 1000 mm.
2. The coarse grain soils are of unfavorable plasticity and fluidity, which will bring serious abrasions to shield machines. In view of the high abrasivity of the sandy cobble/gravel soils with boulders, the primary wear on cutting tools and the secondary wear on the support structures such as the cutterhead, if not controlled, are excessive, and especially the cutting teeth made of hard metal (cemented carbide) is liable to flake off when subject to impacts of the over-sized particles.
3. Being loosely cemented and with less cohesion, most coarse grain soils exposed are typically unstable strata and their mechanical properties are unfavorable for force generation and transmission, and the ground surface is liable to cave in if improper manipulations of the shield machines exist.

Fig. (1). Typical sandy gravel/cobble soil with boulders encountered in Beijing area.

Though configurations of the cutterhead and tools of a shield machine must be customized for specific ground conditions, some options with respect to key parameters of the configurations can be deduced from the above-mentioned construction cases.

4.3. Use of Single Disc Cutters with Inserts

It is very common for shield tunneling in the coarse grain soils that the installed center disc cutters and single disc cutters suffer excessive abnormal wears. At a low rotation speed of the cutterhead, the center disc cutters are easily jammed by clogging of the cutterhead (see Fig. (20)). The single disc cutters have a single point of contact at the face and tend to skid or be stagnant in the soft or not compact soils, wearing flat on one side and making excavation of the over-sized particles difficult. Frequent work of changing cutting tools at more time and expenses has to be executed during the whole tunnel construction. Whereas, disc cutters with tungsten carbide inserts can excavate cobbles and boulders with a higher efficiency because they have multiple points of contact at the face. The multiple buttons act like traction, allowing the carbide cutters to ‘grip’ in soft or loosened ground and keep rotating. As compared to performances of the twin disc cutters, it was found that the twin disc cutters were more easily worn out than single disc cutters when building a railway tunnel in Beijing sandy gravel/cobble soils with boulders [32]. So, it is recommended that the plate type cutterheads equipped with the chip inserted single disc cutters in combination with the more aggressive soft ground tools are adopted when boring tunnels in the water-rich coarse grain soils.

Fig. (20). Clogging at the cutterhead center.

Based on the above statements and discussions, the following is concluded:

1. The spoke type cutterheads equipped with precut bits, shell bits, scrapers and a center fish-tail bit are strongly recommended for the shield tunneling in loose coarse grain soils without groundwater.
2. The spoke-plate or plate type cutterhead with reasonable opening ratio is a better choice if the first priority is to guarantee stability of the cutting face or mechanized driving will be performed in ground conditions rich in groundwater. The reasonable opening ratio of a cutterhead can be decided based on a good estimate of the size of particles in soils as well as experiences from the previous similar construction projects.
3. Reasonable combinations of tools and proper height differences among tools for the specific particle size distributions are of paramount importance to realize the efficient boring of tunnels in the coarse grain soils. Especially, the precut bits are suggested to be installed on both spoke type and plate type cutterhead on account of good performances revealed. The disc cutters, if mounted on cutterheads, will be single disc cutters with tungsten carbide inserts.
4. Enough aggressive tools and robust cutterheads allowing for longer driving in the coarse grain soils are suggested to be prepared or reserved to ensure the scheduled construction time lest the unforeseen scenarios or more difficult ground conditions will be encountered
5. In the case that interventions and chamber access for changing tools are indispensable, the tools replacement plan should be well planned beforehand and the operation procedure should be well prepared, as is helpful to reduce adverse effects caused by the over-sized particles on shield tunneling.