- Original Article
- Open Access
Fragmentation Energy-Saving Theory of Full Face Rock Tunnel Boring Machine Disc Cutters
© The Author(s) 2017
- Received: 30 March 2017
- Accepted: 2 June 2017
- Published: 20 June 2017
Attempts to minimize energy consumption of a tunnel boring machine disc cutter during the process of fragmentation have largely focused on optimizing disc-cutter spacing, as determined by the minimum specific energy required for fragmentation; however, indentation tests showed that rock deforms plastically beneath the cutters. Equations for thrust were developed for both the traditional, popularly employed disc cutter and anew design based on three-dimensional theory. The respective energy consumption for penetration, rolling, and side-slip fragmentations were obtained. A change in disc-cutter fragmentation angles resulted in a change in the nature of the interaction between the cutter and rock, which lowered the specific energy of fragmentation. During actual field excavations to the same penetration length, the combined energy consumption for fragmentation using the newly designed cutters was 15% lower than that when using the traditional design. This paper presents a theory for energy saving in tunnel boring machines. Investigation results showed that the disc cutters designed using this theory were more durable than traditional designs, and effectively lowered the energy consumption.
- Newly design
- Disc cutter
- Specific energy
- Energy consumption
A full face rock tunnel boring machine (TBM) is a large underground device for full-scale boring of rock. The cutting of rock requires considerable power (usually up to several thousand kW) and involves massive energy consumption. For example , excavation in certain geological strata with either un- or underdeveloped surrounding rocks consumes as much as 3000 kW/h per meter in electrical energy. Researchers around the world have therefore focused on approaches to reduce energy consumption of TBM.
As far back as 1965, Teale  proposed the concept of specific energy, defined as the amount of energy required to cut through a unit volume of rock, and thereby introduced the start of a new era in terms of energy-saving designs for TBM. The distinctive feature of this design was the concept of an optimal cutter spacing, which was utilized to determine the position of the disc cutters (the difference between the radii of adjacent disc cutters), i.e., the cutter spacing was determined by the minimum specific energy requirement. In 1978, by using a TBM indentation test, Wang, et al  found that an optimal cutter spacing existed for the layout of disc cutters. In 1985, Mao, et al  also discovered the existence of an optimal cutter spacing by using a disc-cutter rolling test. In 2007, Gertsch, et al  used linear rolling test, determined the optimal spacing of disc cutters used in hard rocks, such as Colorado Red Granite, for which the optimal spacing was 76 mm. Acaroglu, et al  developed a fuzzy logic model to predict specific energy requirements for TBM performance. In 2012, Moon, et al , through simulations and results obtained in real linear cutting machine (LCM) tests, revealed that the effective rock-cutting condition corresponding to the minimum specific energy could be estimated by an optimized ratio of disc spacing, s, to penetration depth, p (the s/p ratio), which, in turn, is linearly proportional to the square of the material brittleness, B 2, and cutter tip width, t (i.e., s/p = cB 2 t, where c is a coefficient). In 2013, Cho, et al  studied the minimum specific energy required during TBM excavation in a Korean granitic rock using LCM testing and photogrammetric measurement and provided a three-dimensional (3D) digital comparison. In 2015, simulation by Hadi, et al  revealed that eroded disc cutters increased the specific energy requirement. Simulations by Mohammad  showed that the specific energy requirement of a double disc was less than that of a single disc and that the optimum s/p ratio was about 10. These studies all focused on constant cross-section (CCS)-type disc cutters and those used earlier. At present, the energy saving method is mainly focus on the traditional cutters , and no researches can be found from the public information about designing a new cutter to reduce the energy consumption of TBM. The large energy consumption by use of traditional cutter in the excavation process enhanced the vibration of the cutterhead, increased the disturbance variable in the control of the cutter head system, and influenced the stability of the cutterhead .
In 2012, the 3D fragmentation theory of disc cutters was developed . The following year, it was reported that disc cutters designed according to this theory had an apparent enhancement in their lifetime  and the specific energy required for fragmentation was lower [15, 16]. Alteration of the angles of the disc cutters during fragmentation was found to be capable of reducing the force required for fragmentation .
This work presents fragmentation models of traditional (CCS-type) and newly designed (according to 3D fragmentation theory) disc cutters based on the above research and with consideration of the effects of alternating cutter edge angles. Coupled with a field study, research has been carried out concerning the energy consumption of penetration, rolling, and side-sliding fragmentations. Related field data revealed that the amount of energy required by the newlydesigned disc cutters was 14.8% less than that of traditional cutters.
Fragmentation by disc cutters involves the process of a resultant forceacting between a disc cutter and its cutting object—the rock, and includes extrusion, stretching, and shearing forces. The complex mechanism and physical properties of rocks, such as anisotropy, fracture, and brittleness, render it difficult to develop a mechanical model of fragmentation by disc cutters: despite much research, this issue remains incompletely resolved.
What interested us was that even before leap-frog fragmentation occurred, deformation of rocks under the action of the disc cutters possessed some features of plasticity, which are shown in Fig. 2. A mechanical model was developed for the mutual interaction between rocks and disc cutters. Theoretical study of the model demonstrated that modification of the disc-cutter edge angles leads to effective reduction of the specific energy of fragmentation, as discussed in Refs. , .
The thrust of a disc cutter is the force on the cutter applied by the TBM hydraulic cylinder through the cutter head.
3.2 Energy Consumption
Energy consumption of penetration cutting
Energy consumption of full-scale linear cutting
Energy consumption of side-slip cutting
In Fig. 6, O 1 O 2 is the axis of a disc cutter, O 2 O 3 is the axis of the cutterhead, his the penetration, D is the instantaneous disc-cutter maximal penetration point, and C is the instantaneous disc-cutter cutting point. Because it is transport motion (revolution of the cutterhead) that leads to side slip, an analysis of transport motion is presented.
4.1 Basic Theory
4.2 Case Study and Verification
The parameters adopted for the disc cutters in this case study, such as locus and disc-cutter radii, are given in Ref. . Substituting the corresponding parameters into Eqs. (25)–(28), we respectively obtained: λ = 6.808%, ε = 3.477%, β = 4.516%, ζ = 8.372%. The fact that these results satisfy Eq. (29) demonstrates that the theory presented is essentially correct. It can also be concluded that energy savings by employing newly designed disc cutters could reach 6.808% + 3.477% + 4.516% = 14.801%.
Equations are proposed for the fragmentation energy consumption of single traditional and newly designed disc cutters;
Equations are developed for fragmentation energy consumption of the traditional and newly designed disc cutters positioned on the cutter head;
Comparative study showed that a TBM equipped with the newly designed disc cutters used less energy than one with traditional cutters; i.e., under the same fragmentation conditions and for the same penetration, the energy consumption of the newly designed disc cutters was about 15% lower than that of the traditional cutters.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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