Rock+fragmentation+under+different+installation+po.pdf

J. Cent. South Univ. 2017 24 2306−2313 DOI https//doi.org/10.1007/s11771-017-3642-2 Rock fragmentation under different installation polar angles of TBM disc cutters CHENG Yong-liang程永亮1, 2, ZHONG Jue钟掘1, MEI Yong-bing梅勇兵1, 2, XIA Yi-min夏毅敏1 1. State Key Laboratory of High Perance Complex Manufacturing, Central South University, Changsha 410083, China; 2. China Railway Construction Heavy Industry Co., Ltd., Changsha 410100, China Central South University Press and Springer-Verlag GmbH Germany 2017 Abstract The disc cutters of tunnel boring machine TBM are installed with different polar angles. This causes the cutting depth difference between adjacent disc cutters on the tunnel face. A rock-cutting model was established to study the rock fragmentation law between adjacent disc cutters with different polar angles based on particle flow code PFC. The influence of polar angle of adjacent disc cutters on rock cracks and stresses under different cutter spacing and penetration was studied. Research shows that polar angle difference leads to the discontinuity of rock-fragmentation process by adjacent cutters. The effect of rock-fragmentation is influenced by the cutting depth difference between adjacent cutters. The effect of rock-fragmentation pered best, meanwhile large rock blocks were flaked when the difference of cutting depth is half of the penetration. Too large or small difference of the cutting depth will cause high specific energy consumption of rock fragmentation. The specific energy consumption is relatively small when the difference of cutting depth is half of the penetration. Key words tunnel boring machine; disc cutter; polar angle; particle flow code; rock fragmentation 1 Introduction Tunnel boring machine TBM occupies a very important role in tunnel construction projects nowadays. TBM moves forward by disc cutters of cutter-head rolling and breaks the rock [1–5]. Construction quality is influenced directly by fragmentation of tunnel face. Many studies have been presented on rock crack ation and propagation patterns under the action of disc cutter. The crush zone is generated beneath the disc cutter, and then the intermediate cracks, radial cracks and lateral cracks develop outwards from the disc cutter [6]. LIU et al [7, 8] and SU et al [9] simulated the ation and development of rock cracks under the action of TBM cutter and pointed out that the effect of rock fragmentation was determined by the law of rock cracks’ development. The growth of rock cracks under the action of disc cutter is influenced by geological factors including rock strength, rock internal structure, mineral content, joints and confining stress [10]. Two- dimensional numerical study was applied by TAN et al [11] to explore the influence of joint strength, spacing and orientation on rock fragmentation. LIU et al [12] adopted a 2D discrete element to explore the effects of embedded cracks with different dip angles on the rock fragmentation process, cutting characteristics and breaking efficiency. Combined with numerical simulation, the influence of confining stress on cutting process, fracture conditions and cutting efficiency of soft and hard rock has been conducted on the triaxial testing machine by MA et al [13] and LIU et al [14]. TBM cutter-head and its tunnelling parameters are set up reasonably according to the geological conditions in a construction project. Among those parameters, disc cutter spacing and cutter-head penetration have a great influence on rock fragmentation effect. BALCI [15] studied the effect of disc cutter penetration and specific energy consumption by using the linear cutting test. Based on the principle of minimum specific energy consumption, the appropriate cutter spacing and penetration were selected and the feasibility was verified by comparing with the actual engineering data, which guided the selection of TBM parameters. CHO et al [16] made a simulation of the linear cutting machine test LCM by using AUTODYN-3D. The rock fragmentation process was analyzed and the optimum cutting spacing was acquired Foundation item Project2012AA041801 supported by the Hi-tech Research and Development Program of China; Project2013CB035401 supported by the National Basic Research Program of China; Project51475478 supported by the National Natural Science Foundation of China Received date 2016−03−21; Accepted date 2016−08−15 Corresponding author CHENG Yong-liang, Senior Engineer; Tel 86–15873319850; E-mail yongliangcheng 万方数据 J. Cent. South Univ. 2017 24 2306–2313 2307 based on the principle of the minimum specific energy consumption. TAN et al [17] used the discrete element to analyze the dynamic response mechanism and rock crack propagation law under the action of double disc cutters. The optimum disc cutter spacing was acquired and the simulation was verified on the rotary cutting test machine. The position layout of TBM disc cutter is usually represented by its installation radius and the polar angle on the cutter-head. The influence of disc cutter’s installation polar angle on rock fragmentation was not considered in the above stated researches. The order of cutting is influenced by installation polar angle. MOON and OH [18] simulated the process of disc cutter sequentially pressing into the rock with different penetrations and cutter spacing based on the discrete element software and verified the result with linear cutting machine test. TAN et al [19] also simulated and studied different rock fragmentation models of disc cutter cutting simultaneously or sequentially based on the discrete element numerical simulation. HUO et al [20] established a mapping relationship between sequential angle and rock fragmentation energy under optimum cutter spacing of multiple disc cutters, and pointed out that the rock fragmentation efficiency is higher with multiple disc cutters sequentially breaking the rock. The installation polar angle of disc cutter not only affects the cutting sequence, but also leads to crack interaction in the circumferential direction and cutting depth’s difference in the direction of tunnelling. This work aims to investigate the effect of different polar angles of adjacent disc cutters and different interaction models of adjacent disc cutters on TBM cutter-head. Using discrete element simulation software, the process of crack growth and rock fragmentation effect under different cutting depths were analyzed. 2 Rock fragmentation s of disc cutters with different polar angles 2.1 Cutting depth between adjacent disc cutters The whole TBM disc cutter rotates in a spiral . On the one hand, it moves in a uni circular shape along the cutter-head. On the other hand, it moves forward with the same speed in the direction of tunnelling. The position of TBM disc cutter on the cutter- head is usually determined by its installation radius and the polar angle. As shown in Fig. 1, adjacent disc cutters are often arranged in different polar angles. The adjacent disc cutters pass by one region with a time difference when cutter-head rotates to cut rocks. It means that when the adjacent disc cutters pass by any radial line sequentially, there will be some cutting depth differences between the previous and the next cutter on this radial line. As shown in Fig. 2, because rocks are flaked unevenly in rock-breaking process, the tunnelling face is often uneven when TBM disc cutters break the rock. Cutting depth difference of adjacent disc cutters aggravates the unevenness of tunnelling face. Fig. 1 8 m open type TBM cutter-head Fig. 2 Tunnel face of Liaoning northwest water diversion project The polar angle difference between two adjacent cutters is denoted as θ θ≤π, and the cutter-head tunnels forward for h with one revolution. Then there will be two cases on adjacent disc cutters’ cutting depth in one rotation 12 ; 2π2π hhhhh θθ - 1 There might be three rock fragmentation s between adjacent cutters in the process of one rotation. 1 Cutting depth difference h1 and h2 appears alternately; 2 Rock is broken only with cutting depth h1; 3 Rock is broken only with cutting depth h2. If the cutter cannot break the rock with cutting depth h1, it will break the rock with the cutting depth h2 later. In the following, the result of rock cutting in some cutting depth will be studied and the effects of rock fragmentation with different cutting depth are compared. As shown in Fig. 3a, the cutter-head rotates in a 万方数据 J. Cent. South Univ. 2017 24 2306–2313 2308 counter clockwise direction and the polar angle difference between the inside disc cutter and the outside disc cutter is π/6. In zone A, the outside cutter cuts the rock ahead of the inside one by π/6 and the inside cutter is h/12 lower in depth than the outside one. In zone B, the inside disc cutter cuts the inside rock along the arc LN. When the outside disc cutter rolls in zone B, it has fallen behind the inside disc cutter by 5π/6 and the outside cutting depth is smaller than the inside one by 11h/12. In one rotating process of cutter-head, if h1 and h2 appear alternately, the proportion of the area of the rock fragmentation will be 12 21 Ah Ah A1 and A2 represent the area of zone A and zone B respectively, which means that there will be a bigger area of rock fragmentation when the cutting depth difference is smaller. When the polar angle is smaller, although h1 and h2 vary greatly, the rock fragmentation face is relatively flat with a smaller cutting depth difference in most part of the area. 2.2 Discontinuous rock fragmentation process TBM breaks the rock by the crack interaction produced by adjacent disc cutters. The existence of the polar angle difference between adjacent disc cutters determines that there is not always crack interaction between adjacent disc cutters, and the process of rock fragmentation is discontinuous. As still shown in Fig. 3a, when the inside disc cutter is in zone B and the outside disc cutter is in zone A, the cracks produced by adjacent disc cutters do not interact and the effect of rock fragmentation is not obvious. When the inside disc cutter is in zone A or the outside disc cutter is in zone B, the cracks produced by adjacent disc cutters interact and large rocks peel. In one rotation of the cutter-head, interaction time between adjacent disc cutters is t2 and non-interaction time is t1, then 1 2 2π t t θ θ - 2 This shows that non-interaction time t1 is longer and the discontinuity of the process of rock fragmentation is more obvious while the polar angle difference is bigger. Figure 3b is shows a schematic view of the rock fragmentation process when the polar angle difference between the inside and the outside disc cutters is π. During the first half of the cutter-head’s one rotation, the outside disc cutter rolls along the arc ML and the inside disc cutter rolls along the arc HN, and there is no collaborative rock-breaking between the inside and outside cutters. During the second half of the cutter-head’s one rotation, Inner and outer cracks intersect, which resulted in that two disc cutters broke the rock collaboratively. The cutting depth of the inside cutter is lower than the outside one by h/2 in zone A, and Fig. 3 Sketch of rock cutting process by adjacent cutters a Polar angle difference of π/6; b Polar angle difference of π higher by h/2 in zone B. During the first half of the cutter-head’s one rotation, there is not much rock peeling off between adjacent disc cutters, therefore rock fragmentation by adjacent disc cutters is mainly in the second half of the rotation. This is an alternate rock fragmentation process. The discontinuity of rock fragmentation process exacerbates the cutter-head vibration and load instability. 3 Rock fragmentation effect of disc cutters with different polar angle 3.1 Establishment of rock-breaking model by disc cutter Rock will act in diverse crushing patterns under different cutting depth difference of adjacent disc cutters. With discrete element simulation software, the effect of rock-breaking under different cutting depths is simulated, and the model of rock-breaking by disc cutter is established as shown in Fig. 4. The rock samples are made of a large number of flexibly-glued rigid particles. 万方数据 J. Cent. South Univ. 2017 24 2306–2313 2309 Contact failures are classified into tensile failure and shear failure, which are respectively shown in red and blue Fig. 5. The macroscopic parameters of marble are shown in Table 1. The size of the rectangular model is 400 mm in length and 120 mm in height. According to the cutting depth difference, some part of the rock is ignored and the ramp boundary is applied between two cutters. The cutters are regarded as rigid bodies, which are pressed into the rock samples sequentially. The right cutter is pressed into the rock with a depth of h firstly, and then the left cutter is also pressed into the rock with a depth of h which is lower than the right cutter by Δh. Fig. 4 Schematic diagram of simulation model Fig. 5 Discrete element of rock particles 3.2 Process of crack development with cutting depth differences The cases that cutter spacing is 80 mm and 90 mm respectively and the penetration is 8 mm were selected. The processes and effects of rock-fragmentation at varying cutting depth differences were compared. As shown in Fig. 6a, two adjacent disc cutters were pressed into the rock at the same cutting depth. Shear cracks and tensile cracks generated where the cutter and the rock contacted with each other and the crush zone generated under the cutters. With the cutter intruding into the rock continuously, intermedian and lateral cracks were generated around the crush zone. Intermedian cracks continued to develop to a greater depth, while lateral cracks developed along the direction which is approximately parallel to the free face until the lateral cracks generated from adjacent cutters intersected and rocks flaked. Then the process of rock-fragmentation was completed. Due to the anisotropy of the rock, the fragmentation conditions near adjacent cutters were not all the same. When there was a polar angle between adjacent cutters, two cutters could not go through any cross section simultaneously and there was a sequence. The latter cutter had a larger cutting depth. Figure 6b shows the rock-fragmentation situation at a cutting depth’s difference of 4 mm. Due to the influence of left lateral ramp, the limiting effect to cutter cracks was less comparing to a plane case, and left lateral cracks were fully developed and extended further when the first cutter was pressed into the rock as shown in Fig. 6. The cutter load undertaken