Experimental Study on Rock Fragmentation by the 19-inch TBM Cutter and Statistical Analysis of Debris

Transcription

1 Experimental Study on Rock Fragmentation by the 19-inch TBM Cutter and Statistical Analysis of Debris Wenliang. Yang a,b, Yadong. Xue a,b * and Xueqiang. Zhang c a Department of Geotechnical Engineering, Tongii University, Shanghai, China b Key Laboratory of Geotechnical and Underground Engineering of Education Ministry, Tongji University, Shanghai, China c Shandong Techgong Geotechnical Engineering Equipment Co., LTD, Shandong, China * yadongxue@126.com Abstract The inspiration of this paper stems from the increasingly wide use of 19-inch disc cutters in recent TBM tunnel projects. The cutting mechanism of the 19-inch cutter is different from that of the 17-inch cutter. To explore the working performance of 19-inch cutter, a series of preliminary full-scale disc cutting tests are conducted with a single disc cutter (483mm diameter and 13mm tip width). This cutter was installed on the TJ-TS500 linear cutting machine (LCM), and a concrete specimen with a size of 980mm 980mm 500mm was cast in the model box. A database composed of specimen properties, cutter forces (normal, rolling and side), penetration depth, and spacing is established using the data collected from the LCM. Both normal and rolling force will increase with the increase of spacing, as well as penetration. Specific energy is a significant parameter in TBM performance predicting. The result shows that the lowest SE is not only correlated with spacing, but also penetration. The weight and size of chip debris are measured after one group of tests. Characteristics of chip debris are somewhat correlated with cutting process. SE and chip percentage have a rough negative correlation. Nominal size increases with spacing, especially for the largest chip debris. The results of the model test are valuable for the performance prediction of the field TBM. Keywords: 19-inch Disc Cutter, Linear Cutting Machine, Cutter Forces, Rock Debris 1. Introduction China is a rapidly developing country and there are many tunnel projects under construction, including metro tunnels, highway tunnels, railway tunnels and water supply tunnels. An increasing number of these tunnels are being constructed by rock Tunnel Boring Machine (TBM). Compared to New Austrian Tunneling Method (NATM), TBM has the advantage of higher safety, quality and efficiency (PENG Qi, 2013). The inspiration of this paper stems from the application of 19-inch TBM cutters in the excavation of a water supply tunnel in Northwest Liaoning, China. The field TBM type is Robbins MB , with a cutter-head of 51 cutters and a diameter of 19 inch (483mm). The 19-inch cutter has higher carrying capacity and cutting efficiency. An increasing number of tunnel projects have started to use 19-inch cutters. However, the cutting property and process of the 19-inch cutter have not been completely determined, as few laboratory tests have been conducted. In fact, 17in cutters are more commonly used in the field and tested in the laboratory (M. Sapigni et al., 2002; Prasnna Jain et al., 2014). The cutter force (normal force, rolling force and side force) and cutting mechanism of the 19-inch cutter differ from those of the 17-inch cutter. This kind of cutter is only weakly supported by the academic community and engineering experience. Thus, it is significant to conduct a full-scale model test for 19-inch cutter. Rostami and Ozdemir developed the Colorado School of Mines (CSM) model to predict the performance of a hard-rock TBM (Rostami. J. and Ozdemir. L, 1993). The CSM model was further developed by Rostami in 1997 (Rostami. J, 1997). Furthermore, Bruland introduced the NTNU model for TBM performance prediction (Bruland. A, 1998), and Barton used Q TBM to evaluate TBM performance (Barton. N, 1999). Each model has its own different origins, backgrounds and applicable conditions. For example, Kazem Oraee once proved that Q TBM and NTNU method were more reliable in the excavation of the Karaj Tehran water supply tunnel in Iran (Kazem Oraee and Bahram Salehi, 2013). Actually, it is difficult to predict TBM performance. This is because TBM performance prediction involves the understanding of rock fragmentation process across a wide range, from

2 micro-scale (i.e. the interaction of surface contact of rock material and cutter tip) to macro-scale (including the interaction of rock mass and TBM) (Jafar Khademi Hamidi et al., 2010). Cutter forces are significant parameters of the TBM performance prediction model. Many variables influence the cutter force, such as penetration depth, cutter spacing, cutter types, cutting velocity and rock mechanical properties. Many models were developed to relate cutting forces to rock parameters and cutting geometry (spacing and penetration)(ebrahim Farrokh and Jamal Rostami, 2008). In a specific TBM tunnel, penetration and spacing are two important parameters that influence the cutter force and cutting efficiency (R. Gertsch, L.Gertsch and J.Rostami, 2007). If penetration depth is small, the cutting force will also be accordingly small, leading to a lower cutting efficiency. However, if penetration depth is too large, then the cutter may be damaged because of the huge cutter force. Spacing also sensitively influences the cutting efficiency. To fully explore the influence of penetration and spacing on cutter forces, a series of cutting tests were designed with the penetration of 4mm, 6mm, 8mm, and spacing of 4cm, 6cm, 8cm, 10cm. Namely, there are 12 group of tests in total. It is widely accepted that cutting velocity has a strong influence on cutter forces (M. Entacher, 2012), different cutting velocity will lead to different types of fragmentation, dynamic or quasi-static. The cutting velocity in this test was 400mm/min, and the typical cycling time of the loading path diagram of normal force is less than 1 second. According to Z. X. Zhang s computational formula (Z. X. Zhang, 2004), this model test turned out quasi-static results. Despite several decades of academic rock mechanics and practical rock engineering, there is still a lack of fundamental knowledge on the rock-cutter interaction (Nicola Innaurato and Pierpaolo Oreste, 2011). The TBM tunneling process in hard rock is actually a rock or rock mass breakage process, which determines the efficiency of the tunnel boring machine (Q.M. Gong and J. Zhao, 2009). The rock failure beneath the cutters is a complicated multi-crack-propagation process rather than a single-crack-propagation problem (Z. X. Zhang, 2004). Rock debris is formed when the cracks developed from adjacent cuts have been propagated and connected. Two different types of rock debris are formed, one type is powder and the other is chip. Regarding the development of cracks, the weight and shape of debris are somewhat correlated with cutter forces, spacing and cutting efficiency. This paper will explore the relations between debris characteristics and cutting process. 2. Experiments Full-scale rock cutting tests combined with physical property tests are probably the most reliable estimation technique currently available. The laboratory full-scale rock cutting test is a very useful tool in determining design parameters and performance prediction of a TBM for a specific job (C. Balci, 2009). Totally, 12 group of cutting tests are performed by the TJ-TS500 linear cutting machine (LCM) in the Key Laboratory of Geotechnical and Underground Engineering of Education Ministry in Tongji University (Fig. 1). Cutting process is simulated by the relative movement between concrete specimen and cutter in a process powered by the y-hydraulic cylinder. Penetration is adjusted by the movement of z-hydraulic cylinder, and cutter spacing is controlled by the location of x-hydraulic cylinders for each cut. The stiffness of LCM is high enough to ensure that the actual penetration is very close to the designed one. The disc cutter installed on the LCM is a commercial constant cross-section (CCS) cutter with a 483mm diameter and a 13mm width. A high quality aircraft aluminum cylinder equipped with three-dimensional strain gauges is installed upon the top of the cutter holder. During the test, data of cutter force, penetration, spacing, cutting velocity, and cutting length are collected from the LCM. The working performance of the LCM is listed in Table 1. Table 1 Working performance of TJ-TS500 LCM LCM Rock Range of cutter force (kn) Range of cutting distance (mm) Size(m) Size(mm) F z F y L z L y The force data is used for the exploration of cutting process and prediction of cutting performance. Cutting efficiency is calculated by specific energy (SE), which is the energy required to fragment a unit volume of rock (Teale R, 1965). Lower specific energy implies a higher cutting efficiency. Optimal spacing, the cutting spacing of lowest SE, is an important parameter in TBM design. In fact, optimal ratio of s/p is a more reasonable approach than optimal spacing. Cutting coefficient (CC) is the ratio of the rolling force to the normal force. This parameter reflects the ratio of total torque to

3 thrust of a TBM; the higher the CC, the higher the torque needed for a given amount of thrust. After one group of tests, debris is collected and weighed. Fig. 1 General view of TJ-TS500 linear cutting machine. During the full-scale model test, rock was simulated by cast-in-place concrete. The concrete specimen is designed large enough (980mm 980mm 500mm), so that edge effects can be avoided. Additionally, the surface of the specimen should be smoothed flat before the start of next test group, in order to avoid measuring errors of the cutter force. The TBM performance is directly related to rock strength (Prasnna Jain et al., 2014). M. Sapigni et al. noted that the TBM thrust will increase linearly with rock mass strength (M. Sapigni, 2002). This paper does not focus on the influence of rock strength, so the strength of specimen will not act as a variable. Uniaxial compressive strength test of this concrete specimen was performed and the result is 30MPa in average. 3. Results and Discussion 3.1 Normal and Rolling Force The ultimate bearing capacity of normal force is an important parameter for disc cutters. The disc cutter used in this test and the Northwest Liaoning water supply tunnel has a bearing capacity of 311kN. Bearing capacity is mainly based on the shaft strength of the disc cutter. The 19-inch cutter has a higher bearing capacity than the 17-inch because of thicker shaft. This is the reason why 19-inch cutter has become increasingly popular. Spacing and penetration are two major factors that influence the normal force and the rolling force. With the increase of spacing, normal force will increase at an accelerating rate on the whole, regardless of penetration (Fig. 2(a)). It is commonly acknowledged that the strength of intact rock is higher than that of rock with cracks (Q.M. Gong and J. Zhao, 2009). The increasing spacing will lead to a weaker interaction between cuts, thus causing a decrease of crack density. As a result, carrying capacity of rocks, as well as normal force will both rise. In Fig. 2(a), we can find normal force increases more sharply at the spacing of 8cm, which reflects that there exists an influential sphere within the radius of about 4cm, where the density of cracks is relatively higher than the outer space (Fig. 3). Normal force will increase nearly linearly with penetration depth at any spacing (Fig. 2(b)). This is commonly expected and easily accepted. Make a comparison between Fig. 2(a) and Fig. 2(b), it is easily concluded that normal force is more sensitive to the variation of penetration than that of spacing. This is because normal force and penetration are both in the vertical direction. Thus, the relation between normal force and penetration is direct. While the influence of spacing lies in the effect of crack density, which acts in an indirect way. Increase of spacing also results in an increase of rolling force (Fig. 2(c)). The reason is similar to that for normal force. As has been discussed, wider spacing always leads to higher integrity and higher residual strength of rock. The designed thrust of TBM reflects the combined action of the normal force of all cutters, and the designed torque is mainly correlated with the rolling force. In brief, if normal force and rolling force increase, both thrust and torque will increase. Judging from the relationship of normal/ rolling force and spacing, if the cutting spacing of one specific TBM

4 cutter-head is wide, then both thrust and torque of TBM should be designed in an appropriate value large enough. Rolling and normal force of the 19-inch cutter both increase with increasing penetration (Fig. 2(b),(d)). However, they response differently when penetration is near zero. If penetration approaches zero, the rolling force is also close to zero. But normal force is a different situation. If we reversely extend the lines in Fig. 2(b), normal force will not be zero even if penetration equals zero. It can be explained that very small penetration quickly causes the normal force to rise to a high level, but has a far weaker effect on rolling force (R. Gertsch et al., 2007). To deeper explore the relationship between normal force and penetration, the vertical penetration test (VPT) for a 19-inch disc cutter was conducted. Normal force and penetration have an nearly linear relationship (Fig. 4). According to the linear fitting equation in Fig. 4, when the penetration is 4mm, 6mm and 8mm, normal force equals 67.2kN, 99.5kN and 131.8kN, respectively. It is obvious that the normal force of vertical penetration test is a little higher than that for the linear cutting test, because the contact area between specimen and cutter in the vertical penetration test is larger. Field penetration index (FPI) is the ratio of normal force to the penetration depth, which reflects a stable relationship between normal force and rock properties. It is a significant parameter in TBM performance prediction (Q.M. Gong and J. Zhao, 2009; J. Hassanpour et al., 2011). Fig. 5 shows the increase seen in the relationship of mean FPI and spacing, which is in accordance with the result of J. Hassanpour et al. (J. Hassanpour et al., 2011). (a) (b) Fig. 2 (c) (d) Relationship between normal/rolling force and spacing/penetration.

5 Fig.3 Example of an influential sphere. Fig. 4 Relationship between normal force and penetration of VPT. Fig. 5 Relationship of FPI and spacing. 3.2 Specific Energy and Cutting Coefficient Specific energy is one of the primary performance parameters of TBMs (O. Acaroglu, 2008). SE is calculated by equation (1). In this formula, F r has greater influence on SE than F n. W F p F L n r SE (1) V m / Where SE is expressed in J/cm 3, F n and F r are the mean normal and rolling force, p is the depth of penetration, L is length of cutting path, m is the mass of debris, ρ is the rock density. It is commonly accepted that the lowest SE is only related to the optimal spacing. However, the result of this test indicates that the lowest SE varies with both different depth of penetration and spacing. In fact, spacing is not the only variable to determine the lowest SE, the effect of penetration shouldn t be ignored when determining the optimal spacing (TAN Qing et al., 2012). Optimal spacing increases with the increase in penetration (Fig. 6). For example, if penetration is 4mm, the optimal spacing is about 6-7cm; if penetration is 6mm, it would be about 9cm.

6 As a matter of fact, the relationship between SE and penetration/spacing is complicated. Thus, it is quite difficult to tell whether the SE will increase or not with increasing penetration or spacing. If the spacing is relatively narrow, SE will increase with the increase of penetration. However, if spacing is wide, SE will first decrease before increasing (Fig. 6). Thus, it is more reasonable to determine the lowest SE by selecting the optimal area defined by both penetration and spacing. In this test, the optimal area is a diagonal band split by a centerline: P=0.625s Similar ideas were found using the ratio of s/p in a 2010 paper by Jung-Woo Cho et al. ( Jung-Woo Cho et al., 2010). In the s/p method, the optimum s/p ratio of this test is about 16, a similar result to that of Jung-Woo Cho. Fig. 6 Contour of specific energy Cutting coefficient (CC) is defined by formula (2). It can reflect the relation of torque and thrust in a way. CC increases obviously with the increase of penetration regardless of spacing (Fig. 7(a)). As Fig. 2(b) and Fig. 2(d) show, both normal and rolling force will increase with increasing penetration. According to the relationship between thrust/torque and normal/rolling force that has been discussed in 3.1, both thrust and torque of the TBM cuter-head will increase with the increase of penetration in the field TBM excavation. But considering the increase of CC (Fig. 7(a)), torque will increase faster than thrust. In most cases, the performance of TBM is evaluated by the working thrust, which means the working torque is easily ignored. Therefore, if penetration exceeds the maximum design value, efforts should be made to ensure that working torque does not exceed the accepted limit. Different from penetration, spacing has little influence on CC. It will remain almost stable on the whole no matter how spacing varies (Fig. 7(b)). This result shows that spacing has similar effects on normal and rolling force. Fr CC (2) F Where F r and F n are the mean rolling and normal force. n (a) Fig. 7 (b) Relationship between CC and penetration and spacing

7 3.3 Mass Percentage of Chip Debris Destruction nucleus is formed after the indentation of the cutter into rock, which acts as a fluid matter that is subjected to a hydrostatic pressure. Chips will form due to the interaction between the nuclei of adjacent cutting grooves (N. Innaurato et al., 2007). If the pressure is high enough, chip formation will take place between grooves (Fig. 8). Fig. 8 Formation of destruction nucleus and chips. The formation process of two types of debris is different. Powder debris is formed by the heavy pressure from the cutter ring, while the formation of chip debris is a result of the connection of cracks stimulated by adjacent cuts. Chip debris is defined as the debris that has a maximum dimension size of 2cm in this test. A study of chip sizes was valuable, which can be used to evaluate the ground conditions, rock mass properties, and their impact on the performance of field TBM (Ebrahim Farrokh and Jamal Rostami, 2008). In this chip weighting test, chip debris is first collected from the surface of specimen after each group of cutting test, and then measure the weight (Fig. 9). Chip mass percentage is the percentage ratio of chip mass to total mass of debris. Fig. 9 Classification and weighing of chip debris. Mass percentage of chip debris is correlated with SE and spacing. If SE is high, chip percentage is low; if SE is low, chip percentage is relatively high (Fig. 10(a)). To form one unit volume of debris, powder debris will cost more energy than chip debris. Therefore, chip percentage and SE have a negative correlation. If spacing is wide, cracks between adjacent cuts will be denser and deeper, thus, more chip debris will form. This is the reason why chip percentage will increase with the increase of spacing. However, if penetration is small (p=4mm), wide spacing may cause a drop in chip penetration (Fig. 10(b)). This is because cracks do not develop well at both shallow penetration and wide spacing. Only a small amount of debris can form in this situation.

8 (a) (b) Fig. 10 Relationship of chip percentage and SE and spacing. 3.4 Geometric Characteristic of Chip Debris The geometry of chip debris varies substantially. It is difficult to propose generalized parameters to characterize geometry of the whole group of chip debris. In order to show the geometric characteristics of 12 group tests, the largest chip and the most typical chip of each group were collected, and the length, width and thickness of both chips were measured. For the convenience of using these data, nominal size was defined as the arithmetic average value of length and width and flat degree was defined as the ratio of thickness to nominal size. Average nominal size of different penetrations increases with spacing, especially for the largest chip debris (Fig. 11). Narrow spacing confines the volume of chip debris. When spacing decreases, chip size decreases, too. This phenomenon is more obvious for the largest chip of each group. Increase of normal force decreases the flat degree (Fig. 12). High normal force leads to a relatively high horizontal confining stress field between adjacent cuts. The high confining pressure makes the rock between cuts fracture into thin chips. In the field TBM tunnel in northwest Liaoning, chip debris become thinner with the increase of thrust. In the case when the TBM encounters a broken rock layer, the thrust decreases, and the proportion of thick chips or rock blocks increases simultaneously. Fig. 11 Relation of nominal size and spacing. Fig. 12 Relation of flat degree and normal force.

9 4. Conclusions Both normal and rolling force will increase with the increase of spacing as well as penetration. However, penetration has a more sensitive influence on cutter forces. Cutter forces increase almost linearly with penetration. The curve slope of vertical penetration test is steeper than that of linear cutting test, because contact area is larger. SE is correlated not only to spacing, but also to penetration. The effect of penetration should not be ignored when discussing the optimal spacing. Optimal spacing increases with increasing penetration. It is more reasonable to discuss the optimal area consisting of penetration and spacing rather than to look at optimal spacing alone. CC increases strongly with the increase of penetration at each spacing, but it mainly remains stable during the change of spacing. When penetration increases, the torque of TBM ascends sharply, and this should not be undervalued. SE and chip percentage have a rough negative correlation. The relationship between chip percentage and spacing is somewhat complicated, but on the whole, chip percentage increases with spacing. Nominal size increases with spacing, especially for the largest chip debris. This is because debris size mostly depends on the space between two adjacent cuts. Increase of normal force decreases the flat degree. This is a similar result to what has been observed in the field TBM tunnel in northwest Liaoning, China. Acknowledgements The authors acknowledge the support of National Natural-Science Foundation of China (Grant No , No ) and the support of science and technology project plan of Guizhou Provincial Transportation Department (Grant No , 013). The authors appreciate the generosity of Shandong Techgong Geotechnical Engineering Equipment Co., LTD. for providing disc cutters and valuable data. References [1] Barton, N. TBM performance estimation in rock using Q TBM. Tunnels and Tunneling International,1999, 31 (9): [2] Bruland, A. Hard Rock Tunnel Boring. Ph.D. Thesis, 1998, vol. 1 10, Norwegian University of Science and Technology (NTNU), Trondheim, Norway. [3] C. Balci. Correlation of rock cutting tests with field performance of a TBM in a highly fractured rock formation: A case study in Kozyatagi-Kadikoy metro tunnel, Turkey. Tunnelling and Underground Space Technology,2009,24: [4] Ebrahim Farrokh, Jamal Rostami. Correlation of tunnel convergence with TBM operational parameters and chip size in the Ghomroud tunnel, Iran. Tunnelling and Underground Space Technology,2008,23: [5] J. Hassanpour, J. Rostami, J. Zhao. A new hard rock TBM performance prediction model for project planning. Tunnelling and Underground Space Technology,2011,26: [6] Jafar Khademi Hamidi, Kourosh Shahriar, Bahram Rezai and Jamal Rostami. Performance prediction of hard rock TBM using Rock Mass Rating (RMR) system. Tunnelling and Underground Space Technology, 2010,25: [7] Jung-Woo Cho, Seokwon Jeon, Sang-Hwa Yu and Soo-Ho Chang. Optimum spacing of TBM disc cutters: A numerical simulation using the three-dimensional dynamic fracturing method. Tunnelling and Underground Space Technology, 2010,25: [8] Kazem Oraee, Bahram Salehi. Assessing prediction models of advance rate in tunnel boring machines a case study in Iran. Arabian Journal of Geosciences,2013,6: [9] M. Entacher, K. Lassnig and R. Galler. Analysis of Force Path Diagrams of Linear Cutting Machine Tests regarding Geotechnical Parameters. Proceedings of the GeoCongress 2012, San Francisco, USA: [10] M. Sapigni, M. Berti, E. Bethaz, A. Busillo and G. Cardone. TBM performance estimation using rock mass classifications. International Journal of Rock Mechanics and Mining Sciences,2002,39: [11] N. Innaurato, C. Oggeri, P. P. Oreste, and R. Vinai. Experimental and Numerical Studies on Rock Breaking with TBM Tools under High Stress Confinement. Rock mechanics and Rock Engineering, 2007,40(5):

10 [12] Nicola Innaurato, Pierpaolo Oreste. Theoretical Study on the TBM Tool-Rock Interaction. Geotechnical and Geological Engineering, 2011,29: [13] Nuh Bilgin, Melih Algan. The performance of a TBM in a squeezing ground at Uluabat, Turkey. Tunnelling and Underground Space Technology,2012,32: [14] O. Acaroglu, L. Ozdemir and B. Asbury. A fuzzy logic model to predict specific energy requirement for TBM performance prediction. Tunnelling and Underground Space Technology, 2008,23: [15] PENG Qi. Application,Research and Future of Tunnel Boring Machine Technology. Tunnel Construction, 2013,33(6): [16] Prasnna Jain, A.K. Naithani and T.N. Singh. Performance characteristics of tunnel boring machine in basalt and pyroclasticrocks of Deccan traps A case study. Journal of Rock Mechanics and Geotechnical Engineering, 2014, 6(1): [17] Q.M. Gong, J. Zhao. Development of a rock mass characteristics model for TBM penetration rate prediction. International Journal of Rock Mechanics and Mining Sciences,2009,46:8-18. [18] R. Gertsch, L.Gertsch and J.Rostami. Disc cutting tests in Colorado red granite: Implications for TBM performance prediction. International Journal of Rock Mechanics and Mining Sciences, 2007,44(2): [19] Rostami, J. Development of a Force Estimation Model for Rock Fragmentation with Disc Cutters Through Theoretical Modeling and Physical Measurement of Crushed Zone Pressure. Ph. D. Thesis, 1997,Colorado School of Mines, Golden, Colorado, USA. [20] Rostami, J., Ozdemir, L. A new model for performance prediction of hard rock TBM, In: Bowerman, L.D., et al. (Eds.), Proceedings of RETC, Boston, MA, 1993, [21] TAN Qing, YI Nianen, XIA Yimin, XU Zijun, ZHU Yi and SONG Junhua. Research on Rock Dynamic Fragmentation Characteristics by TBM Cutters and Cutter Spacing Optimization. Chinese Journal of Rock Mechanics and Engineering, 2012,31(12): [22] Teale R. The concept of specific energy in rock drilling. Int J Rock Mech Min Sci Geomech Abstr, 1965,2: [23] Z. X. Zhang. Estimate of Loading Rate for a TBM Machine Based on Measured Cutter Forces. Rock mechanics and Rock Engineering, 2004,37(3):