Towards the prediction of rock excavation machine performance

 Recent rock cutting laboratory experiments and field studies on the performance of rock cutting trenchers has provided a better understanding of the processes and factors affecting tool consumption and excavation rates of rock excavation machines. On the basis of this, a model has been developed to assist in the prediction of trenching rates and tool wear in various geological situations. The paper provides an overview of the set-up and results of both laboratory and field studies. It describes a basic framework model of the processes and mechanisms involved in assessing excavation rates and tool consumption and discusses how the acquired knowledge can be used to assist with predictions for future excavation works. It then considers how this knowledge could be applied by practitioners who have to work with a scarcity or absence of good quality and reliable data. Received: 29 July 1997 · Accepted: 3 January 1998     

Linear stone cutting tests with chisel tools for identification of cutting principles and predicting performance of chain saw machines

Different natural stone quarries are visited for collection of stone samples, determination of geological conditions, specifications and operational conditions of the chain saw machines and recording machine cutting performance with a data acquisition system. The samples are tested with a linear cutting test rig using chisel-type cutting tools with 0°, 15°, 30° and 45° sideways angles at different depths of cut and tool spacings, to determine stone cuttability, cutting characteristics of chain saw machines and effect of unsymmetric and symmetric sideways angles and different cutting patterns on cutting performance (tool forces, specific energy, optimum cutting geometry). A deterministic model is suggested for predicting performance of chain saw machines using the results of linear stone cutting experiments, and the laws of kinematics. The results of experimental studies and in-situ investigations indicate that the cutting action of chain saw machines can be successfully simulated by linear cutting experiments and the suggested model is proven, though requiring some additional study, to be a useful and reliable tool for selection, design, and performance prediction and optimization of chain saw machines.     

Assessment of pick wear based on the field performance of two transverse type roadheaders: a case study from Amasra coalfield

A successful excavation of roadheaders depends on the cutting performance and the tool life of conical picks. Tool life is important in terms of wear rate which is affected by different rock parameters such as equivalent quartz content, mineral grain size, as well as cutting parameters on the cutterhead. In this study, analyses among wear rate, specific energy, advance rate, and cutter consumption were carried out. The wear mechanisms of two different models of conical picks were examined from different aspects depending on rock and machine parameters. Their relation with the mechanical and abrasivity properties of rocks and petrographic analyses were investigated. In addition, the metallurgic content and Rockwell hardness of conical picks were determined to describe the metal alloys and their effects on the wear of cutting tool. The results showed that the metallurgic content, pick positions, and other environmental conditions influence the wear mechanism. Finally, two different models were proposed to estimate the pick consumption in sandstone and siltstone rocks based on actual data obtained from coalfield.

     

Predictive plots for conical pick performance using mechanical and elastoplastic properties of rocks

Conical picks are by far the most widely used drag type cutting tools employed on partial face rock excavation machines. The cutting force and specific energy are two important design parameters for the conical pick performance, and the rock cutting testing is considered as the promising tool for determining these parameters. In the absence of an instrumented cutting rig, researchers generally rely on empirical predictive plots. For this, this paper suggests predictive plots for estimating the cutting force and specific energy, in consideration of the cutting depth to define the cuttability with conical picks. In this context, rock cutting tests were carried out on six volcanic rock samples with varying cutting depths using the unrelieved and relieved cutting modes. The cutting force and specific energy were correlated with the uniaxial compressive strength, Brazilian tensile strength, elasticity modulus, and plasticity index. Predictive plots were proposed for different cutting depths in the unrelieved and relieved cutting modes and exponential relationships were obtained among the cuttability parameters, and mechanical and elastoplastic properties of rocks.     

The effect of machine design parameters on the stability of a roadheader

A “roadheader” excavator has a special set of characteristics among tunnel excavation machines and the determination of the stability states of a roadheader is important for effective and continuous excavation. For roadheaders having equal power, if one is more stable than another, it can respond to higher boom forces. A new computer program was developed to analyze the stability states of roadheaders. The developed method allows analysis for both longitudinal and transverse cutting head type machines either at one point on the excavation face or in terms of the whole face. Values of the four stability states of the roadheader, namely: turning around the vertical axis, turning to the side directions, turning to the back direction and sliding, can be obtained using this program. In this study, the effects of machine design parameters were investigated by applying this method to the stability of a longitudinal type roadheader. The studied machine design parameters are the machine weight, boom length, machine width, track width, distance between the back leg of the machine and the center of gravity, distance between the start of the boom and the center of gravity, distance between the horizontal point and the vertical rotation point of the boom, and the distance between the ground and boom axis when it is parallel to the ground. The effect of the machine design parameters on stability has been illustrated in detail.     

Analysis on the damage behavior of mixed ground during TBM cutting process

Accurate prediction of rock cutting forces of disc cutters is especially significant for the design and construction of tunnel boring machine (TBM). Through the analysis of motion trajectory of TBM disc cutters, a three-dimensional model of rock breaking process of disc cutters is established. In terms of the rock strain which is resulted from the interaction between disc cutters and rock during the process of rock breaking, a three-dimensional cutting forces model is proposed with disc cutters set at certain parameters and in certain sizes. Subsequently, the equation of contacting forces between rock and disc cutter is derived. Moreover, a new method has been presented for the study of the rock breaking theory of the disc cutter and it also provides guidance for the design and application of TBM in tunnel excavation. The three-dimensional model for the rock breaking mechanism is used for predicting the cutting force for the situation of mixed ground.The damage field and the rock failure zone induced by disc cutter for mixed ground are also discussed in this study. In detail, the rock damage zones are divided into two parts, one is the left damage field which located in the outside of disc cutter. The other is the right damage field which located in the outside of disc cutter. The influence of the rock ground dip on the rock failure zone was also studied by parameter analysis.     

Dominant rock properties affecting the performance of conical picks and the comparison of some experimental and theoretical results

Conical picks are the essential cutting tools used especially on roadheaders, continuous miners and shearers and their cutting performance affects directly the efficiency and the cost of rock/mineral excavation. In this study, in order to better understand the effects of dominant rock properties on cutter performance, 22 different rock specimens having compressive strength values varying from 10 to 170 MPa are first subjected to a wide range of mechanical tests. Then, laboratory full-scale linear cutting tests with different depth of cut and cutter spacing values are realized on large blocks of rock specimens using one type of conical pick. Specific energy, cutting and normal force values for relieved and unrelieved cutting modes are recorded using a triaxial force dynamometer with capacity of 50 tonnes and a data acquisition system. Cutter force and specific energy values are correlated with rock properties and theoretical force and specific energy values obtained from widely used theoretical approaches.The results indicate that uniaxial compressive strength among the rock properties investigated is best correlated with the measured cutter performance values, which is in good agreement with previous studies. However, it is also emphasized in this study that Brazilian tensile strength, Schmidt hammer rebound values, static and dynamic elasticity modulus are also dominant rock properties affecting cutter performance.Theoretical specific energy defined by different researchers has a meaningful relationship with the experimental specific energy, which is an essential parameter for predicting the instantaneous cutting rates of mechanical excavation systems. It is also demonstrated that the experimental cutter forces obtained for 5 mm depth of cut are in good agreement with theoretical force values, if the friction angle between rock and cutting tool is included in the theoretical formulation. It is emphasized that, to some extend, laboratory tests can help to minimize high cost of a trial–error approach in the field.     

Effect of cutting tool blunting on the performances of various mechanical excavators used in low- and medium-strength rocks

Full-scale laboratory cutting tests that measure the specific energy (SE) are widely used to evaluate rock cuttability by mechanical excavators, and in particular roadheaders fitted with radial or drag-type bits. Radial or drag-type bits are often changed during operation as they wear and become blunt. In this study, full-scale cutting tests were carried out on different rock types using bits with varying degrees of wear in order to assess the impacts of pick bluntness on cutting forces and the SE. The relationships between wear flats and cutting forces, SE, and various rock properties such as uniaxial compressive strength, tensile strength, indentation index, Shore hardness, Schmidt hammer hardness, and density were examined and are discussed in this paper. The mean cutting force increased 2- to 3-fold and the cutting SE rose 4- to 5-fold with a 4-mm wear flat as compared to a sharp pick. Critical wear flats were plotted for different rock property values, and 25 MJ/m³ was considered the threshold SE above which cutting performance was considered to be poor. Best-fit predictive models based on statistical analysis of the laboratory cutting test results are introduced as a means to estimate SE as a function of bit type, wear condition, and various mechanical properties of the rock. These models can be used to predict the performances of mechanical excavators that use radial tools, especially roadheaders, continuous miners, and longwall drum shearers.     

Thrust and torque components on mixed-face EPB drives

In this communication we propose a method to estimate net thrust and torque applied at the excavation front by cutting tools from EPB machines when they are working in mixed faces constituted by soils and soft rock. The method is inspired by similar simplifications used for the analysis of TBM drives in rock. The proposal is validated using a database of EPB registers which were gathered from more than 35 km of tunnel drives excavated in soils, soft rock and heterogeneous media. The results allow us to assess the influence of the type of ground excavated and its geotechnical properties on the net thrust and torque applied.     

Development of a mechanical rock breakage experimental platform

With social and economic development and improvement of science and technology, tunnel boring machines, raise boring machines, reaming machines and road headers have been unprecedentedly applied to underground engineering construction. Mechanical rock breakage experiments became an indispensable mean for mechanical excavation of underground engineering. It can also guide the design and operation of excavation machines. This paper introduces the new mechanical rock breakage experimental platform developed by Beijing University of Technology. The design and manufacture of this platform considered the influence of high in situ stress, cutter head rotation and rock breakage by multi-cutters at the same time on the rock breakage process. Different tools, such as rolling cutters, scraper, ripper, and pick, for different excavation machines can be tested in the platform. It can also take into account the different rock mass structure in the sample box. Different construction conditions with different cutting tools can be realistically modeled. The experimental platform consists of a mechanical system, a hydraulic system, an automatic control system and a data acquisition system. After installation of the designed experimental platform, several tests with different construction conditions were conducted. The experimental results showed that the platform reached its design requirements.     

The RUB Tunneling Device – A newly developed test method to analyze and determine the wear of excavation tools in soils

The efficiency of a tunneling project is mainly associated with the abrasivity of the acting soil and the wear resistance of the cutting tools. Heavy wear can dull the cutting tool, which negatively affects the penetration rate and therefore the efficiency of a tunneling process. Completely worn tools with a short service life have to be replaced by newer ones. This circumstance results in unplanned machine shutdowns and higher maintenance costs. It is thus of high economic as well as technical interest to obtain a deeper understanding of soil/cutting tool interactions during tunneling. To meet this challenge, a large number of different testing devices to estimate the abrasivity of soils have been developed within the last two decades. An innovative and promising experimental setup is presented in this work. A horizontal implementation offers the possibility of simulating a tunneling process as well as the tribological system of a TBM tool. The interactions between all system components can be mapped and analyzed in detail. This method offers a unique setup, which allows wear prediction of TBM tools in a homogeneous soil with project-specific parameters (soil composition/condition, soil mechanics, tool material and machine/tunneling data).     

EPB tunneling challenges in bouldery ground: a new experience on the Tabriz metro line 1, Iran

Underground construction of the Tabriz metro line 1 involved boring twin tunnels with two earth pressure balance machines in alluvial deposits. Although this type of machine is recommended for excavation in fine soils, its performance was evaluated in this project where part of the ground consists of alluvial deposits with particle size ranging from silt to boulder and even rock blocks. It was found that the tunnel boring machine (TBM) performance was highly reduced due to the variation in ground conditions. Many types of problem were observed during the excavation in coarse grained alluvial deposits including high cutter-head torque, excessive wear of tools, breaking of TBM parts and ground instabilities in the form of large settlement and ground collapse. A trial effort was implanted to stabilize the overburden ground by the injection of cement grout, but this proved to be unsuccessful. In both cases, the excavation operation was delayed. With the experience gained from the excavation of the first tunnel and use of foam with a high foam expansion ratio as a soil conditioner in closed mode, the performance of the TBM in the second tunnel improved.     

锦屏Ⅱ级水电站深部大理岩板裂化破坏试验研究及其对TBM开挖的影响

针对锦屏Ⅱ级水电站TBM引水洞及排水洞深部大理岩发生的剧烈板裂化岩爆与非剧烈板裂化片帮2种板裂化破坏现象,通过采集该工程区深部大理岩岩样,采用真三轴岩爆试验设备,对其在不同高应力作用下的板裂化破坏现象进行室内试验。通过对试验过程中发生的各种现象的全面描述和试验结果的分析表明,试验中岩样发生的板裂化破坏现象与TBM开挖隧道围岩的板裂化破坏具有很好的吻合性,未来围岩的主要破坏方式将以板裂化片帮与岩爆为主。围岩板裂化破坏将影响TBM开挖运行,分别探讨掌子面与洞壁板裂化破坏对TBM开挖的影响。     

Cutter force measurement on tunnel boring machines – Implementation at Koralm tunnel

Loading of a disk cutter on tunnel boring machines (TBM) is usually estimated from global machine thrust. In order to be able to measure cutting forces of an individual disk cutter, a new measurement method was developed and published in a previous paper (Entacher et al., 2012). This study presents the implementation process of the measurement method at the first TBM of the Austrian Koralm tunnel. Results show that peak forces are a multiple of average forces and that the distribution of forces across the tunnel face can be very irregular. The occurring forces are in very good agreement with geological features of the tunnel face such as highly fractured zones and orientation of schistosity.     

坝基岩体开挖卸荷与分带研究——以小湾水电站坝基岩体开挖为例

 小湾坝基开挖引起了一系列岩体卸荷变形破坏现象,包括岩体中已有结构面的回弹松动与剪切错动,以及完整岩体的岩爆拱裂、薄板状破裂等新生破裂。开挖中岩体的新生破裂面在空间分布上有如下特点：破裂面产状与开挖面具有较好的一致性;在深度分布上,开挖卸荷破裂面相对集中在开挖面之下4～6 m的深度以上。岩体的卸荷变形破坏导致了岩体完整性的显著降低,但卸荷松动过程的主体部分多在几个月内完结。无论是岩体开挖卸荷还是河流天然切割卸荷都是一个能量释放过程,并且卸荷变形破坏具有显著的空间分布规律,因此采用能量方法研究岩体的卸荷分带是合适的。给出了岩体卸荷分带的应变能方法,小湾水电站的实际应用表明该方法是可行的、有效的。     

A new method to evaluate roadheader operational stability

Roadheader, one of the mechanized excavation equipments, has an exceptional place among the other excavation methods. Determination of the stability states of the roadheader is an important matter for the efficiency of excavation. For this reason, a new method has been developed to obtain numerical values that indicate stability states of such machines. The method has been developed for both longitudinal and transverse cutting head type machines by establishing stability equations for states of turning around the vertical axis, turning to the side direction, turning to the back direction and sliding in all cutting modes. A computer program based on these equations has been written in C++ programming language. The stability analysis can be made either for one point on the excavation face or whole face. Obtained values can be used to compare the stability states of various roadheaders in the same cutting condition as well as using to compare the stability states of a roadheader. A stability index that has also been established can only be used to compare a stability state of a roadheader in different cutting conditions. The developed methodology was tested in a transverse cutting head type roadheader for arcing, lowering and lifting modes. It has been obtained that the stability state of turning around the vertical axis of the roadheader is the most critical state especially in arcing mode. The effects of the machine design parameters, cutting head and tunnel parameters on the stability of the roadheaders can also be investigated by this method.     

柔性边界加载试验机研制

现有的土力学试验机都是刚性边界加载，也就是等位移边界加载。这种试验机基本上是以研究均匀材料为目的，并且，它主要用于研究材料内部一点的应力-应变状态和强度准则。而实际的土石混合体和破碎岩体内部包含有结构面和节理面等，它们是非均匀、非连续的，在外力的作用下其内部的应力分布是不均匀的。对于这种材料，其真正的受力条件是等应力边界条件。为此，介绍了一种自行研制的三轴试验机——柔性边界加载试验机。该试验装置可以更好地从均匀加载的角度研究非均匀、非连续介质的变形及破坏规律。对这种试验机的原理和设计作了说明；推导出了轴压、围压、半衡压之间的协调关系：同时，还介绍了超声波位移测量系统以及压制试样的装置和压制过程。     

Experimental study of the effect of conditioning on abrasive wear and torque requirement of full face tunneling machines

One of the main operational parameters in pressurized full face soft ground tunneling with machines such as earth pressure balance (EPB) TBMs is soil conditioning. Soil conditioners which are proper foam or conditioning agents, are often injected through the cutterhead or into the cutting chamber to make the muck flowable, lower inner friction between the soil particles, control soil stickiness, prepare the excavated soil to be compressible during the tunneling operation, control the water inflow, reduce the torque on the cutterhead, and lower power and torque of the screw conveyor. Foams can also be used in hard rock tunneling to reduce the wear and torque to replace conventional dust suppression by water sprays. This paper reviews the common practice in soil conditioning and focuses on assessing the influence of the conditioning on tool wear and torque requirement of the machines. Tests performed on many standard type soils and few samples from soft ground tunneling projects around the US on Penn State Soil Abrasion testing device. The results of tests performed in dry, moist and conditioned soil will be discussed and compared in this paper. The preliminary results show that the use of proper soil conditioning can reduce the wear of cutters and machine components by orders of magnitude while it can reduce the torque by over 50%.     

Ground–liner interaction in rock tunneling

Tunnel constructions are gradually increasing because of the development and upgrade of infrastructures such as highway, subway, railway, and many other facilities. Most of tunnels are excavated either by using drilling and blasting or by using tunnel excavation machines such as TBM (tunneling boring machine) or Shield. NATM (new Austrian tunneling method) is one of most frequently used tunneling methods and it uses drilling and blasting to excavate a tunnel in rock. While tunnel excavation using TBM or Shield machines produces quite a regular and smooth tunnel excavation surface, the tunnel excavation using drilling and blasting results in a very irregular and rough excavation surface. The stress behavior in a shotcrete tunnel liner installed along the excavation surface is very dependent on the surface status and tunnel engineer should consider the surface condition for the design of a shotcrete tunnel liner.Numerical analyses are conducted to investigate the effect of the irregularity of tunnel excavation surface on the response of the shotcrete tunnel liner. For the investigations, the controlled parameters include the irregularity of the excavation surface, the stiffness of the surrounding ground, and the coefficient of earth pressure at rest. The investigations show that the response of a shotcrete tunnel liner is highly dependent on the parameters and for the same earth pressure condition the effect is more evident when the irregularity is more severe and the surrounding ground is less stiff.     

Examining the influence of TBM-ground interaction on electrical resistivity imaging ahead of the TBM

Tunnel excavation by means of tunnel boring machines (TBMs) is susceptible to unknown changes ahead of the tunnel face. Geophysics offers a technique called electrical resistivity that can continuously, in real-time, spatially map the formation in front of the TBM. Electrical resistivity has been successfully established for many applications including vadoze zone hydrology, oil/gas location, mineral location and failure detection in geo-structures. Yet it has not been well-established for TBM excavations. This is in part due to the complexity of the TBM tunneling environment and the uncertain influence this environment may have on the success of TBM-integrated-electrical resistivity to predict changes ahead of the tunnel face. One significant uncertainty lies in the interface region that exists around the TBM created during the modification of the virgin formation by a mechanical mixing action of the rotating cutterhead and the injection of additives used to aid in the extraction of the muck and protect the cutting tools from frictional wear. In this study, we investigate the influence of this interface region on TBM-integrated-electrical resistivity for both hard rock and soft ground tunneling conditions through finite element modeling. Regarding the performance of TBM-integrated-electrical resistivity to detect changes ahead of the cutting face, the interface region holds significant influence for both earth pressure balance (EPB) and open mode tunneling conditions. Electrical resistivity for slurry based tunneling is not influenced by the interface region. Simulations suggest that TBM-integrated-electrical resistivity can be sensitive to a formation change that is located up to five diameters in front of the TBM.