Tunnel Boring Machines: Benefits, Operaing And Design Considerations

The Importance of Cutter Head and Driving System

Tunnel boring machines are high tech equipment that are used in construction projects because of its many benefits in environment conservation projects. A tunnel is an underground passage that is used for passage of trains, people and vehicles. It may also be an underground mining passage.

The most important element of a tunnel boring machine is called the cutter head. The cutter head is used to crush the ground with its powerful discs, (Huo et al 2012). It is operated by a cutter head driving system (CDS). A cutter driving system is a complicated system that is mainly compose of motors, pinions and ring gears.  

The cutter driving system works by supplying torque to the cutter head through ring gears. The more the driving torque is increased, the more the cutting power. T alter the torque power depending on the load, the cutter driving system has a vector control (VC) and torque control (TCS) system that are often applied to make or alter inverter responses to alter the torque as required. 

A TBM cutter head and its driving system often suffer a great load fluctuation which may reduce its working condition during tunnel construction, (Liu, H., Li, P. and Liu, J., 2011). The gear transmission system may also be limited in working performance due to tear and wear, stiffness, backlash and or transmission errors. These often results excessive damage and often failures.

There are few conditions under which cutter head can fail. Failure often occurs due to vibrations and shock. To cut down the system vibration, a coupling model approach of a CD s is used to. analyze dynamic characteristics of  the system. Current technologies that have been used to reduce these failures include using non-linear models of CDs and using non-linear gear meshing factors, for example the time variant transmission gears, (JAVAD, G. and NARGES, T., 2010).

Changing geological factors and the Cutter Driving System are one of the few factors that impact the failure of the Cutter head. These leads to excessive tear and wear, torque imbalance, gear tooth breakage. Hence, to curb these problems, there needs to be an earnest investigation of the design, the gear transmission system and the operating conditions of inverter motors.

This particular project is aimed at establishing an appropriate tunnel boring machine operating conditions.

1. To investigate the operation principles and efficiency of tunnel boring machines.
2. To investigate the impact of TBMs on underground transportation.
3. To analyze the usage of control systems to monitor TBM operations.
4. To investigate how altitude affects the working of TBMs.
5. To come up with more advanced and efficient tunnel modelling approaches as to overcome current challenges.

Tunnel boring machines are often used for urban tunneling especially those with a length of over 1.5 km. Applications of TBMS however requires careful considerations. Application of TBMs for mining and municipal projects requires careful considerations, many which are technical and some non-technical. Wherever TBMs have been applied, one thing can be recognized – every tunnel project and site location is unique in terms of access, geology, terrain and project completion expectations and demands. Due to the varied needs of different sites, the design and operation of TBMs needs to be carefully considered. There also needs to be a careful consideration of the tunnel construction requirements, and apply them properly to the state of the TBMs.  It is the purpose of this paper to give an exposition of the design of TBMS and tunnel construction considerations. Related work that has been done before on the technical considerations of TBMs and tunnel construction include the following. (Yazdani-Chamzini, A. and Yakhchali, S.H., 2012), designed a model for gear driving system and they investigated of inertia on load-sharing characteristics.

Factors Affecting Tunnel Boring Machines Failures

 Zhang et al. analyzed the characteristic of a tunnel boring machine (TBM) in mixed face conditions.

Quin and Zhao came up with an optimization model for dynamic analysis and optimization of the control motor. They came up with a control method to reduce vibration on the gear transmission system.

While all these researches have made significant contributions aimed at improving the working efficiency of TBMs, fewer researches have suggested new designs of TBM. Most designs are based on the old paradigms of designing TBMs and there is need t come up with new designs that will revolutionize the way TBMs are manufactured and even maintained. There is still limited study on this subject since the cutter heads designs and variations are developed by designers. Furthermore, designing of cutter heads involves complex algorithms developed by manufacturers. To end users, the design of TBMs has been obscure to them, and their choice of TBMs will be limited.

The studies that have been done in the recent years have not made significant engineering propositions to improve the efficiency of TBMs, yet these studies have not fully addressed the external factors that could be integrated into the TBMs design to increase its operation efficiency. In this paper, explicit investigation of the design of TBMS will be made and a proper model will be developed that will be used in the design of more efficient TBMs, (Sun et al 2011). Several elements that contribute to the efficient functioning of TBMS will be looked at, that include influence of attitude to working efficiency of a TBMs, and the integration of an Integrated Control System to manage the CDS.

The efficiency of a tunnel boring machine depends much on many factors which include the entire functionality of all system components and the entire rolling stock. The cutter head however plays the most important role in the CDs, (Entacher, M., Lorenz, S. and Galler, R., 2014). Therefore, the cutter head design is key to the operation of the CDs. Overly, the cutter head designs greatly impacts rate of operation (ROP) and daily advance rate. (AR).

To increase the stability of the TBM in a rocky site, the may be need to use a double shielded TBM, that will well endure the immense vibrations from such a site.

The cutter discs length and tonsil strength are often varied according to the power of a TBM. In improved and more powerful TBMs, the enduring strength of the blades is generally high as opposed to low power TBMs, (Font-Capó et al 2012). In dealing with the stiffness of hard rocks, the torque is of necessity to be high so as to withstand the reaction from hard surfaces.

Recent Studies on Tunnel Boring Machines and Tunnel Construction

A generic TBM is made of a cutting head which is composed of cutting discs as shown in Fig two below. The cutting disc can be up to 8 meters in diameter. The cutting discs increase the efficiency of the cutter head, (Peila et al 2011). 

The cutting discs that a cutting header has dictate the overall performance of a TBM. Using the linear cutting test of a TBM, various variables established through the operation of the machine can be determined. (Yagiz et al 2009). The main element in the linear cutting test is the space that needs to be between any two cuts. The linear cutting test majorly helps in determining external factors that may be considered to increase the efficiency of the TBM. Cutting forces may also vary depending on the type of tunnel and kind of soil in the construction site. . Furthermore, the TBMS calls for many other external features that would increase its efficiency too. The linear cutting test majorly helps in determining external factors that may be considered to increase the efficiency of the TBM. Cutting forces may also vary depending on the type of tunnel and kind of soil in the construction site.

To measure the cutting forces, the load shell helps and also gives TBM operators ability to alter operating speeds of the cutting blades and therefore control the cutting process.

The discs need to be positioned in a certain position in order to achieve grater cutting depths. In order to achieve the desired function, the cutting force should always be applied perpendicular to the rolling direction, (Jiujiang, C.U.I., 2009 ).

The energy considerations of the machine include torque and power requirements as needed by the machine. These conditions need be established in order to determine specific energy required by a TBM, ( Li et al 2008 ).

In order for a TBM to achieve its working efficiency three combinations. The cutter-head ought to have bracing systems operated by a hydraulic system to ensure efficiency a TBM. Moreover, a central conveyor system should be installed to remove source chips from the source.

In order for the head source to rotate at 360 degrees, several discs interlock with one another, which leads to an increase in penetrating force of the TBM. The surface gets loosened as the TBM penetrates.

A TBM works differently on different occasions, depending on various factors. A tunnel boring machine will always show different characteristics in different conditions. In connection, its performance is affected too. The machine tends to advance into the ground more in low altitudes than in higher altitudes, (Sousa, R.L. and Einstein, H.H., 2012 ). This can easily be proved in the boring of granite. Understanding the forces that the machine will counter is important in determining the type of cutters that must be used. For example a generic tunneling machine would work more efficiently in low altitude areas because of the weak forces that the machine will have to encounter. Many operators of TBMs would therefore prefer low altitude to high altitude areas.

Designing More Efficient Tunnel Boring Machines

Control systems play a very important role especially to monitor working of a conveyor belt. Controlled monitoring will ensure that risks are properly mitigated and even eliminated. Therefore, there must be established regulations that should be set to monitor the installation and maintenance of the control system, (Mengshu, W.A.N.G., 2008 ). These may include the following:

1. The control system should be properly labeled and should be visible and easily identifiable.
2. They should be positioned for safety operations so as to avoid unnecessary actuations.

• They should be located next to one another if the start and stop functions are not located on the same controlling device.

1. They should be guards on each controlling system so as to avoid unnecessary actuations.

The control, systems should be designed such that:

1. The hydraulic pumps that switch the drive motors shall not cause danger to other machines or persons.
2. There shall be no danger in case the voltage control fails.

• Failure of any part of the machine shall not cause in deterrent movements to other parts of the machinery.

1. There shall be a master control unit on each control unit to stop the entire system in case of any alarm.

The main control should be installed in such a manner as to control all other control units. The machine should not start if the min control i=unit has not started. All other auxiliary controls should be subject to the main control unit.

The control machinery should also be fitted with a control that shall safely bring all the operations of the control unit to a stop. Each control point should be guarded with an emergency switch to stop all operations in case of an emergency.

The type and nature of a tunnel constructed in a certain area will depend on many factors. The following are the major types of tunnels:

1. Bordered tunnels
2. Immersed tube – underwater crossings

• Cast-in-situ – waterway

1. Cut and cover – shallow depths
2. Tunnels excavated through rocks

According to (Mahdevari et al 2012), The type of tunnel to be constructed will depend on the type of material found in the site. Before a tunnel is constructed, several considerations have to be, and because tunnel construction involves complex engineering conditions, is costly and risky too. Following are some of the considerations:

1. The effect of the surface removal may cut off access to people and vehicles.
2. Relocation of public facilities.

• Noise from construction site

1. Vibrations on property
2. Air pollution
3. Effect on settlement buildings.

Depending on the excavation grounds, the cutter heads may be altered differently in order to achieve the desired efficiency. The diameter of the cutter may often need to be readjusted depending on the hardness of the rocks. The Rotational speeds may also need to be altered by the Variable Drive (VFD) system, (Chehade, F.H. and Shahrour, I., 2008 ).

The performance of a TBM depends on four major factors:

1. Penetration per revolution
2. excavation specific energy, €,

• Thrust per disk,

1. utilization co-efficient, 

Penetration per revolution of a TBM depends on the rock mass properties and the applied thrust.. From a theoretical perspective, a relationship between these factors may not be evident. This is because the TBM may have an insignificant value of thrust which may be too low to help obtain the rock mass stability, (Mroueh, H. and Shahrour, I., 2008 ).

To get penetration depth of a TBM, we will use predictive laws, whereby we will consider the Uniaxial Compress Strength (UCS) of the rock-mass.

If p= 30 and σc-0.4365 – 0.047 and RSR+ 3.145        

RSR = rock-mass rating

σc (MPa) =   Uniaxial Compress Strength (UCS)

P = penetration rounds value

Net advance rate (NAR) can be evaluated by:

N A R = P*n

n= number of cutter head revolutions per minute

If taken that σc = 100 MPa, and using the relationship R S R = 0.765 R M R + 12.35, the TBM penetrations value can be easily deduced.

Cutter-head power can be viewed as a function of the rock-mass condition and disc cutter diameters and penetration speed of the TBM. Force on the cutter discs increases as the rock strength increases, (Huo et al 2012).

Historically, tunnel boring machines have always been used for construction of water diversion channels, roads, drainages, conveyance etc. tunnel excavation is a complex and risky procedure and in recent years major technical advancements have been made so as to improve the design and efficiency of TBMs. These machines are based on old designs of more than 50 years ago are majorly characterized by huge dimensions and enormous weights and high power consumption. The cost of maintenance can be incredibly high. Replacement costs and technical assistance cost are normally high due to the limited number of trained technical staffs. Recent major technical advances have been developed for TBMs to enhance future considerations in excavation of tunnels for complex mining procedures and layouts. On this note it is important that tunnel boring machines and tunnel construction be properly investigated so as to avoid unnecessary errors and reduce risks during tunnel construction. There is need to come up with new TBM designs that will break up the old paradigms and allow for the design and exploration of new designs of developing TBMs and new mechanisms and regulations of constructing tunnels.

It requires more power to bore on jointed rocks than on disjointed rocks. The excavation energy of a TB a function of the torque value and power of the cutting discs. Specific energy is mainly evaluated in two ways: firstly, you may related to the cutter head location and secondly, you may choose to relate it to advancement. Hence the specific energy is the ration of power of the cutter head and volume of the total excavate per unit time. This is expressed in the following relationship:

Es = (C·ω) / (v·A)  

C = torque of cutter head

ω = rotation speed of cutter head

v = advance rate  

A = cross section of the tunnel

Evaluating the equation above, it is observed that torque (ω) is constant while the specific energy is correlated inversely to advancement rate, (Yagiz et al 2009). In many TBMs works, this can be statistically expressed. Based on the large amounts of evidence from mathematical calculations and data, new systems can be developed that are able to predict TBM advancement rate, given that the geological and geo-mechanical conditions have been established.

The analysis carried out in this project explores the design and working principles of a TBMs, and tunnel construction considerations. It is clear that TBMs working efficiency can be optimized by modifying various factors and components in a typical TBM. It is not necessary to hold onto the traditional designs of TBMs. Based on the adjustments that have been suggested in these projects, better designs of TBMs can be achieved resulting in modern, safe and cozy tunnel structures.

References

Chehade, F.H. and Shahrour, I., 2008. Numerical analysis of the interaction between twin-tunnels: Influence of the relative position and construction procedure. Tunnelling and Underground Space Technology, 23(2), pp.210-214.

Entacher, M., Lorenz, S. and Galler, R., 2014. Tunnel boring machine performance prediction with scaled rock cutting tests. International Journal of Rock Mechanics and Mining Sciences, 70, pp.450-459.

Font-Capó, J., Vázquez-Suñé, E., Carrera, J., Martí, D., Carbonell, R. and Pérez-Estaun, A., 2012. Groundwater inflow prediction in urban tunneling with a tunnel boring machine (TBM). Engineering Geology, 121(1-2), pp.46-54.

Huo, J., Sun, W., Chen, J. and Zhang, X., 2012. Disc cutters plane layout design of the full-face rock tunnel boring machine (TBM) based on different layout patterns. Computers & industrial engineering, 61(4), pp.1209-1225.

Huo, J., Sun, W., Chen, J., Su, P. and Deng, L., 2010. Optimal disc cutters plane layout design of the full-face rock tunnel boring machine (tbm) based on a multi-objective genetic algorithm. Journal of Mechanical Science and Technology, 24(2), pp.521-528.

JAVAD, G. and NARGES, T., 2010. Application of artificial neural networks to the prediction of tunnel boring machine penetration rate. Mining Science and Technology (China), 20(5), pp.727-733.

Jiujiang, C.U.I., 2009. Risks and Countermeasures for Construction of Shield-bored Tunnels [J]. Tunnel Construction, 4, pp.377-396.

Li, S., XUE, Y., ZHANG, Q., LI, S., LI, L., SUN, K., GE, Y., SU, M., ZHONG, S. and LI, X., 2008. Key technology study on comprehensive prediction and early-warning of geological hazards during tunnel construction in high-risk karst areas [J]. Chinese Journal of Rock Mechanics and Engineering, 7, p.003.

Liu, H., Li, P. and Liu, J., 2011. Numerical investigation of underlying tunnel heave during a new tunnel construction. Tunnelling and Underground Space Technology, 26(2), pp.276-283.

Mahdevari, S., Shahriar, K., Yagiz, S. and Shirazi, M.A., 2012. A support vector regression model for predicting tunnel boring machine penetration rates. International Journal of Rock Mechanics and Mining Sciences, 72, pp.214-229.

Mroueh, H. and Shahrour, I., 2008. A simplified 3D model for tunnel construction using tunnel boring machines. Tunnelling and Underground Space Technology, 23(1), pp.38-45.

Mengshu, W.A.N.G., 2008. Construction Scheme of Taiwan Strait Sub-sea Railway Tunnel [J]. Tunnel Construction, 5, p.002

Peila, D., Borio, L. and Pelizza, S., 2011. The behaviour of a two-component back-filling grout used in a tunnel-boring machine. ACTA geotechnica Slovenica, 1, pp.5-15.

Sousa, R.L. and Einstein, H.H., 2012. Risk analysis during tunnel construction using Bayesian Networks: Porto Metro case study. Tunnelling and Underground Space Technology, 27(1), pp.86-100. 

Sun, W., Huo, J., Chen, J., Li, Z., Zhang, X., Guo, L., Zhao, H. and Zhao, Y., 2011. Disc cutters’ layout design of the full-face rock tunnel boring machine (TBM) using a cooperative coevolutionary algorithm. Journal of Mechanical Science and Technology, 25(2), p.415.

Yagiz, S., Gokceoglu, C., Sezer, E. and Iplikci, S., 2009. Application of two non-linear prediction tools to the estimation of tunnel boring machine performance. Engineering Applications of Artificial Intelligence, 22(4-5), pp.808-814.

Yazdani-Chamzini, A. and Yakhchali, S.H., 2012. Tunnel Boring Machine (TBM) selection using fuzzy multicriteria decision making methods. Tunnelling and Underground Space Technology, 30, pp.194-204.