Traditionally, tunnels have been excavated by drilling and blasting method (DBM), but now with the advent of road headers and TBMs, there has been a significant increase in the rate of excavation and improved safety record. Often in problematic reaches, drill and blast methods come to the rescue and are handy (Ramamurthy, 2008). When unfavourable or changed conditions are encountered without warning, it has a far greater impact on the rate of advance, construction costs and schedule delays in a TBM driven tunnel than in a drill and blast tunnelling.
It appears that TBMs and DBM are expected to provide constructability options for contractors to be competitive. In the tunnelling industry, where market conditions continue to demand higher advance rates and lower costs, TBMs offer numerous benefits, including higher advance rates, continuous operation, less rock damage, uniform muck characteristics, greater safety and potential for remote automated operation.
On the other hand, the DBM is very flexible and adaptable. The definite answer to which tunnelling method should be chosen is always a tough question.
Proper choice of the tunnelling method is crucial for the engineers and contractors, as mistakes or misjudgements can have serious consequences, both for the economic viability and the overall success of the project.
Tunnelling engineers have to make judicious choices on a case-by-case basis considering the site conditions and expected outcome. When both TBM and DBM are feasible, a careful assessment of the risks must be made, particularly, in terms of safety, economy and productivity.
Factors affecting the choice of tunnel method:
A. Tunnel design parameters
B. Rock mass characteristics
C. Performance factors
D. Contract related factors
Tunnel Parameters
Diameter
Although TBMs have excavated tunnels more than 15m diameter, yet it is better to limit the size of the tunnel due to the following reasons:
The success potential of a TBM in hard rock decreases with increasing diameter (Kovari et al., 1993; Bruland, 1998).
There are technological limits for the maximum dimensions of some major TBM components e.g., the bearing and the head (Nord, 2006).
The intensities of both the instability phenomena and the induced convergence also increase with increasing diameter of excavation (Tseng at al., 1998; Barla G. and Barla M., 1998)
A TBM drive requires a pre-determined (fixed) tunnel diameter but it can excavate a circular profile with a high degree of accuracy. However with the drill and blast system, the tunnel cross-section can be driven to any required size or shape and most importantly the tunnel size and shape can be changed along the length of the drive.
Length
Since the mobilisation cost of TBM is high, it requires a long tunnel to justify a large capital investment. Therefore TBMs will be used where tunnels are to be long and of uniform crosssection and profile.
The conventional DBM is therefore most often used on shorter tunnels.
In the case of long tunnels with favourable geology, relatively high advance rates can be achieved with a TBM.
However as soon as the geology becomes complex and there are zones of disturbance, drill and blast performance can become significantly better as compared with a TBM.
A simple indicator on when a TBM solution might be suitable is to make a simple estimate as shown below. The formula simply says:
Tunnel length (m)/Tunnel diameter (m) x (UCS in Pascals)1/3 > 1.5 (Nord, 2006)
That if the tunnel length divided by the tunnel diameter and the unconfined compressive strength of the rock at power of one third and the result is larger than 1.5 it might be worthwhile to check the TBM alternative. The trigger value of 1.5 using the above formula is not as accurate as it might seem and perhaps it would be better to say that when the result is 3, the TBM option is definitely a viable solution and when the value is less than 1, the TBM option should be considered less favourable than the DBM. Please note that this expression has no scientific back up. Poor ground conditions are not foreseen here and nor is abrasive rock considered (Nord, 2006).
Based on the research at the Swiss Federal Institute of Technology, TBM technology shows excellent cost efficiency in the case of tunnels longer than approximately 3km. The exact length depends upon the rock mass characteristics, tunnel parameters, labour cost and utilisation factor.
Shape
DBM is very adaptable and flexible in regards to the excavation of any tunnel cross-section (Grimscheid and Schennayder, 2002). A circular profile can be excavated with a high degree of accuracy by a TBM. However, with drill and blast system the tunnel cross-section can be created to any required shape or size and most importantly the tunnel shape and size can be changed along the length of the drive.
The suggestions for choice between tunnel boring machine and drill and blast system have been presented in Table 1.
Rock mass characteristics
Strength
The TBM excavation with respect to advance rate is by far much more depending on the strength characteristics of the rock than drill and blast.
Geological features
Geological conditions to be encountered such as, faults and groundwater can have a major impact on machine performance, application, operation and the production rate. These parameters must be accounted for when estimating the machine utilisation, which is a key parameter in scheduling.
Analysis of field performance of different TBM projects is the foundation for estimating the effect of these geological features in the rock mass.
The opinion is that drill and blast method offers a higher flexibility and consequently better opportunities to cope up with unforeseen conditions.
According to Nord and Stille (1988), variable rock conditions favour the choice of the blasting method. Water conditions affect both methods, but the TBM is more hampered than the drill and blast system if pre-grouting has to be done. The variation in tunnelling speed when excavating in favourable versus unfavourable ground conditions is also less for the drill and blast than the TBM method.
In the case of TBM, massive rock is unfavourable for fast penetration, while for DBM, it is obviously favourable due to the lack of tunnel support needs and can be drilled at reasonable speed despite the lack of jointing.
Rock type
The overall composition of the rock mass holds a first order control on TBM penetration. The more mafic (iron and magnesium rich) the rock mass the lower the penetration. Some rock types (such as fine grained or glassy dike rocks, amphibolites, pegmatite, intrusive, garnetiferous zones, quartz veins) have important bearing on TBM penetration and these should be identified and categorised accurately. Unique igneous and metamorphic textures can make or break a contract (Merguerian, 2005).
Abrasiveness
The abrasiveness of a rock or soil is its potential to cause wear on a tool. It is an important parameter to assess the technical and economical aspects of a tunnelling method.
Rock mass rating
Nick Barton (2000) found that the TBM technique is most competitive time-wise versus drill and blast when rock conditions are in the Q-range 0.1 to 10 on his rock quality scale (Figure 2).
It should be pointed out that this is a hypothetical statement but it does points on to the difficulties the TBM excavation faces when entering into a very poor ground. Many cases have been recorded where TBM technique has to be abandoned in favour of the drill and blast technique. But also on the very end of the quality scale the TBM excavation will be difficult due to monolithic character of the rock yielding only few joints.
In low quality rock, the penetration rate can be potentially very high but the support needs, rock jams and gripper bearing failure result in slow advance rate, with utilisation coefficient as low as five to 10 per cent or less (Barla and Pelizza, 2000). Grandori (1995) correlated the advance rate of the TBM with RMR value. It showed that RMR class III provided a peak in production for a double shield TBMs, while they would not be recommended for neither class V (very poor) nor class I (very good rock masses).
The choice between TBM and DBM on the basis of geological and hydrogeological considerations have been suggested in Table 2.
Performance factors
Rate of advance
In the case of drill and blast system, equipment is available in various sizes and is selected to fit the actual tunnel size. In a larger tunnel, more drilling machines can operate in parallel and larger units can be deployed for mucking and hauling. Therefore, there is no direct relationship between tunnel size and advance rate for drill and blast operations.
Barton analysed a large number of TBM driven tunnels and has concluded that there is a major variation in the rate of advance and penetration rate depending on the rock quality. He suggested a tunnel stability relationship based on Penetration Rates vs. Rock Quality Designation for TBMs (Barton, 2000). Since the time that this was developed, we have not seen any recent research to suggest that the TBM technology has advanced in terms of penetration rates based on Barton’s work.
Barton (2000) also made a comparison between advance rates of TBMs and DBM as shown in Figure 2.
Although this relationship suggests a relationship based on project-based information. That being said, TBM and DBM equipment improvements over the past decade have increased the equipment efficiencies, and as such the relationship between Rock Quality and advance rate for TBM and DBM should be updated.
Boreability
When the TBM cannot penetrate the face to a sufficient rate and or the wear of the cutting tools exceeds an acceptable limit, it is an indication that rock is not borable. The penetration rate per revolution of the cutter head that can be achieved under the maximum thrust is the main index describing the capacity of a TBM to excavate a given rock. A limit of penetration per revolution below which a rock shall be considered nonborable is influenced by the abrasivity of the rock, the diameter of the tunnel and the geology of the rock formation. The high abrasivity associated with low penetration dictates frequent changes of cutters, increases the cost of excavation per unit of rock, in addition to the time lost in replacing the cutters.
The penetration rates below 2 to 2.5mm/rev of the cutter head is a signal of boreability problems. An excavation process starts to be efficient when the penetration rate crosses 3 to 4mm/rev.
When the diameter of the tunnel increases, three different effects make the situation worse (Barla and Pelizza, (2000):
- The rotational speed of the cutter head should decrease for an equal penetration per revolution, because the bearings and seals of the disk cutters permit only a maximum speed equivalent to 150 m/min.
- The number of cutters to be changed per meter of tunnel advance increases, therefore, increasing the stopping time required for such operations.
- The state of average wear of the cutters mounted on the head increases, thus decreasing the penetration per revolution.
Under extreme conditions, each one of the above three factors excites the other one bringing the progress rate down to unacceptable values. For these reasons, a rock type may be borable for a TBM of small diameter, but not for a TBM of large diameter.
- If ROP is the average rate of penetration, then
- ROP = boring length in meters/boring time in hours
- Penetration per revolution, Pr = (ROP x1000)/(RPM x 60) mm/rev.
- RPM is cutter head revolutions per Minute
- Field Penetration Index, FPI =Fn /Pr KN/cutter/mm/rev.
- Fn is the cutter head load or normal force in KN
The choice between TBM and DBM on the basis of work done by Barla and Pellizza, (2000) and Hasanpour et al, (2011) is given in Table 3.
Support requirements
Most tunnels will require support to ensure its long-term stability. The type and magnitude of the support is determined by the rock mass characteristics, water conditions and state of stress.
In general, less support is needed for a TBM than a drill and blast operation. In cases where drill and blast requires little support, the TBM in similar conditions may require no support. In cases where heavy support is needed for drill and blast operations, the support measures and stabilisation ahead of the face will not be less for TBM technology.
In fact, they may be even larger and certainly take much more time due to the difficulties with installations of supports right behind and ahead of the cutter head.
When heavy support is needed, TBM operations will provide lower advance rates than the DBM system (Barla and Pelizza, 2000).
Equipment utilization
The TBM operations experience downtime due to changes of cutters, regripping, maintenance and downtime, etc. All this down time adds up to 40 to 60 per cent of available operating time.
Skilled labour
One crew is required for a single TBM working face but a TBM crew will be larger. Crew needs higher skill level, but are easily trainable because operations are more consistent and continuous.
The suggestions for the choice between TBM and DBM on the basis of operating requirements are given in Table 4.
Environmental and Safety Constraints
Overbreak
Overbreak is the excavation of the rock beyond the designed profile. Overbreak increases the cost of mucking, support and concrete lining. Overbreak is generally influenced by the lithology, rock mass properties and quality of blasting.
Overbreak caused by geological instabilities is generally larger when tunnelling by drill and blast than TBM. In some cases, however, it is more complicated during TBM excavation to support ahead and right behind the tunnel face and as a result of that support is installed at a very late stage resulting in larger collapses. These collapses have sometimes led to the complete burial of the TBM.
Out fall behind the gripper pads of the TBM is another form of geological overbreak linked to the TBM operation. The overall experience is that TBM excavation will generate less geological overbreak (Nord, 2006).
Vibrations
This is a major concern when tunnelling by DBM in an urban environment. If the surroundings are highly sensitive to vibrations, there may be constraints in the amount of explosives that can be used per delay. This may limit the progress of the DBM.
However the problem is alleviated with latest advances in drill and blast technology. In case of TBMs, there are significantly less disturbances to the surroundings.
Safety and environmental risks
Tunnelling is not a risk free technology. Drilling and blasting system is quite challenging when tunnelling in populated areas. Not only is the work closer to people, structures and utilities, but environmental concerns about blasting effects on flora, fauna and water resources need to be considered. In addition, government scrutiny of commercial explosives activities due to terrorist incidents and continuing threats have increased public fears regarding the applications of explosives in urban environment.
On the other hand, premature surrender to TBMs sometimes becomes a costly decision. The sensitivity of TBMs to changes in actual conditions increases the probability of involved risks.
During excavation, the situation can become critical at any minute, meter and under any circumstance. In some cases, the failure of a TBM necessitates the last minute switch to DBM. When blasting methods are introduced at the last minute without having proper planning and controls in place, the risks of blasting problems are increased. During TBM excavation, the rock support in general is installed from within protected and shielded areas. Absence of blasting fumes and related problems inside the tunnel provides improved working environment.
Suggestions for the choice between TBM and DBM on the basis of environmental and safety requirements have been given in Table 5.
Cost
A TBM tunnel project requires more demanding infrastructure in terms of roads, power supply, muck handling, work areas for storage and robust transportation needs, there are normally higher costs and longer times required for TBM mobilisation.
Transporting of the equipment to the site also needs additional time and cost. TBM tunnel projects require more electric power than DBM projects.
Tunnel quality
During TBM excavation, it may be easier to ensure accurate alignment. The periphery of a TBM tunnel is smooth and usually has less overbreak. As such, it is possible to maintain excavation preciseness with TBMs.
Based upon cost and quality requirements, suggestions for the choice between a TBM or a DBM tunnel are given in Table 6.
Conclusions
TBM tunnel excavation represents a large investment in the decision making process with inflexibility with regard to changes in diameter and small radius curves and challenging vertical and horizontal alignments. As such, the use of TBMs for near horizontal excavation alignments can be a potentially rapid excavation and rock support method for rock tunnels.
On the other hand, DBM is very flexible and adaptable with comparatively lower advance rates. That being said, there is a need for careful planning for the optimum selection of tunnelling alternatives, because a wrong choice can lead to costly and time consuming consequences.
In this study, the suggestions for the selection of a tunnelling method based upon tunnel parameters, boreability, geological conditions, equipment operating requirements, power needs, environmental and safety constraints, costs and tunnel quality requirements have been made.
The suggestions made in this paper may help facilitate the selection of the tunnelling method for a project or produce further investigation into the selection criteria and viability for each method during the design and contract bidding stages.
Further research and review of project specific case studies in North America should to be investigated to determine the validity of the penetration rates when rock quality has been a factor in the tunnel equipment selection decision making process
