TBM vs Drill & Blast for Tunnels

International Conference and Exhibition on Tunnelling and Trenchless Technology 7-9 March 2006, Subang, Selangor, MALAYSIA

TBM VERSUS DRILL AND BLAST, THE CHOICE OF TUNNELLING METHOD Gunnar Nord, Senior Advisor Construction Atlas Copco Rock Drills AB Sweden ABSTRACT The choice between Drill & Blast and TBM tunnelling is a non-ending topic. It is not surprising as the two methods have been competing for more than 30 years. During this time period both methods have made major technical advances. In this paper some specific differences of the two methods are dealt with from a general standpoint. Focuses are on time for the excavation, risks involved and to a minor extent costs. Statements are to some extent supported by references to cases. A simple guide line is also given for a first stage decision what excavation method to prefer. It is concluded that there are only few major rock tunnelling project where the excavation method can be established before a proper investigation and estimations have been made. The method is an open question before the bottom line on time and cost is arrived at. 1 INTRODUCTION The choice of excavation method for a defined tunnel is in many cases far from obvious. There are many conditions that have to be established and evaluated before a decision can be taken on the excavation method. Sometimes crucial facts are missing and replaced by guesses. In this paper the essential conditions for the choice of excavation method will be highlighted and discussed. The choice has been restricted to the two excavation methods Drill & Blast and TBM (Tunnel Boring Machine). Other mechanical methods like road header and hydraulic breaking have been omitted. The two selected methods are the dominating ones when it comes to tunnelling in rock Here focus will be on hard rock excavation and the TBM technology using EPB (Earth pressure balance) and slurry technology will not be dealt with. The purpose of the paper is to give tunnel builders that are used to the drill and blast technique a better understanding of what tunnelling by TBM means and vice versa and therefore this paper has been given the actual title. The two methods will be compared and discussions will be held on the suitability of one or the other from various aspects. By TBM is here meant a hard rock excavating machine with a circular rotating head equipped with disc cutters. The cutters are rolling along circular path on the tunnel face. The load on the cutters makes the rock fail and the loosened pieces fall by gravity down to the tunnel invert where they are picked up by buckets mounted along the periphery of the cutter head of the TBM. Only atmospheric pressure is acting on the tunnel face. By Drill and Blast is meant loosening of the rock by use of explosives that are applied in holes drilled from the tunnel face and ahead. The explosives in the individual holes are detonated in a preset sequential order. The loosened rock is loaded and transported away from the face. 205

International Conference and Exhibition on Tunnelling and Trenchless Technology 7-9 March 2006, Subang, Selangor, MALAYSIA

For both methods regular support measures are taken to secure the stability of the underground opening. The paper will start with a general discussion on what conditions will influence the choice of method, after that a more detailed presentation will be given on some key issues and finally a few cases will be given where both TBM and Drill & Blast have been employed. 1. GENERAL In tunnelling as in most other manufacturing the most cost effective is the preferred one. The ambition of a buyer is to pay as little as possible for a defined product irrespective of it is a bicycle or a tunnel. The problem when it comes to tunnelling is that in many cases it is difficult to accurately predict the price tag. There are so many cost influencing conditions that are difficult or expensive to define beforehand. Furthermore a tunnelling project is normally running over several years and new unforeseen conditions may arise during this period. The calculated price tag varies depending on who is producing it and how the timing is. This statement is support by the results from bid openings on tunnel projects around the world. The amount of money that various contractors are asking for to produce a defined tunnel is very variable with spans as large as twice the price between the lowest and the highest bidder. In general it can be said that the drill and blast method gives less variation of the price. The opinion is that the drill and blast method offers a higher flexibility and consequently better opportunities to cope with unforeseen conditions. 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 tunnelling there is a very clear relation between time and money and this statement is valid for both of the two methods. When the tunnelling costs are split on time related and material related some 2/3: rd of the cost would be characterised as time related when it comes to drill and blast and the figure for TBM is in the same range. The variation in this split on time and material is though larger for the TBM alternative and that depends to a large extent on the status of the TBM itself. Is it hired, fully owned with no new commitments in sight, is it old or new and finally is the excavation carried out in a low cost or high cost region. 2. ADVANCE RATE OF THE TUNNEL HEADING Higher excavation speed is normally associated with TBM excavation and so are also the variations of it. What conditions are affecting the speed of the TBM excavation? This question is split in two. The first is, what sets the advance rate when the machine is running properly and the second how much time must be allocated for other activities that require a standstill of the machine. The advance rate is ruled by the penetration per revolution of the cutter head and the rotation speed of the cutter head. The easier the disc cutters penetrate the rock per revolution of the cutter head and the higher the revolution is, the faster is the advance rate. The penetration per revolution is ruled by the strength characteristics of the rock material and load on the cutters.

206

International Conference and Exhibition on Tunnelling and Trenchless Technology 7-9 March 2006, Subang, Selangor, MALAYSIA

2.1 Rock strength and advance rate What are now strength characteristics in this respect? There are different means to measure and evaluate them. Certainly the unconfined compressive strength is an essential component and it is valid also for regular percussion drilling used in the drill and blast tunnelling method. Furthermore the brittleness of the rock is of importance. The higher the brittleness is the larger chips or fragments of rock are loosened when the rock fails under the load of the cutters. Weaknesses in the rock as joints and foliations will boost the excavation rate and there are empirical ways to establish what the penetration rate will be. The problem is to establish what the strength characteristics will be along the tunnel line. When the same strength characteristics affect the drilling rate in the two tunnelling methods they should face the same uncertainty with respect to prediction of the advance rate. However the percussion rate is only marginally affected by the weaknesses but the TBM cutter penetration is highly affected by it. In addition it is difficult to predict the character of the weaknesses especially when there is a large cover of rock along the tunnel line. When considering how much of the total time is spent on drilling and boring in the two methods a generalised figure is that the boring time in the TBM excavation is normally at least 3 times larger than in the drill and blast method. The conclusion of the discussion above is that 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. This makes the estimation of the advance rate and tunnelling cost much more difficult and uncertain. 2.2 Advance rate and tunnel size The cutters mounted on the cutter-head have two limitations namely the load that can be applied on them and the rotation speed. The load applied on the cutters and the strength characteristics of the rock are ruling how large the penetration per revolution of the cutter head will be. The maximum cutter load is though not always fully utilised. When a good penetration is achieved the limiting factor is often the installed power for the rotation of the cutter head. The TBM advance rate is than limited by the torque that can be applied on the cutter-head. The rotation speed is limited by the capacity of the bearings inside the cutter. A faster rotation than limited would reduce the service life of the bearings drastically. This means that the outer most placed cutter on the cutter-head is setting the periphery speed of the cutter head. Consequently the diameter of the TBM is directly affecting the RPM of the cutter head. A somewhat simplified conclusion is that a 7 meter TBM will only advance at half the speed of a 3,5 meter diameter TBM. What is now the situation for the drill and blast technique with respect to advance rate and tunnel size? The equipment being used in drill and blast tunnels are available in various sizes and are selected to fit the actual tunnel size. This means also that the larger the tunnel is the more drilling machines can operating in parallel at the tunnel face and for loading and hauling bigger units can be employed. This means that there is not a direct relation between tunnel size and advance rate for drill and blast tunnels.

207

International Conference and Exhibition on Tunnelling and Trenchless Technology 7-9 March 2006, Subang, Selangor, MALAYSIA

Metres per month

Average Monthly Advance rates

Figure 1. Average advance rates for a number of TBM tunnel

700 600 500 400 300 200 100 0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

Classified according to diameter

1 10,00 excavation cases

What does this mean on the choice of the two excavation methods? It is as simple as that the smaller the tunnel is the more difficult will it be for the drill and blast method to compete with TBM. Figure 1 above is meant to support this statement. Here a large number of tunnelling cases have been classified with respect to the diameter and each bar represent half a meter of the tunnel diameter. The diagram has two peaks, one at bar 6 and the other at bar 13. This is surprising as the rate should be continuously falling from the smallest diameter. The small TBM:s