Pre-grouting works have a large impact on all tunnelling in Scandinavia. Almost no civil tunnel work is performed without the pre-grouting component. This is still not the case in most other parts of the world and therefore a brief explanation should be given.
The bedrock in Scandinavia is 80 per cent of crystalline character and of Precambrian age. It is in general of good quality for construction as it is stable and durable. Almost no conventionally excavated tunnels are given a secondary concrete lining. The permanent support is normally based on grouted bolts and sprayed shotcrete.
The drilling and pre-grouting work is carried out rapidly and to a high standard. The sealing work constitutes a large share of the total tunnelling time. The drilling of grout holes at the Hede – Alvangen rail tunnel in Sweden is carried out with a newly developed drill rig specially designed to deal with pre-grouting operations. With the project’s quality and capacity achievements, it is suitable for presentation of the drilling technique.
The project
The 130m2 tunnel is part of an upgrading of the railway line between Gothenburg and Trollhattan. It is a 1.1km dual track high speed rail tunnel. An emergency escape tunnel of 35m2 runs parallel for 0.6km. To speed up the excavation but also to provide access to the emergency tunnel a 500m long additional tunnel has been constructed.
The Swedish division of the Norwegian company Veidekke is subcontractor to Sweden-based Peab, which has been given a larger share of the upgraded railway line.
All tunnel works including permanent support are due to be completed within 21 months. The excavation itself is expected to take 18 months. Of these 18 months, roughly six months are typically allocated for sealing of the tunnel against water seepage by use of pre-grouting.
Three major joint sets have been identified and the recorded rock quality designation values are generally very high. In the portal areas the rock cover will go down to 4m, which is low for the 15m span.
Drilling for pre-grouting
The most common drilling method is to drill in a funnel shape along the periphery of the tunnel face. In this case the number of holes is 38. The number of holes will vary depending on the ground conditions and the demand on the sealing effect. The target of the sealing is specified as ‘litres of water inflow over 100m of tunnel and minute’. The accepted maximum ranges normally from 0.5 to 10. In this case the target is 8l/100m/min. The procedure has for many years been to start with a moderate number of grout holes and monitor the result by use of water loss measurements in a number of additionally drilled holes, a relation between the water loss measurement results and the expected inflow is then established. New means to monitor the ground conditions makes it possible to pinpoint the number of holes needed to ensure that the maximum allowed inflow of water is not exceeded.
The long grout holes require drilling with extension rods. That means a number of joints and decouplings are performed for every hole. This has until a few years ago been performed manually by tunnellers standing on the service platform of the rig. The safety regulations state that no drilling is permitted when people are in front of the drill rig. As the application of the regulations becomes strict, the drilling procedure becomes much more time consuming and the tunnelling more costly. These facts pushed for the development of the fully mechanised handling and jointing of the drill rods.
The standard procedure for drilling grout holes is to have two or three boom rigs in the heavier range equipped with strong and stiff booms to rods. The storage should be large enough to hold a drill string with a length of 25 to 30m. The rig will be used for regular blast hole and bolt drilling.
This standard drilling procedure applied on a set of grout holes will take some 10 hours. In addition to this, the time for grouting and hardening of the grout will consume another 10 hours. This means that a full day is spent on the sealing work. Now some 18m of tunnel can be excavated. Normally 18m requires three (3 x 6m) rounds, with the time for one round is some 12 hours for a tunnel the size of Hede – Alvangen. This means that the time for grouting takes about 35 per cent of the total time for tunnelling for a tunnel of this character and the time for drilling is roughly 17 per cent.
The Veidekke concept
Bearing in mind that the drilling for grout holes may take as much as 17 per cent of the total tunnelling time it is not a stretch to think of the option to add more rock-drills to the rig. A doubling of the rock drilling capacity would theoretically reduce the construction time by 9 per cent.
Assuming a tunnelling cost where one rig is involved of SEK 150M (USD 23.69M) the time related costs will roughly be SEK 100M (USD 15.79M). The nine per cent (17 per cent over two) would mean a saving of SEK 9M (1.42M) for one project only. Certainly there are additional costs involved and the reduction of time may be less but the idea to go for more rock-drills on the rig still seems viable. These ideas brought the Veidekke Company to form a kind of joint venture with Atlas Copco in Orebro to design and build a four-boom rig equipped with high capacity rock drills all suited for the drilling of long grout holes.
The rig was supposed to be used for all types of drilling that will occur in drill and blast tunnelling. The rig was given four equal booms and a capability to cover a cross-section of 200m2. The feeds are 21ft (6.4m)-long and all are equipped with a rod handling system (RHS) with capacity to drill 30m-long holes using T45 rods suitable for 64mm bits of retrac type. The rock-drills are of type Cop 3038. The rig control system (RCS) is such that it should be possible for one man to operate it. It is based on the ABC total system where the rig performs all the pre-programmed working sequences with the option for the operator to interfere if needed.
On this new rig-concept the operator has been given a more active role. He is given the tool to alter the design of the drill holes and drill settings of the rig. This feature has been designated ‘active operator design’. That means the operator will play a more important role, providing a better sealing result. As well as a more precise location of the grout holes, the drilling is faster.
In order to get as accurate positioning of the rig as possible it is given the ‘total station’ navigation. That means that the rig position can be established with an accuracy of 10mm.
The rod handling is automatic. This system has been given the name ‘Auto RHS’.
The specification used for the rig makes it suitable for the Hede – Alvangen project. The project is designed with continuous pre-grouting and a fairly short construction time. The threshold for accepted water leakage is far from the lowest as it set to 8l/min and 100m. The accepted deviation of the grout holes was up to four per cent and that means that the bottom of the 24m-long grout holes must not deviate more than 1m from the planned position.
Four-boom rig performance
The accuracy of the grout-hole drilling is continuously monitored and the result from one round of holes is given in figure 2. All the holes fall within the stipulated tolerance of 960mm. There are rounds where one or two holes have an unacceptable deviation. This might be explained by structural changes in the rock-mass. Sets of weakness planes crossed by the drill bit is known to have a great impact on the hole deviation. Typical weakness planes are joint sets of somewhat higher frequency, foliations and bedding planes when dealing with sedimentary rock.
The percussion rates in the grout hole drilling are in line with what can be expected from a 30kW rock drill and a 64mm bit when drilling in this type of ground. The penetration rate varies between 1.5 and 2m/min. For the 48mm blast hole the penetration rate is doubled.
Already at this early stage the expected speed improvements have been realised. For blast hole drilling some may dispute the time saving when going from three to four rock-drills on the boomer. Theoretically, when disregarding mobilisation and demobilisation time and losses, due to the higher complexity to coordinate when the number drilling units is increased the capacity cannot be increased by 33 per cent (4/4=1.33). Experience indicates that a more correct capacity increase should be 3.4/2.7 = 1.27 or an increase of 27 per cent.
This figure matches the results achieved at this site compared to another site with very similar conditions using the XE3C rig with the rock drill Cop 3038.
Concerning the capacity increase for grout hole drilling when going from two to four rock drills on the boomer no algorithm has been established to evaluate such an alteration. It is however easy to apply the formula for blast hole drilling and the result is an increase by 80 per cent. The result is close to reality.
Manning and rig safety
In this Hede-Alvangen case one man operates the four-boom rig. He is dealing with the whole process from transport up to the tunnel face, hook up of services, navigation and drilling. This is considered a very safe way to work, as there can be no miscommunication. Furthermore there is no labour working from the service-platform as the rod-handling system makes that activity obsolete. So far the anticipated improvements of the safety have come true. The number of accumulated man-hours at the rig is still too small to confirm that the safety has definitely improved.
The economy linked the four-boom rig
A quick look at the effect of time saving indicates the following:
The blast hole drilling typically covers some 20 per cent of total time at face. A 20 per cent reduction of the drilling time, which will be the case when going from three to four booms, would mean a four per cent reduction of the total excavation time or 2.7 per cent of tunnelling time. The time related cost for this project that can be related to this rig is given above and was estimated at SEK 100M (15.8M). The savings will then be SEK 2.7M (USD 0.4M). The discussion on savings as performed above for the grouting work ended at SEK 9M (USD 1.4M). What it means to cut down on the number of operators is probably another two million for a project of this nature. That means that savings in the range of SEK 14M (USD 2.2M) can be identified.
Conclusions
The driving force behind the development of this rig was a strong request from contractor Veidekke to the drill-rig manufacturer Atlas Copco. It should be a rig capable of performing all the drilling tasks asked for in tunnelling but with a competitive edge to grout-hole drilling.
The rig was manufactured and delivered mid 2010 and in autumn the same year performed as expected. The rig concept developed at an early stage and there was plenty of time for sending ideas back and forth and also to make proper checks. The project started with very ambitious plan, which were beneficial in the long run.
Figure 1, location of the Hede-Alvangen tunnel project in southwest Sweden The Boomer XE4C rig at the tunnel face of Hede-Alvangen Figure 2, deviation at the bottom of the 24m long grout holes from one round of holes
