THE PERFECT SHAPE
OF THINGS TO COME

By GUNNAR NORD, Senior Construction Advisor, Face Drilling and Mechanized Bolting

An inaccurate contour, or planned outline, of a tunnel following drilling and blasting has long been a problem for tunnellers the world over.

The dilemma is created by inexact drilling of the blast holes. This results in overbreak, or excess rock, being blasted out, leaving irregularities in the planned contour of the tunnel.

In simple terms, the more overbreak, the more time and money is wasted, primarily in mucking out. Transporting the excess rock - perhaps a considerable distance - and restoring the tunnel to its planned smooth contour with reinforcement and lining all means time lost.

Facts and figures

The considerable costs of overbreak can be illustrated by this hypothetical example: A 70m2 two-lane road tunnel, 1 km long, is being built and, according to the tender documents, only theoretical volumes of excavated rock as well as secondary lining will be paid for. So-called geological overbreak is compensated for and the perimeter of the tunnel is typically 22 m.

Gunnar Nord: Dealing with overbreak brings big savings.

A reduced overbreak of 10 cm resulting from more accurate drilling gives the contractor these savings in Euros: Concrete secondary lining 2,200 m3 @ EUR 120/m3 = 264,000. Reduced mucking 2,200 m3 @ EUR 5/m3 =
11,000. Reduced shotcrete 200 m3 @ EUR 300/m3 = 60,000. Reduced construction time 100 hours @ EUR 700/hr = 70,000. Total savings: EUR 405,000.

These figures illustrate the magnitude of the overbreak problem. However, placing holes accurately and with a proper alignment is not new. It could be achieved even in the era of hand-held drills, using short rounds. But the problem was the amount of time it took.

Best technique

As far back as the mid-80s, an overbreak of less than 10 cm was recorded on drill-and-blast sewage declines in Sydney, using both hand-held and mechanized drilling.

During the same period, on the Route 5 tunnel project in Hong Kong, mechanized drilling operations employed the best technique then available for accurate contours. It involved accurate setting out of the tunnel face, TAS (Tunnel Angle System) for the alignment of the feeds, an immediate monitoring of the profile - plus a bonus to the rig operators. The results are shown in Figure 1.

More light at the end of the tunnel came in recent years in the shape of a manual contour control system introduced by the Norwegian company Bever. It has been mounted on a large number of drill rigs - mainly Atlas Copco Boomer units - and the operator is guided by a TV screen in manually positioning the booms to achieve more accurate holes.

And in 1998, following years of development, Atlas Copco launched its own contour control system, which is being integrated into its new generation of rigs. So far, it is operational only in manual mode but is due to be available for robot drill rigs later this year.

Figure 1: Overbreak improvement at the Route 5 tunnelling project in Hong Kong.

Progress at projects

The following four examples of what can be achieved with Atlas Copco Boomer rigs equipped with Bever control are from projects over the past five years. The first two cases involve drilling with Robot Boomer rigs.

• The Henriksdal sewage treatment plant, Stockholm, Sweden: The contractor wanted very high drilling standards, although there was no secondary concrete lining in the design. The length of the rounds was 5 m and the size of the opening some 80 m2. The rock was hard crystalline granites and gneiss, which was not expected to cause a drill steel deviation problem.

The skill of the operators was rated highly, conditions were favourable - but incentives were not strong, as there were no savings to be made on concrete. The average overbreak was 14 cm outside the collaring line and the distribution is shown in Figure 2.

• The Escalette road tunnel project in southern France: The two tunnel tubes were to be concrete lined with a strong incentive to keep overbreak down. Excavated sedimentary material was dominated by limestone, which had a clear bedding with a gentle dip.

Tunnels were some 700 m long with an excavated cross-section of about 70m2. Rounds were 4 m or less, if ground conditions were poor. Tunnel sections with geological overbreak were excluded from the study.

Results, shown in Figure 2, were very similar to Henriksdal - although conditions were quite different - and showed what can be achieved by the equipment and good operators in moderate-size tunnels without difficult geological conditions.

• The Mitholz tunnel in Switzerland: This adit tunnel to the Lötschberg-Basistunnel showed the most encouraging results on overbreak using the Bever control. The 1.5 km adit has an excavated section of 66 m2 and the rock has so far been limestone and shale. Normally, 4 m rounds were drilled and the pull was 3.8 m. In the 0.6 m-spaced perimeter holes, smooth blasting explosives were used with electronic detonators.

The drill rig was a Rocket Boomer 353 S and the contractor wanted as little overbreak as possible as he had to pay SFr 300 for every cubic metre of it that was more than 6 cm outside the theoretical line. Over the entire tunnel length, this meant the penalty for every extra cm of average overbreak amounted to SFr 93,000.

For the first 100-200 m of the tunnel, average overbreak was 25 cm. But by the time work had progressed to 700 m, it had come down to 10 cm as the skill of the drilling crews improved.

Figure 2: Similar overbreak advances at the Henriksdal and Escalette projects.

• Boliden's Garpenberg Norra Mine, Sweden: An Atlas Copco Rocket Boomer 352 S has been used on the latest section of the ramp tunnel and overbreak was reduced from more than 20% to 9%, or an average of 15 cm. This resulted in a reduction of the muck volume of 3 m3 per lineal metre of tunnel and savings on transport from a depth of 800 to 1,000 metres.

According to the mine management, the savings achieved in one year paid for the extra cost of improving drilling operations at the mine.

The reasons for these excellent results include young and dedicated rig operators and the fact that the rock itself does not generate any noticeable overbreak.

Important factors

So what are the main factors in reducing overbreak?

• First, the ambition of the site managers and operators is extremely important. Close monitoring of the tunnel face is a must in order to register undue overbreak and take counter measures - and a small bonus to the people involved can lead to great results.

• Geological conditions can have a great effect on hole direction. The drill string has a tendency to deviate perpendicular to the foliation in anisotropic rock like phyllite, schist and gneiss. The problem is more acute in surface drilling when far longer drill rods are used. And a high frequency of jointsets in the rock can also influence the drilling direction.

• Blasting is extremely important, and employing the smooth blasting technique with electronic detonators in the periphery holes can contribute greatly to a smooth tunnel contour.

• Tunnel size affects drilling accuracy. When booms and feeds are extended to the full, they are not so rigid and deflections can occur. Exact computerized compensation is not easy to achieve - and the further out the booms are, the greater the collaring and orientation errors can be.

• Alignment of the feed is critical at the start of drilling and a tunnel face which is not orientated perpendicular to the drill rod may cause bending and an inaccurate starting point. Low feed force and reduced impact at the start saves drill tool wear and, after that, feed force should be just enough to maintain tight joints without bending the rod and risking deviation.

Fast and accurate drilling requires dedication, experience and reliable, state-of-the-art drill rigs. With these prerequisites, there are not many projects where the economic advantages of contour control can be ignored.