Tool system drills straighter hole

Paul Boughton

Swedish bench-drillers prefer Sandvik T35 tool system for cautious blasting

A drill-and-blast contractor working on Stockholm’s Southern Link millennium project needed to drill very straight, small-diameter holes to enable blasting to be carried out safely in close proximity to buildings and traffic. He chose to change to T35, the Sandvik drilling system for cautious blasting. As a result, hole straightness has been improved, ground vibration has been minimised, and tool costs have been almost halved.   

AB Schakt och Sprängning (‘Rock Excavation and Blasting Ltd’) was founded in the 1940s by Hilding Fredin. When Fredin passed away in 1974, his son Kjell-Arne took over the affairs of the company. Throughout this time, the company has carried out rock excavation work in Stockholm and the surrounding municipalities, specializing in cautious and very cautious blasting. Having run the company for nearly thirty years, Kjell-Arne has decided to hand over the reins to project engineer Magnus Thyberg in the spring of 2003. AB Schakt och Sprängning has been a subsidiary of Skanska since 1993.

Since the start-up of the Southern Link project (part of Stockholm’s long-awaited orbital motorway scheme) in June 2000, Kjell-Arne has been responsible for the drilling and blasting operations in stage SL 23. This involves lowering a busy thoroughfare called Nynäsvägen by approximately six metres, and constructing ramps to connect the road with two recently excavated tunnels that run east-west around the south of the city. AB Schakt och Sprängning is using four Atlas Copco ROC 542 drill rigs to help remove approximately 110 000 m3 of surface rock. The blasting work is scheduled for completion in January 2003.

Many types of drilling

“We are drilling benches between zero and eight metres in height,” Kjell-Arne explains. “Above eight metres, we do the benching in stages. The greatest bench height is 16 metres. The rock varies, consisting mostly of granite and gneiss, jointed toward the surface. We have to work very close to buildings the whole time, and try to disrupt the traffic as little as possible. Beneath one section, there is a water tunnel quite near the surface.”

Numerous holes, amounting to several hundred so far, have also to be drilled into the walls of the excavation at two-metre spacings for the installation of rock bolts. Moreover, environmental stipulations require watertight grout-curtains to be installed at the sides of and beneath the access ramps, which are located very close to the city’s Globe Arena (the world’s largest spherical building), and this requires a great number of holes to be drilled for grouting purposes.

“About20 per cent of the rock is crushed on site and re-used,” says Kjell-Arne, “and the rest is transported to various crushers in the area. We always try to tailor the rounds to suit the rock conditions. We use Dyno-Nobel Dynamit cartridges for the bottom charges and Kemix emulsion pipe-charges for the column charges, and fire the rounds using the Nonel® initiation system, of course. The burden is between 1.2 and 1.4 metres, and the spacing between 1.3 and 1.7 metres. The holes in the rounds are inclined between 5:1 and 6:1. For the contours, we use the smooth-blasting method, with the seam ‘lagging’. In the case of greater depths, the benches are taken down in steps in order to ensure maximum straightness.”

Precise measurement of vibration

Owing to the location of the work site, there are great restrictions on ground vibration. Nitro Consult, responsible for measuring vibration, has installed around 40 measuring points, the nearest of them just 8-10 metres from the edge of the bench. At one of the tunnels, the peak particle velocity limit was set at 18 mm/s; elsewhere at between 18 and 50 mm/s. Kjell-Arne recalls only a handful of occasions when the limit has been exceeded. He is able to read the measured values directly off his mobile phone after each blast.

New tools

“At the beginning of the project,” recalls Kjell-Arne, “we planned to use the rock-drilling tools available at the time. Regulations in Stockholm state that holes bigger than 51 mm in diameter may not be drilled on this type of job, in order to limit the dimensions of the explosives. In practice, this means using R32 equipment, but that gave big problems with hole deviation from time to time. We therefore had no option but to change to T38, for which 64 mm is the smallest possible bit diameter. The holes were straighter, but then we had problems charging the holes with the small-diameter cartridges, due to the risk of overlapping. At the time, Sandvik had tested a new tool system that seemed capable of solving the problem. The new equipment enabled 54 or 57-mm holes to be drilled using the same rod dimension as T38, and Sandvik promised it would give straight holes.”

What Sandvik technicians had done was to develop a new, 35 mm T-thread and apply it to a 39-mm rod. The advantage was that the outside diameter of the female end of a 39-mm MF rod could be reduced from 56 mm – as is the case for T38 – to a mere 48 mm with T35. This enables a 54-mm bit to be used with 39-mm rods, which have twice the rigidity of 32-mm rods.
Straight holes
“We tested the equipment and saw at once that it lived up to Sandvik’s promise,” says Kjell-Arne. “With T35 you can drill small-diameter holes straight, and this makes blasting safer, with less vibration, which is what this job is all about.”

Olof Birgersson of Skanska’s Gothenburg division, who operates of one of the ROC 542 crawler rigs working at the Globe Arena, confirms Kjell-Arne’s view: “T35 drills very straight and is very easy to uncouple,” he says. “It is an ideal solution for this type of drilling, and I shall certainly be recommending it to my colleagues back in Gothenburg.”

Long service life

Schakt-och-Sprängning rig operator Leif ‘Åland’ Eriksson, one of Olof’s colleagues on the ROC 542 rigs, says the service life of the T35 tool-system is 'very good', as he displays a shank adapter not yet fully worn out, even though it has clocked up 118 hours of drilling. Since the average penetration rate is 1.3 metres per minute, this indicates a service life of over 9000 metres.

The Sandvik CAPP drill-bits last for between 800 and 1500 metres. They are reground in accordance with Kjell-Arne’s philosophy: “It is much more economical to grind the bits lightly, but often,” he says. “That way you utilize the cemented-carbide optimally. In other words, you use it for actual drilling, rather than just grinding it away.” Consequently, all bits on site are ground, or “dressed”, rather, after every 20-25 metres of drilling.

A technical explanation for the long service life lies in the fact that the dimensions of the T35 tool system harmonise very well with COP 1238, COP 1838 and HL 1500 hydraulic rock drills, which are the models used most commonly for this type of drilling. The piston diameter in all of them approximates the diameter of T35 rods. This means that, provided the machine settings are made accurately, high efficiency can be achieved in the whole system. In other words, the bulk of the impact energy is utilized for drilling the rock, rather than for heating up and wearing out the rod joints!

Much lower costs

An ostensibly small initiative by Sandvik has therefore made life easier for all bench drillers working in densely built-up areas. “Even if we may not have followed up the exact service lives of all tools,” concludes Kjell-Arne Fredin, “I estimate that, by using the new T35 tool-system instead of R32, we have, by and large, cut our tool costs in half.”

Fig. 1. Olof Birgersson of Skanska’s Gothenburg division:“T35 is an ideal solution for this type of drilling”

Fig. 2. The T35 equipment enables 54 or 57-mm holes to be drilled using the same stiffer rod

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