This project profile discusses how one company drills small-diamater holes in congested areas.

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 minimized and tool costs have been almost halved.

AB Schakt och Sprängning (Rock Excavation and Blasting Ltd.) was founded in the 1940s. The company conducts rock excavation work in Stockholm and the surrounding municipalities, specializing in cautious and very cautious blasting.

Since the start-up of the Southern Link project (part of Stockholm's long-awaited orbital motorway project) in June 2000, Kjell-Arne Fredin has been responsible for the drilling and blasting operations. This involves lowering a busy thoroughfare by approximately 20 feet, 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 365,000 square feet of surface rock.

Many Types of Drilling

"We are drilling benches between zero and 25 feet in height,” company president Fredin explains. “Above 25 feet, we do the benching in stages. The greatest bench height is 53 feet. 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, also have to be drilled into the walls of the excavation at 6-foot 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.

“About 20 percent of the rock is crushed on site and re-used,” says Fredin. “The rest is transported to various crushers in the area. We always try to tailor the rounds to suit the rock conditions. We use dynamite cartridges for the bottom charges and emulsion pipe-charges for the column charges. The burden is between 4.0 feet and 4.6 feet, and the spacing between 4.3 feet and 5.6 feet. 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

Owing to the location of the work site, there are great restrictions on ground vibration. Nitro Consult, responsible for measuring vibration, installed around 40 measuring points, the nearest of them just 30 feet from the edge of the bench. At one of the tunnels, the peak particle velocity limit was set at 0.72 inches per second - elsewhere at between 0.72 inches and 2 inches per second. Fredin recalls only a handful of occasions when the limit was exceeded. He is able to read the measured values directly off his mobile phone after each blast.

“At the beginning of the project,” Fredin recalls, “we planned to use the rock-drilling tools available at the time. Regulations in Stockholm state that holes bigger than 2 inches 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 2.5 inches 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 2.2-inch or 2.3-inch 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, 1.4-inch T-thread and apply it to a 1.56-inch rod. The advantage was that the outside diameter of the female end of a 1.56-inch rod could be reduced from 2.24 inches - as is the case for T38 - to a mere 1.92 inches with T35. This enables a 2.16-inch bit to be used with 1.56-inch rods, which have twice the rigidity of 1.28-inch rods.

Straight Holes

“We tested the equipment and saw at once that it lived up to Sandvik's promise,” says Fredin. “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.”

A Schakt-och-Sprängning rig operator says the service life of the T35 tool-system is “very good,” as he displayed a shank adapter not yet fully worn out, even though it has clocked up 118 hours of drilling. Since the average penetration rate is more than four feet per minute, this indicates a service life of 30,000 feet.

The Sandvik CAPP drill-bits are said to last between 2,600 feet and 5,000 feet. They are reground in accordance with Fredin'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,” after every 75 feet or so of drilling.

A technical explanation for the long service life lies in the fact that the dimensions of the T35 tool system harmonize 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. ND