Neutrino lab requires own special shape

A unique local experiment in pure science could prove to have down-to-earth applications for operators of deep mines around the world, according to industry insiders here.

For the past 32 months, miners at the Creighton No. 9 shaft of Inco (TSE) have been excavating one of the largest underground openings ever attempted at depth. The 100-ft. high by 65-ft. in diameter barrel-shaped cavity at the mine’s 6,800-ft. level will eventually house the Sudbury Neutrino Observatory (SNO), due for completion in spring, 1995.

The depth, size and shape of this vast underground cavern have posed a host of new challenges even for Inco’s veteran mines engineers, rock mechanics and hardrock miners.

The most unusual feature of the cavity, according to Phil Oliver, the dean of Inco’s rock mechanics department, is its shape: a beer-barrel configuration with a domed roof.

“This shape is very foreign to miners,” Oliver said. “While there may be deeper large underground openings elsewhere, most of them are crusher stations, and they’re rectangular, which is not a good shape, from a ground control point of view.”

The beer-barrel profile was adopted because it conformed to the precise design specifications of the neutrino-hunting scientists. (The cavern will eventually house a large spherical vat full of heavy water which will serve as the heart of the detector.) But it had benefits to the excavators, too, according to Oliver, because it reduced the volume of rock displacement and hoisting required to complete the project.

Oliver also believes that the circular shape, and especially the domed roof, will help attenuate the danger of rockbursts.

The site of the excavation was carefully selected to avoid active mining areas and geological anomalies. After rejecting two other possible locations, an area 150 ft. into Creighton’s hangingwall was chosen by Inco’s mining staff and SNO scientists.

Using an existing crusher station at the 7,000-ft. level as a model, Oliver simulated the worst-case scenario in terms of ground failure and its impact on the neutrino lab, which will house $300 million worth of heavy water on loan from Atomic Energy of Canada Ltd. He then devised a ground control scheme which should protect the observatory from even the strongest conceivable rockburst, earthquake or mining blast.

On-site excavation began in April, 1990, with the blasting of a 20-ft. bench at the top of the cavity, followed by a 12-ft. diameter vertical slot running from the top to the bottom of the eventual opening. The slot served as an orepass to a scooptram-serviced haulage drift on what will become the cavern floor.

The first round was also carefully drilled to shape the observatory’s domed ceiling. Three 8-ft. benches were then blasted, followed by two 20-ft. slices. By the end of this year, the excavation should be 90% complete, according to Inco officials.

Because of the observatory’s precise specifications, including the need for smooth walls to help guarantee an ultra-pure laboratory environment, each round was tailored to ensure there was absolutely no underbreakage, and the overbreakage was kept to a minimum. Despite the size of the opening and its concave shape, the worst overbreakage was no more than 24-36 inches. When the excavation is complete, the walls will be smoothed off to the uniformity required by SNO’s researchers.

The drilling pattern featured 295 two-and-an-eighth-inch holes, 8 ft. or 20-ft. deep, arranged in concentric circles. Oliver personally designed both the pattern and the blasthole loadings in an effort to minimize fly rock and to crack the walls of the host rock.

“You try to control ground failure at your convenience rather than letting it happen at its convenience,” Oliver explains with reference to his de-stressing attempts in cracking the walls. After minimal scaling, a series of cable bolts are driven 25-ft. into the walls in a 5×5-ft. pattern to create a lattice work of cracked, de-stressed ground, which is then anchored into the surrounding undisturbed rock.

Monitoring of the cavity walls to date has revealed that the stresses are heavier in the horizontal dimensions than in the vertical, according to Oliver, but little ground movement or stress buildup have been recorded so far.

Each blasthole is loaded with alternating layers of ANFO (an ammonium nitrate explosive) and a sand deck. The alternating pattern is employed “so that the rock breaks in a desired and predictable pattern, minimizing fly rock or side breakage while at the same time guaranteeing that there will be failure material in the walls,” Oliver explains.

Fly rock from the blasting is of more than passing concern on the SNO project because of a series of korbels (concrete piers), structural steel and an overhead crane which are already in place in the ceiling of the cavity. Excessive fly rock could damage these installations, which are a critical — and permanent — part of the observatory structure.

Oliver also designs the blasting sequence for each round with painstaking care. “All caps have to be cooking before the first shot fires,” smiles Oliver, who began his mining career as a longhole blaster. “The timing `scatter’ is critical, and the strength of the rock is time dependent.”