By Stew Slater
If you’ve had the nagging feeling there hasn’t been as much blasting over the past few weeks at St. Marys Cement’s Thomas Street Quarry, you’re not just imagining things. According to Quarry Supervisor Tony Black, in the last month, there have been just two blasts — something which, typically, is done once per week to supply sufficient limestone to keep up with Cement Plant demand.
That’s because the company has been taking a month-long test drive of a “surface miner” manufactured by German company Wirtgen. Instead of extending the aerial footprint of the quarry by blasting rock off its walls, the company has instead been extending the depth of the quarry by using the machine to cut away layers of rock from its floor, eight inches at a time.
“The majority of the material (the Cement Plant has) used over the past 3-4 weeks has been from the floor of the quarry,” Black explained, adding the machine has extracted approximately 40,000 tons of rock over that time. It’s rated to produce 600 tons per hour.
The Cement Plant typically requires between 800,000 and one million tons of extracted limestone per year. Plus, some limestone from the quarry is used by St. Marys Cement’s Canadian Building Materials (CBM) division.
“It’s doing much better than we thought it would do, production-wise but also in terms of noise, and in terms of dust levels,” reported Production Manager Robin Manzer, during an up-close visit to see the machine on Friday, Nov. 16 — the final day of the trial run.
According to Black, when quarry operators first put the surface miner to the test, things didn’t look good. First, they tried using it on the upper floor — beyond the current quarry extraction site, on untouched surface limestone. But that rock — which has a different chemical make-up from the floor rock — was too hard for the machine.
Then, they tried a lower floor area that had been driven over and exposed to the elements for decades. It proved to be too brittle, and broke almost instantly into particles too small for the Cement Plant to utilize efficiently.
But when that brittle layer was cleaned away, the newly-uncovered rock was perfect for the Wirtgen’s carbide extraction cutters. With some adjustments to the number of cutters in the machine’s hydraulically-driven rotating drums, Black and crew created a size of end product that — unlike the rock extracted by drilling and blasting the quarry wall — needs only to go through a “secondary crusher” before being loaded on the Thames-crossing conveyor en route to the Plant.
By the time the trial run was complete, it had broken away four eight-inch layers on one section of the quarry, three eight-inch layers in another section, and a single layer in another location. Rows of broken rock — similar in appearance to the windrows left by hay-cutting equipment in a farmer’s field — lay ready to be picked up and hauled to the crusher.
As Manzer noted, if the surface miner makes a return to the quarry on a longer-term basis (which depends, largely, on whether St. Marys Cement decides it can justify the cost of purchasing the equipment), nearby residents will definitely notice a decrease in the amount of dust and noise. But that doesn’t mean it’s without potential environmental impacts. After all, every eight inches removed from the quarry floor is eight inches closer to the groundwater aquifer.
That’s why a Permit to Take Water was secured from the provincial Environment Ministry. And that’s why water in the quarry’s settling ponds and well — along with a number of privately-owned wells in the vicinity — is monitored regularly. During the recent trial run, Black notes, no changes were noted during any of that testing.