Having watched the drilling machines operate on the Vaughan Metropolitan Centre site, when I compare my observations with the wreckage photos at York University, I see a number of differences, so I wonder whether there are different operating philosophies at the two sites.
First off, let me describe a drilling machine to the best of my ability. Two large hydraulic pistons are mounted between the machine's main body and the drilling rig post. These pistons are adjusted so that the drill is plumb. The rig post has two functions: it holds a set of pulleys at the top that run a cable down to the drill shaft, and it holds a hydraulic-powered drilling motor that can be positioned anywhere along the height of the rig post. The drill shaft is a hollow hexagonal-cross-sectioned metal tube that telescopes into two sections. Various styles of drill heads and buckets can be attached to the shaft. The holes they're drilling are lined with metal tubes that I estimate have a two-foot diameter. The drill motor has a cylindrical sleeve that is used to force the tube into the earth. Simply stated, they bore down a few feet, then they push the tube down while turning the tube, and repeat the process till they get to the required depth.
At Vaughan, I have never seen the drill motor hoisted more than 20 feet up the 60-foot post. (These measurements are estimates.) The wreckage photo shows the drill motor is hoisted to the very top of the post. This position means that the machine's centre of gravity was the highest from the ground that it could possibly be.
At Vaughan, they insert 15 feet of tubing at a time. Therefore, at any point in time, there is never more than, maybe, 17 feet of tubing protruding from the ground. The wreckage photo shows an extremely long tube that fell with the machine, about 50 feet long. This suggests that at York U they pre-assembled the tube on the ground before attempting to bore it into the ground. Such a large tube will also raise the centre of gravity and create lots of wind resistance. (Was it windy that day?)
The wreckage photo shows the left hydraulic piston badly bent. The question I have is whether that bend occurred before or after the fall. The forces from the post hitting the excavator and the ground could have caused that bend, so we cannot rule that out. But, it is also possible that torque forces from the drill motor exceeded design because, perhaps, the drill got seized on a hard rock. The drill motor can run both clockwise, to drill, and counter-clockwise, to loosen the drill shaft for extraction. A clockwise drilling motion, suddenly seizing, would cause the whole machine to rotate counter-clockwise and could cause the hydraulic piston to bend the way it shows in the photo. The counter-clockwise motion would also move the whole machine's center of gravity far enough so that the machine tipped.
