Driving a 70 Ton Dump Truck
Each bucket Dwight dumps into the truck holds 12 tons of granite.
I wanted to drive a big truck, so I drove a tractor-trailer. Once I had tasted the big rigs, I wanted bigger. I rode in an 18-ton tug while towing a 100-ton airplane – better, but I couldn’t drive it. How about driving a 70-ton mining dump truck with a gross loaded weight of 250,000 pounds? ALL RIGHT!
Martin Marietta Materials of Raleigh, North Carolina invited me to spend a day in a granite surface mine. The mine opened in 1981 and produces one million tons of crushed stone a year. After a facility tour, I joined Dan Dohm in the cockpit of a Terex TR70 Rigid Haul Truck. First, I had to climb a seven-step staircase. Once seated, the driver’s view is from about 13 feet above the ground. The engine is a 24-litre Detroit Diesel V12 with 760 hp and 2,400 lb-ft of torque at 1,200 rpm. The transmission has six forward and two reverse gears and is fully automatic. The fuel tank holds 938 litres of diesel, with an average day consuming about half the tank. The vehicle is 32’ 6“ long, 16′ 2“ wide and 15′ high.
Mining is basic in concept. Blow out a wall to free rock, load into a truck, haul up the pit road to the crusher hopper, dump and repeat about 100 times a day. Driving down into the pit, my first impression was how bumpy the ride was and at 40 kph, how quick we were moving. Because the road descends clockwise into the mine, it’s safest to drive on the left. The driver sits on the left, and the truck’s 16-foot-wide girth makes it hard to judge where the right wheels are. If you drive on the right and hang the right wheel over too far, you can run over rocks. This can tear a $5,800 tire. Driving on the left means the right tires are in the middle of the road, which is safest, and you are directly over the left tires so you can see what you are driving over.
At the bottom, Dwayne Bell was waiting with his Cat 988G Wheel Loader, the bucket high and ready to dump 12 tons of rock into the truck bed. Over the next hour, Dan explained the steps as he drove. Then it was my turn. My first drive was the easier climb up the road. First I put on a four-point seat belt. Dwight sounded the horn, signaling the loading was complete. I put the transmission in D, released the air parking brake and pressed the gas pedal. While the steering is very light, the gas pedal required some pressure.
Since the road was fairly steep and the truck just under 250,000 pounds, we raced up at a roaring 15 kph. At this weight and speed, the ride is smooth. At the top, I pulled into a corner, shifted into reverse and backed up to the crusher. What surprised me most was how easy it was to back up this “building on wheels.“ With the large, well-placed mirrors, I was comfortably backing the truck in minutes. To dump, I had to wait for the green traffic-light, race the engine in neutral and hold the dump lever. The sound of 140,000 pounds of granite boulders rolling out is deafening, and the process creates a lot of motion in the truck. I touched the dump lever forward, put it in drive, released the parking brake and we were off.
Driving down the hill was scarier. Rather than 10 kph, Dan would hit 40. Also, braking tends to be done with the retarder, which is a lever by the driver’s right hand. Since this is an air system, it takes time to apply. After three runs, I was feeling confident, but they had wet the road to reduce dust. Dan warned me to be careful to not lock the rear wheels. As I rounded the last corner, I was going a bit too fast so I applied the retarder. Forgetting it takes time for the pneumatic system to work, I pushed harder and locked the back wheels. The rear wheels slid out for maybe 10 feet. You haven’t lived until you’ve slid out a 110,000-pound truck! I had made my first rookie mistake and got razzed royally on the radio about it. Dwight kindly offered me a towel if I needed to clean up anything. I climbed back on the horse and did another run – much better.
The day ended with a bang as I watched them blow a wall releasing 10,000 tons of new rock to process. I was expecting a Hollywood scene and was surprised how quiet the explosion was.
I wanted to drive something big and I sure got to do that. I felt like the new rookie on the crew, but a very welcome one. My thanks to Dan for teaching me and Dwight for making me feel welcome with his jabs and barbs. It was sure fun to be part of the crew for a day.
The Terex TR70 is one vehicle my Smart car can out-accelerate!
Published in Dec 31, 2006 Toronto Sun Autonet.ca Sunday Edition