High Centered

by kfrego

Gates and Fox bought two new Dosco Roadheader mining machines to excavate the two miles of linear collider tunnel at Stanford University. The shallow depths and nearby structures of the university required mechanical excavation of the tunnels. The new Dosco’s were manufactured in England with European electrical systems and required large transformers to use our US electricity. Each roadheader weighed about seventy thousand pounds, was eight feet wide, and close to thirty feet long. They looked like a big metal box with caterpillar tracks underneath and a massive straight boom sticking out the front with a rotating cutterhead on the end of it. The nearly two hundred horsepower electric motor to drive the cutterhead was built into the center of the boom. An open operators cab was built into one side of the machine. A flat apron on the front of the machine gathered the excavated muck onto a conveyor belt. The conveyor belt transferred the muck through the center of the machine and into a mucker bucket at the rear. The rubber tired mucker hauled the muck outside. The machines were installed with hydraulic jacks that allowed the tracks to be lifted for repair and inspection. We learned the jacks were very important as the work progressed.

We used shotcrete to line the tunnels. Shotcrete is a cement mix with sand and peastone which is pumped through a hose system with a nozzle on the end. We batched our shotcrete with hot water at Stanford. The nozzle blows air which propels the shotcrete mix against the ground in layers up to three inches thick. A chemical accelerator is used at the nozzle to make the shotcrete set up fast. We used type 5 cement at Stanford to reduce the salts in the rock from deteriorating the shotcrete. Water was used to wash down the rock before the shotcrete was applied.

The further the tunnels advanced, the water we used saturated the weak rock in the floor of the tunnel behind us. Every low spot or puddle eventually turned into mud. Our rubber tired equipment negotiated the mud with no problem, but cut deep ruts. The seventy thousand pound roadheaders with only a foot or so of ground clearance would get high centered and not be able to move. You’re in a tunnel and there is only one way out. No matter how much we graded or plated the floor of the tunnel with clean crushed rock, we were frequently buried in mud with the roadheader high centered. We used the on board jacks to lift the machine and install blocking beneath the tracks. When we put the machine down the blocking would disappear into the mud. We spent many an hour fighting the mud.

It was pretty bad. Our roadheader could mine rock forever…………….. but we couldn’t back it up a couple hundred feet.