Hoover Dam Stories: The Jumbo Rig

My father worked his entire life in the trucking industry and I eventually did, too, working twelve years in companies that specialized in land, ocean container and air freight.  So I have been around trucks most of my life and frankly, I like trucks.  Stick with me on this one.     

Before the actual construction of the Hoover Dam could begin, the Colorado river had to be diverted around the site. This required the drilling of four tunnels, each 56 foot in diameter and approximately 3/4 of a mile long through solid rock.  Using pneumatic drills, a few dozen holes were drilled into the wall and stuffed with dynamite (a lot of dynamite. It's estimated that it took a ton of dynamite for every fourteen foot of tunnel).  After the blast, all of the rock and debris had to be shoveled out and taken upstream and the process started over.  In the beginning, it took countless men, drilling, dynamiting and shoveling to make an inch of progress.  

Using ladders to drill the holes at the highest points wasn't the greatest idea. The pneumatic drills that were used were big and bulky and weighed about forty pounds.  The vibration that was generated from one of those monsters alone was enough to knock a man off a ladder.

So how do you drill thirty holes at different heights, pack them with dynamite and move all of the valuable equipment and workers to a safe distance before a blast?  This was a question that the engineers working on the project struggled with.

Then in steps the Jumbo Rig. 

 The Jumbo Rig, or Jumbo Truck was the brainchild of Bernard "Woody" Williams.  Why not create a scaffolding system on wheels?  Why not.  Taking one of the 1916 International flatbeds that he'd seen around the site, Williams had planks of timber mounted on the truck's bed that would accommodate up to thirty drillers standing at three levels to drill simultaneously.  At first this was slow, too, as the vibration caused by all of those drills being used at once required that the wooden struts be hammered back together after every round of drilling. But the idea was sound, and steel jumbos soon followed.  These jumbos had two platforms for drillers to stand on, the third level of men working from the ground.  It also had horizontal bars at the three levels, where the pneumatic drills hung by swivels.  The process then became: Back the truck up to the solid rock wall, drill the holes, pack them with dynamite, tie them together and then pull the entire truck a safe distance away while the blowman detonated.  Then clear away the debris and do it again.  

Eventually, there were eight Jumbo rigs used in the construction of the tunnels.  Did they save time? Of course, in fact, the tunnel crews finished a year ahead of schedule.