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Sarasota News Leader November 15, 2013 Page 25 pipe under the bayou. An initial investigation indicated a lack of knowledge of the underground geology was the problem. But Staheli offered a different interpretation Tuesday. If the overcut was insufficient to allow proper lubrication, more pressure ("jacking force") would be necessary to maintain forward movement of the cutter head. That, in turn, would increase the friction, setting up a posiMicrotunneling is analogous to using a drill tive feedback loop. press on its side. Pressure is applied to the bit by a handle, and oil is used to lubricate the Staheli showed those at the meeting a graph bit's cutting end. In a microtunneling rig, the of the jacking forces recorded during the inibit is replaced with a cutting head, and the tial failed attempt. As the tunnel lengthened, pressure comes from hydraulic jacks, which the jacking forces increased to nearly 400 produce "jacking force." The oil is replaced metric tons of pressure. with a lubricating fluid. "The forces on the head were tremendous," The difference between a drill press and a she said. microtunnel is the hole is a bit wider than the cutter in microtunneling. This difference — called an overcut — allows the lubricating fluid to circulate. This is where Staheli speculates the failure occurred. "The overcut is necessary to limit friction," she explained. Other projects used vastly less pressure, somewhere in the region of 200 metric tons, she noted. The additional force caused the pipe to break in the tunnel and, eventually, the release of material from the tunnel into the bayou. Staheli speculated the material An aerial map shows the location of Luke Wood Park in Sarasota. Image from Google Maps