The drilling division of Frontier Kemper Constructors Inc. (FKCI) recently completed the largest blind shaft ever drilled in North America. The DHI-240 rig (Big Blue) was built and tested by the company during 2008 at FKCI’s headquarters in Evansville, Ind. The drill pipe, collars, heads, swivel and traveling block/elevator were built by WIRTH GmbH of Erkelenz, Germany, a leading manufacturer of specialty drilling and tunneling equipment.

Big Blue is capable of drilling holes up to 20 feet in diameter, and is said to be the safest and most efficient machine ever built for this type of shaft construction. Large-diameter (big hole) drilling is a unique method of drilling shafts for construction and mining. The process is a hybrid of rotary drilling and tunnel boring. Similar to conventional rigs, a big-hole rig consists of a drawworks and a rotary table. It uses drill collar weight and liquid reverse circulation for removal of the cuttings.

A small number of contractors have been drilling shafts using this technique for many years on mining and civil projects. Frontier Kemper, a leader in underground construction, shaft sinking, tunneling and raise boring, is significantly moving the technology forward with this new machine. Unlike other big-hole rigs, Big Blue was designed specifically to drill the largest shafts. Until now, a 20-foot blind shaft only could be effectively sunk conventionally using a large crew and the drill-and-blast method.

The first contract for the DHI-240 was to drill a shaft 20 feet in diameter to a depth of 348 feet deep in southern Indiana for a new coal-mining operation, Black Panther Mining LLC. The shaft was planned and designed as a conventional sink, but Black Panther accepted the alternate proposal of a blind-bore operation. The 20-foot bottom-hole assembly was manufactured by WIRTH and DSI/American Commercial Inc. of Greenville, S.C. U.S.-based support for the equipment was provided by Ron Kurta and Bill Brown of ACI, who spent a significant amount of their time in attendance on the job.

The FKCI crew, led by Charles Ernst, FKCI’s blind drill superintendent, built a concrete foundation pad and a collar to a depth of 40 feet to support the unconsolidated ground and provide a launch point for the drill. The rig had very few shakedown problems, but some difficulties were experienced with sticky shale and clay. The correct chemical treatment of the circulation fluid, with the help of Brock Yordy of Halliburton/Baroid IDP, resolved those issues. The drill cut and removed 11.6 cubic yards of material for every vertical foot of advance. The advance rate was, at times, one foot per hour or better.

The rig is equipped with a drawworks powered by a 300-HP AC variable-frequency motor, and a hydraulically driven rotary table capable of in excess of 300,000 foot-pounds of torque. The hydraulic power is created by two 400-HP units. Compressed air for reverse circulation is provided by twin IR high-pressure, electrically driven compressors. The cutter head carries 32 carbide multi-row disk cutters specifically designed for a submerged, high-pressure environment. The nearly 200-ton bottom-hole assembly consists of heavy drill weights and two non-rotating stabilizers to prevent any drift of the cutter head.

This project has shown that blind drilling shafts in this size range is practical, safe and cost-competitive to conventional methods for construction of some of the largest shafts used by the mining industry.