The tunnel boring method is again another technology to evolve in the industry. Tunnel boring is a hybrid of pipe jacking due to the fact that workers must be inside the pipe during operation. While one operator sits just behind the boring head, another operator thrusts the casing from a launching pit utilizing a boring machine. A spoils tube, located at the invert of the casing pipe installation carries the excavated material back to the launching pit.
The major advantage of this technology is the added accuracy of the bore. The TBM operator will monitor and control the line and grade of the bore continuously from his forward control station just behind the cutting head. Monitoring is enbled by the use of a pipe laser mounted to a fixed "hard point" back in the launch pit. Control is maintained through use of steering control cylinders mounted in the forward section of the TBM. The operator notes the position of the laser on a nearby target and "steers" the TBM back on line and grade as any deviations from the proposed path are noted.
• Overhead Power Lines – Generally tunel boring requires the use of an excavator in order to dig the pit and extract spoils from the pit.
• Soil Conditions – Ground conditions are a very important aspect when choosing a cutting head. In solid rock conditions for example, Aaron Enterprises would utilize a disk cutter head.
• Lubrication – Often times soil conditions require the use of a bentonite mud mixture in order to reduce the skin friction between the casing pipe and the material that surrounds it.
• Grade Control – This aspect becomes critical when tunneling around existing structures. Grade is also crucial when installing certain utilities such as gravity sewer systems.
• Ground Water Conditions – After the excavation of a launching pit, crews often find themselves below the water table. A well point can be installed in order to control the influx of water.
• Visible Restraints of Setup Area – Different job site constraints may require special attention during mobilization and pit excavation. Restraints at the job location are an important consideration when using the auger boring method.