Thrust boring is the trenchless method of boring a pilot hole through a short underground distance. While the process may be merely driving a closed-ended pipe through the earth, workers often employ cutting heads followed by a pipe sleeve. The sleeve works to stabilize the hole, which makes it ideal for high-risk borehole projects.
Thrust Boring Procedure
With thrust boring, the first step is to dig out two pits: one for the thrust machinery and the other as a receiving pit. The thrust pit must be large enough to accommodate the mechanism and workers safely. For most equipment, this means the hole will be approximately 10 meters long with a width of 2 meters. Although this may vary depending on the model of machinery and the length of pipe sleeves. If the area is subject to excess water, the pit must accommodate a dewatering pump to ensure it remains water free. (Be cautious when boring large holes. Read How to Avoid Tunnel Collapse When Boring Big.)
Workers install the thrust boring machine in the pit with cradle guilds installed in front of the device to ensure a correct level and line. At the same time, workers also establish a level control point. A surveyor monitors these points before, during and after bore completion. These points help to ensure level accuracy during work.
The thrust boring machine uses a cutting head to bore the hole while a pipe sleeve jacks into place. Additional pipe sleeves are thrust into the position as needed to complete the installation. Workers weld the multiple sleeves together.
The driving operation continues until the cutting head appears in the receiving pit. At this point, workers remove the auger and cutting head along with any residual spoils. After removing the boring machine, workers install the conduits or carrier pipes through the casing.
Thrust Boring Equipment
Thrust boring equipment has several different components. Each component is determined by the project scope and the geological landscape.
The boring machine is the apparatus that powers the auger and cutting head while moving the pipe casing into place. The power and size of the device are dependent on the project needs. The length of run, ground conditions and the outer diameter of the casing all affect what type of device is necessary. (For more, see How Tunnel Boring Machines Work.)
Steering Heads, Receivers and Transmitters
The steering head is part of the thrust boring guidance system. It is a series of conical-shaped heads that work with a receiver and transmitter to control the drill rod paths. The head design is so that they offer the best control in almost any soil.
The digital transmitter is located on the housing in front of the drill rod. This device sends out digital and radio signals simultaneously. The receiver translates these signals for the operator to use to adjust the steering head as needed.
The type of soil determines the cutting head necessary for the thrust boring project.
Heavy duty rock cutting heads are V-shaped to help self-center the bore. Workers use these heads to cut through solid stones with a medium to hard formation. For hard rock formations, heavy-duty cutting heads without wing cutters are the best choice.
The design of dirt cutting heads allows for boring through any soil that does not contain rock. However, it is best to use these cutting heads for relatively small-bore holes of 12” or less in diameter. All dirt cutters have a lead screw head and may have wings, which are optional.
Combination cutting heads work for both soil and rock conditions. The head itself is a combination of the two cutter types, with wing cutters standard on most tools. While this can be used to cut some rocks, it is not designed to bore through medium to hard rock formations. It is only usable for soft rock formations and clay soil conditions.
Thrust Boring Applications
Thrust boring is the best method to use for situations which involve crossing short distances underneath structures that are difficult to disturb. Most frequently, cities use thrust boring for installing underground utilities such as electric, fiber optic, cable and gas lines.
Companies typically choose thrust boring when entry and exit site footprints do not allow for an angled bore, as is typical with horizontal directional drilling. Usually this method is used to dig under rivers, railways and roads. It is also useful in areas with heavy congestion as well as environmentally sensitive areas.
Thrust boring is perfect for high-risk boring projects as the method provides stabilization not necessarily found in other drilling methods. With a pipe sleeve moving into place as the cutting tool clears the soil, it helps to keep the newly formed hole from collapsing in on itself. It is a useful means of laying new underground utilities in urbanized areas.