Trenchless tunneling methods have literally changed the shape of laying and maintaining underground piping and utilities over the last 50 years. From an innovative fringe idea, it has become a specialized sector of the construction environment. As the technology has progressed over the years, many types of new tunneling methods have been developed. Each method has its own advantages and disadvantages and is suited for some applications more than others.
With this wide range of tunneling methods available to choose from, how do engineers assess which is the best method for their project?
Horizontal Auger Boring
A horizontal auger boring machine is like a giant screw that rotates as it moves forward through the soil. Displaced soil moves along the spiral edges of the auger and comes out the rear of the machine. Auger boring usually happens in stages:
- A segment of piping is pushed into the soil.
- The auger bore removes the soil from within the new pipe segment.
- Soil is removed from the tunnel behind the auger machine as it moves forward.
- Then the process begins again with the next new segment. Horizontal auger boring is ideal when pipes are being laid in a straight horizontal line and the soil is relatively soft.
Pneumatic Pipe Ramming
Pneumatic pipe ramming uses the power of compressed air to drive piping or pipe casings into the ground. The process uses successive blows of a rammer to strike the rear of the pipe. A soil cutting shoe is attached to the leading edge of the pipe casing with a sharp edge to penetrate the soil. Sometimes lubricant like bentonite is also used to facilitate the cutting of the soil and the sliding of the pipe casings behind the cutting edge.
The rear end of the pipe casing, which takes the force of the rammer blows, is protected by an adapter. Soil is removed by auger boring, water jetting or compressed air, depending on the size of the pipe bore. Pneumatic pipe ramming is suitable for pipes being laid in a straight line, in shallow ground. The ramming process can also be used with other tunneling methods in order to loosen a pipe that is stuck. (For more on pipe ramming, see The Process of Pipe Ramming.)
As the name implies, guided boring is used to control the path of a pipe through the ground to a high degree of accuracy. Specialized laser guiding equipment guides the drill along the predetermined path to an accuracy of within one inch. Guided boring is a two-step operation. First, a pilot tunnel is drilled along the precise path required. The direction of the drill is controlled using slanted or asymmetrical drill heads or movable fluid jets in the drilling head.
Once the pilot bore is complete, a back reamer enlarges the diameter of the hole to accommodate the piping. Guided boring is an expensive option due to the complexity of the laser guiding equipment and skilled personnel required to use it. But, when accuracy along a chosen path is essential, guided boring can accomplish what other methods can’t.
Down the Hole Hammer
Down the hole hammers are designed to solve problems of hard rock encountered during trenchless tunneling. Sometimes a horizontal directional drilling machine encounters a section of rock that it cannot penetrate. Rather than damage drilling heads by excessive force and wear, construction crews use a down the hole hammer to solve the problem. This equipment is like a jackhammer often seen on the side of the road as construction crews break through road surfaces or concrete.
It is pneumatically powered to deliver hammer blows, driving the hammer into the rock. Because it is attached to the drill string, it also uses the rotational force of the drill to add to the force. Compressed air blows the rock chips and dust away from the cutting surface. Down the hole hammers are used along with other tunneling methods to break up hard rock that is preventing the job from being completed.
Slip boring is a process that uses a pipe casing to make the initial penetration through the earth. Product pipe segments are welded to the rear end of the pipe casing and pulled or pushed into the bore as it progresses. The advantage of this technique is that the product casing takes up most of the force associated with creating the bore, thus protecting the product pipes from excessive stress.
Once the pipe casing emerges at the exit pit, it is cut off from the product pipe, and the job is complete. This technique is helpful when engineers are concerned about the integrity of the product pipe due to the forces of pipe ramming or other trenchless methods.
Surface casing or pipe casings can be used with a variety of trenchless methods. The idea behind this technique is to use the tunneling process to install a continuous casing. Once the casing is in position, the product pipe is inserted inside the casing. Using surface casings is expensive because of the material costs, but the resultant installation offers a lot of protection for product pipes. They are not exposed to the stresses of the tunneling itself and they are also protected in the future against excavations that may damage the pipes.
Tunnel Boring/Pipe Jacking
Tunnel boring and pipe jacking use hydraulic jacks to push segments of piping into a bore, while a tunnel boring machine excavates the soil for removal from the entry pit. The tunnel boring machine has a combination of cutters to excavate the soil as the pipe progresses through the bore.
This technique can be used for any type of ground conditions and is especially useful for pipes being laid in a straight line through rocky and complex soil conditions. Very long pipe lengths and large diameter pipes can be installed using tunnel boring/pipe jacking. (For more on pipe jacking, see Everything You Need to Know About Pipe Jacking.)
Trenchless construction with its various tunneling methods is changing the laying and maintaining of underground pipes and utilities for the better. These methods offer great advantages over traditional excavation. Knowing the difference between tunneling methods and where best to use each one could make the difference between a successful trenchless project or unnecessary delays and cost overruns.
Before embarking on your trenchless project, investigate the tunneling method options available to you and choose the one best suited to your application.