Guided Boring Methods: Pilot Tube
By using a pilot tube guided boring method, engineers can accurately place new pipelines with minimal disturbance to the surrounding soil.
The pilot tube guided boring method is often used for the installation of cables, pipelines, or conduits in municipalities today. The boring equipment uses a guidance system that allows for greater accuracy than other tunneling options. (Read Guided Boring Machine Maintenance Checklist.)
Knowing the equipment necessary, what evaluations are needed, and the benefits and limitations can help project planners understand if the method is right for their needs. (Read The Planning Process: How to Prepare for a Trenchless Project.)
What Equipment Is Required for the Pilot Tube Guided Boring Method?
The guided boring method uses a variety of equipment to complete boring a pilot tube. The size and exact combination of equipment depend on the nature of the project. However, each guided boring method requires a jacking frame.
The frame houses cylinders to thrust the pipe in place while helping to ensure the pipe moves in the correct direction. The frame installs in the entrance pit of the project along with the guided boring machine (GBM).
Additional to the jacking frame, there is a power pack to run the drill. These power packs are often hydraulically powered using the hydraulic fluid to help cool the diesel engine. The packs which use an electric motor do not use liquid to cool the motor. The packs vary in size from 75 horsepower up to 200 horsepower to control the jack.
A guidance system is necessary to ensure that the cutting goes as planned. The operator controls the angle of the cutting head with the guidance system. The system consists of a theodolite and has a remote focus with an illuminated LED target and camera.
The computer allows the operator to control the cutting head while recording the movements every few seconds.
All guided boring method pilot tubes required some form of cutting or reaming head. Some projects, like the one completed in Minneapolis in 2016, use 8-foot by 16-foot shafts to create the pilot tube.
The jacking pipe is formed by vitrified clay, which housed the cutter head and the reaming head. (Read The History of Vitrified Clay Pipe in Trenchless Installation.)
The reaming head followed the pilot tube to make the hole more significant for the final pipeline to slide in place.
What Field Evaluations are Necessary?
Before beginning guided boring, workers must collect soil samples to determine what type of soil is in the area. Traditional auger boring methods are not conducive for crossing mixed conditions. Guided boring methods can adjust to different soil types if the operator knows what to expect.
Soil conditions for guided boring the dirt should be easily displaceable. It works well for wit sand, caliche, or clay. The cutter head is capable of grinding through small boulders, but not more significant, denser rock. It is not suitable for use below the groundwater table.
The same soil investigations used for horizontal directional drilling (HDD) projects should be conducted for pilot tube guided boring methods. These tests should include samples from several positions along the boring track.
In Edmonton Canada, engineers check six drilling indices to ensure they can identify the soil transitions throughout the drive length.
What type of Pipeline Uses the Pilot Tube Method?
Most subterranean pipeline installations can use the pilot tube method. The method works for lengths up to around 500 feet (or 152.4 meters), and pipes can have diameters of up to approximately 60 inches. While it works for most utilities, it is not the choice for installing new sanitary and storm sewers. It is the choice for water, gas, electricity, and telecommunication lines.
Guided boring is an idea for lines that need to lay relatively close to the surface. Pilot tube methods work at depths around four feet or more. While other ways, such as HDD, require greater depth.
As there is little to no fracking involved in pilot tube guided boring, there is minimal disruption to the surrounding structures.
What Are the Benefits and Limitations of the Guided Boring Method?
As with all drilling methods, there are benefits and risks that engineers must weigh when deciding on how to proceed with the project. The following benefits and risks are just a few that should be considered during the planning phase of any project where guided boring may work.
There are many benefits of using pilot tube guided boring methods. The first of which is versatility. The equipment works with a wide variety of conditions. If the operator knows what type of soil is along the digging track, they can adjust the cutting speed and direction as needed.
An additional benefit is an accuracy provided by the guided boring method. With the onboard guiding system, operators can follow the cutting head, with an acceptable margin or error. The cutting head is continuously monitored for accuracy.
Finally, the environmental impact of using guided boring methods is less than traditional drilling methods. There is little to no damage to the surrounding soil. There is a smaller footprint with little in the way of greenhouse gases being admitted.
Pilot tube guided boring methods are an optimal choice for installing underground utility lines. However, even the best options have some limitations. One such restriction is while it works on multiple types of soil, it is not suitable for the ground with copious amounts of gravel, rock, or medium to large boulders, which is why a soil survey is essential before the start of the project.
Rocky ground conditions can cause the pilot tube to deflect off course and affect the guided boring’s overall accuracy. The soil should be above the area’s water table.
Additionally, there is a limit to how far operators can go with their guided boring systems. These devices are not designed for boring more than 600 to 700 linear feet. It is better to keep the drive below 400 feet for the highest accuracy.
What We've Learned
Pilot tube guided boring methods help to ensure the bore is stable and precise. Municipalities can use this technique to lay new underground utilities relatively close to the surface without disturbing the surrounding soil and ecosystem.
Despite their few limitations, with proper planning and soil investigation, this method may be the best choice for new electric, water, and telecommunication lines in an area.