In most cases, trenchless construction workers use a cased boring technique to run the line simultaneously while drilling the borehole. However, there is an uncased application used under certain conditions.
Knowing which method to use for each circumstance requires a closer look at each technique.
What is Cased Boring?
Cased boring is the traditional auger boring version. In this approach, a casing attaches to the drill machine. The cutting tool sits either just ahead, flush or just inside the casing itself, the job dictates placement. A hydraulic jack located at the back of the machine helps to push the casing in place as the bit drills the borehole. The auger removes spoil as it transports it back through the casing to the launch pit.
Most projects that use the process of boring, use cased boring as their default option. While the hole size varies; typically, the pipes run between 8-36 inches in diameter. Workers may use smaller or larger diameter pipe depending on the job requirements.
While many different trenchless applications use cased boring, its use in burying utility lines (read Effectively Using Utility Detecting Devices) or installing storm culverts is most common. In most cases, these lines or channels need to go under rail and roadway crossings, airport runways, or through pipeline right of ways.
What are Different Types of Cased Auger Boring Equipment?
The Cradle System
The cradle system, often used for gas distribution lines, requires assembling of the drill and casing before placing them in the launch pit. This equipment requires a larger launch pit and crane equipment to put the cutting tool into the hole when assembled. However, it is simpler to operate once assembled. No one must go into the launch pit to control the equipment.
The Track System
The other system is the track system. More commonly used than the cradle system, the track system does not require as large of an assembly area. The casing and auger assembly takes place in the launch pit. No cranes are necessary to move these pieces into place. However, because it is a track system, there are more moving parts to consider. Track lubrication is essential.
What is Uncased Boring?
Uncased boring is similar to the cased method. However, instead of having a casing pushed into the hold made by the auger, there is nothing to support the hole and keep it from collapsing. The cutting tool moves along the same type of track and similarly removes the spoil.
There are few opportunities to use uncased boring. More often, the holes required for this method are smaller than eight inches in diameter. Workers may bore larger holes if necessary, especially if the hole is meant to be a temporary solution.
As with cased boring, uncased jobs usually are for utilities. However, due to the potentially unstable nature, they are seldom used. Workers using uncased methods must use caution when performing these jobs. The tunnel could collapse in if not properly secured. (Also see "4 Essential Considerations for Designing a Borehole.")
Are There Safety Considerations to Consider with Auger Boring?
As with any project, being observant is the primary safety resource in performing either cased or uncased boring projects.
A geotechnical investigation of the area alerts workers to any potential hazards they may face when using cutting tools. The geotechnical engineer will then produce a report with information pertinent to how to proceed with the project with a focus on maximum profit, minimum risk and appropriate spend time. (Read Getting Technical: Information Required in a Geotechnical Investigation Report.)
For both cased and uncased boring projects, the launch pit can pose risks. Compromised slope integrity is dangerous to both workers and equipment. Additionally, employees should remain out of the exit pit during the boring operation.
While schematics may be available for underground lines, workers should verify line placement before beginning. (Read The Science of Getting it Right: Locating Underground Utilities.)
The cost of replacing damaged lines can be substantial. Services using cranes should also be aware of overhead utility lines. For workers performing uncased boring projects, collapse is a possibility, as the holes created by the bore are unsupported.
Workers should never enter the tunnels created by the boring tool. The uncased boring project should be limited to temporary tunneling or small diameter holes, lessening the risk of injury due to unstable environments.
What We've Learned
Cased boring is the most traditional method of auger boring used in trenchless excavation. The use of the casing helps to support the tunnel made by the cutting tool. The housing also prevents settling of the ground above. While using uncased boring is necessary for some projects, small diameter or temporary projects are best for worker safety.