Horizontal auger boring, also known as auger boring, horizontal earth boring, or jack and bore, is one of the most common trenchless and tunnelling options. Auger boring can be used to install pipes and casings in various ground conditions and, in many cases, has lower overall costs compared to other trenchless methods.
Basics
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Auger boring is typically used for mining lengths up to about 500ft (152m) and diameters up to about 60 inches (1.5m). It uses a revolving cutting head that is located at the leading end of an auger string to excavate the soil. Standard practice is to jack the steel casing through the hole as boring takes place. The excavated material is then brought back to the shaft area by the rotation of the helical auger flights inside the steel pipe casing being placed.
Auger boring machines come in different sizes and capacities. Ground conditions usually dictate how powerful of a machine is needed but the anticipated application also determines if auger boring is a suitable trenchless method. For example, bores of less than six inches (usually fiber-optic or electric lines and small water pipes) may be more suitable for horizontal directional drilling.
However, for some water and gravity sewer lines that begin and end on grade, using an auger bore rig could be less expensive.
Challenges
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By GlobalDataThere are several challenges associated with auger boring operation. An auger boring machine typically cannot be steered (there are some exceptions that come with additional cost and challenges); therefore, properly designed and constructed launch shafts/platforms are critical to the success of any project.
Slight misalignment at the start could lead to significant deviation from design path as the boring advances.
Operator skill is also critical as the excavation and soil removal process can’t be seen directly. Changing ground conditions that are not detected can cause subsidence on the surface by over-excavation (excavation beyond specified or directed excavation).
While auger boring is an established and cost-effective method for straight and level connections, one of its limitations has previously been accuracy.
However, new technologies, such as steering shield and pilot bore, have made a major impact and provide more precise bores and successful projects.
Considerations
Different types of auger boring machines are available in the market. Project conditions dictate how powerful a machine is required or what auxiliary equipment will need to be used.
Typically, the interpretation of geotechnical investigation data in addition to design requirements, schedule, and cost impact considerations are used to determine if auger boring is a suitable method for a given project.
To improve safety and productivity of mining operations in more challenging ground conditions (particularly for borings under hydrostatic pressure) proper face control through the establishment of soil plugs in the auger is essential and, in most cases, necessary.
In some cases, where ground conditions are relatively loose or soft and friction forces along the pipe are not prohibitive, the hydraulic jacking systems can force the casing into the ground ahead of the lead edge of the auger cutting tools. This approach (which to some extent is like pipe ramming) assists with reducing the risks of excess excavation by maintaining a “plug” of soil between the lead edge of the casing and lead edge of the auger.
The lead end of the steel casing is typically modified such that the maximum diameter is slightly larger than the remaining pipe diameter (i.e., over-cut). The larger diameter cut assists with creating an annular gap within which water and bentonite lubrication fluids can be injected to reduce resistance to jacking forces caused by friction along the pipe.
Typical auger boring equipment presents less flexibility for unexpected ground conditions, such as cobbles and boulders. While some methods and new technologies are available to mitigate these concerns, the equipment remains less adaptable to obstructions than most other methods.
When encountering extremely poor ground conditions, and where common methods of controlling the excavation by the auger boring machine itself is not effective, ground improvement techniques can be used to prepare the ground along the tunnel alignment for more efficient boring. These include grouting, soil mixing, and dewatering.
