Historically, the bore would be cut first, then the pipe casing installed afterwards, but as the technology has improved over time, companies have found ways to install the casing at the same time as the bore is being drilled to reduce the potential for soil collapse.
How Horizontal Auger Boring Works
The rotating cutting head cuts through the ground at the bore face at the front casing. The cutting head normally sits just in front of or just inside the casing. As spoil is created through the cutting action, it falls into the casing onto the auger, which is a rotating screw with helical fluted coils. It's located inside the pipe casing.
As the auger rotates, the spoil is drawn along the coils towards the entrance pit where it is removed. Spoil can be removed from the entrance pit via a conveyer, manually or with mechanical equipment.
Hydraulic jacking equipment is used to supply the force necessary to drive the casings through the ground. Driving the casings into the bore while the auger machine is excavating is important to prevent soil collapse and associated damage to roads or rail tracks on the surface. (Read: Cased Boring Vs. Uncased Boring.)
Entrance and exit pits are excavated openings at the start and end of the pipe installation. Equipment is placed in position in the entrance pit and successive lengths of casing are added via this pit. When the bore is complete, the cutting head and associated equipment is removed from the exit pit.
Although improvements are being made in the technology over time, the directional steering in HAB is not as refined as in other trenchless technologies. It is vital that the equipment be properly aligned in the entrance pit in order to set the bore direction accurately at the start to prevent significant deviation from the desired exit point.
When to Use Horizontal Auger Boring
Horizontal auger boring is one of the oldest forms of trenchless technology and is suitable for use in a number of different applications.
Pipelines that begin and end on grade or have a gravity run off design, are well suited for HAB as the steering requirements for these applications is quite limited.
HAB is well suited for softer ground conditions. Due to the casing installation along with the excavation of soil, there is limited risk of seepage or soil collapse. In addition, as this method does not use drilling fluid, the integrity of the surrounding soil is largely untouched. (Read: Mixing the Mud: The Science of Drilling Fluid in HDD.)
Typical applications for horizontal auger boring operations are oil, gas, water and sewer pipes with a casing diameter between 24-70 inches.
Auger bores as long as 600 feet have been completed, but a typical length is in the order of 400 feet.
Advantages of Horizontal Auger Boring
The costs of this operation are orders of magnitude cheaper than microtunneling or directional drilling. As mentioned, no drilling fluids are used other than for lubricating the pipe casings and auger, therefore, reducing the potential impact on surrounding soil.
As the casing is installed at the same time as the auger operation continues, the surrounding soil is held in place by the casing and there is limited opportunity for soil collapse or damage to roads, rail tracks on the surface.
Although the directional steering is limited, the grade of the bore can be accurately set through the alignment of the equipment in the entrance pit. The operation of the horizontal auger boring machine is relatively simple compared to more technological trenchless methods.
This means that less training is required, and the system can be used in developing countries where skilled resources aren't easily available.
Disadvantages of Horizontal Auger Boring
Some of the disadvantages of HAB include the fact that there is very limited ability to steer a horizontal auger boring machine, hence the alignment in the entrance pit is critical to ensure the correct path to the desired exit. Also, horizontal auger boring is not suitable for wet running ground or ground where there is a high frequency of boulders.
Horizontal auger boring is one of the oldest and most successful methods of trenchless construction. Although some newer technologies offer specific advantages in certain ground conditions and in steering ability, horizontal auger boring remains a low cost and highly effective method for many applications.
As this technology continues to advance and improvements made to the steering ability of auger machines, the auger boring machine will remain one of the common methods of trenchless construction for the foreseeable future.
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