Auger boring is a technique for forming a horizontal bore hole through the ground, from a drive shaft to a reception shaft, by means of a rotating cutting head. The cutting head is attached to the leading end of an auger string. Spoil is transported back to the drive shaft by the rotation of helical-wound auger flights within the steel casing pipe. Vertical control, using a water level, is typical.
It is possible to use auger boring equipment to contract an uncased bore hole by using a cutting head and auger. However, this practice results in an unsupported hole, and the unprotected augers rotating in the drive shaft create special hazards for workers. Therefore, the common practice is to simultaneously jack the steel casing with the boring operation. If uncased auger boring is permitted, it should be limited to soil conditions with sufficient stand-up time and when short, small diameter bores are used.
A. Track type auger boring
The two major types of auger boring systems are (1) track-type and (2) cradle type. The basic components of a track-type system are the track system, boring machine, casing pipe, cutting head, and augers. Optional components include a casing lubrication system, steering system, locating system, and casing leading-edge band. The auger driving device and jacking equipment are usually integrated into the boring machine (Iseley and Gokhale 1993).
The auger string consists of one or more augers connected end to end for the full length of casing pipe. The leading end of the string is connected to the cutting head, and the other end is attached to the auger machine. The machine applies torque to the auger string, which in turn rotates the cutter head. The casing is advanced by hydraulic jacks located at the rear of the machine.
A properly constructed drive shaft is important for the success of a track-type auger boring project. The shaft requires a stable foundation and adequate thrust block. The track system must be placed on the same line and grade as the desired bore hole. Often this foundation will require crushed stone or concrete.
The thrust block transmits the horizontal jacking forces from the tracks to the ground at the rear of the drive shaft. The thrust block must be designed to distribute the jacking force over sufficient area so that the allowable compressive strength of the soil is not exceeded.
The track-type auger boring operation involves the following:
1. Jobsite preparation
This step involves the investigation of underground utilities and designing the layout of the job site to secure sufficient space for equipment and materials.
2. Bore pit excavation and preparation
The entrance and exit pits (or drive and reception shafts) are excavated at this step. The excavation should follow the instructions given by local codes and OSHA manuals for pit wall sloping and sheeting. The typical pit is 11.5 m (38 ft) long and 3 or 3.5 m (10 or 12 ft) wide. The bottom of the pits is 0.8 m (2 ft 8 in) below the center of casing (Miller the Driller 2002).
3. Equipment setup
Different types of equipment may be required on or around the boring site. Excavators or cranes are needed to dig the boring pit and set the equipment. Boring machines and tracks appropriate for the job are required. Augers must be placed in the casing sections. A cutting head is selected depending on the ground conditions and is installed in front of the first auger section.
The most critical part of the bore is the setting of the machine track on line and grade. If the alignment is not right when the bore is started, it is not likely to improve during the boring process. (picture, movie (This wmv file is 3.4 MB. Windows Media Player is required to run this file.)) Once the track system is installed, the auger boring machine and the thrust block are installed (picture, movie (This wmv file is 3.1 MB. Windows Media Player is required to run this file.)).
Other optional systems described below may be employed for the auger boring operation.
Lubrication system: To reduce the friction between the casing and soil, a lubricant may be applied to the outer skin of the casing. This can also reduce the requirement for the thrust capacity of the boring machine. Two basic types of lubricants are bentonite and polymers.
Water level: The water level is a device to measure the grade of pipe casing as it is being installed.
4. Preparation of casing
In most cases, the lead casing is prepared in the yard prior to its transport to the jobsite and arrives at the jobsite with the auger inside and the cutting head attached to the leading end of the auger. A partial band at or near the head end of the casing is recommended when boring in most soil conditions. The band compacts the soil and relieves pressure on the casing by decreasing the skin friction.
5. Installation of casing
When casings are prepared and auger boring is setup, the leading casing is moved onto the track and connected to the boring machine by welding (movie (This wmv file is 3.5 MB. Windows Media Player is required to run this file.)). “Collaring,” which is the first operation, pushes the cutting head into the ground without lifting the casing out of the saddle. When about 1.3 m (4 ft) of casing has entered the ground, the engine is shut down, the saddle is removed, and the line and grade of the casing is checked.
After the first section of the casing has been installed, the casing is cleaned by rotating the auger until all the spoil is removed. The machine is then shut down and the auger pin in the spoil chamber is removed. The machine is then moved to the rear of the track and is again shut down. Then the next section of the casing and auger are lowered into position. The augers at the face are aligned flight to flight, the hexagonal joint is coupled and the auger pin is installed. Once the casing to be installed is aligned with the installed casing, the two are tacked together then welded fully. The process is then repeated until the bore is completed (picture, movie (This wmv file is 3.7 MB. Windows Media Player is required to run this file.)).
6. Completion of drilling
Once the bore is completed, the machine is shut down and the cutting head is removed. The casing is then cleaned by rotating the augers in the normal direction. The torque plates are then removed to detach the machine from the casing and the augers are retracted till the coupling is well outside the casing. The auger section is uncoupled from the machine and the other auger sections, and is then removed. The machine is then coupled to the next auger and the process is repeated till all the auger sections are removed.
7. Site restoration
Once al the augers are removed, the boring machine and the tracks are removed from the pit, the desired utilities are installed through the casing and the required connections are made. The entrance and exit shafts are then backfilled.
B. Cradle type auger boring
The cradle type auger boring method is suitable for projects that provide adequate room. The bore pit size is a function of the bore diameter and the length of the bore. This method is commonly used on petroleum pipeline projects where large rights-of-way are essential.
This method offers the advantage that all work is performed on the ground level rather than in the pit. The bore pit is excavated several feet deeper than the invert of the casing pipe to allow space for the collection of spoil and water as the bore hole is excavated. The method does not require any thrust structures, however, a jacking lug must be securely installed at the bore entrance embankment.
Auger boring can be used to install casing pipe ranging from 100 mm (4 in) to at least 1,500 mm (60 in) in diameter, with the most common diameters ranging from 200 mm (8 in) to 900 mm (36 in). When the diameter of pipe to be installed is less than 200 mm (8 in), other trenchless technologies are more appropriate and economical, especially, where the line and grade are not very critical. For larger diameters where the line and grade are more critical, pipe jacking and microtunneling can be the better alternatives which provide greater accuracy and cost effectiveness. .
Auger boring was initially developed to cross under a two-lane roadway with an average length of 12 m (40 ft) and a maximum length of 21 m (70 ft). However, typical project lengths range from 30 m (100 ft) to 91.5 m (300 ft), with the demand for longer installations increasing. The longest continuous track-type auger boring project is 270 m (886 ft).
Because the augers rotate inside the pipe, the pipe and coating material must resist potential damage caused by rotating augers. Therefore, the typical casing pipe is made of steel. The product or carrier pipe installed inside the casing can be made of any material suitable for the product being carried.
Shafts are required at both ends of the bore. The drive shaft is primary working shaft. The size of the shaft is determined by the diameter of the bore hole and the length of the casing segments to be used. Typically, casing segments are 3.0 m (10 ft), 6.1 m (20 ft), or 12.2 m (40 ft) in length; the most common length is 6.1 m (20 ft). If casing segments 6.1 m (20 ft) in length are used, the shaft size will be 9.1 m (30 ft) to 10.7 m (35 ft) in length by 2.5 m (8 ft) to 3.6 m (12 ft) in width. The surface area should be approximately 23 m (75 ft) by 46 m (150 ft). The absolute minimum surface area should be 9 m (30 ft) by 25 m (82 ft). Sufficient space should be available for loading, unloading and storage of materials and equipment.
Auger boring can be used in a wide range of soil conditions, from dry sand to firm dry clay to solid rock. Firm sandy clay is the most compatible soil condition for using this method. Boulders or cobbles as large as one-third of the casing diameter can be accomplished. In case of unstable soils, care should be taken regarding the cutting edge leading the casing edge as this may result in spoil being removed without any advancement in the casing which means that excessive spoil is being removed. This situation can create a void between the casing and the bore hole which may lead to surface subsidence.
It is important that the drive shaft construction crew understand that the success of the project depends to a large extent on the quality of the drive shaft. Shaft construction may take 1 day for shafts less than 3 m (10 ft) deep when the excavation embankments can be sloped. Shaft construction could take several weeks if the shaft is greater than 10 m (33 ft) deep and the excavation support system is steel sheet piling.
The auger boring operation takes a four-person crew 3 to 4 hours to set up the auger boring equipment for a steel casing project 610 mm (24 in) in diameter utilizing segments 6.1 m (20 ft) in length. A typical production rate for this size project is 33 m (100 ft) in an 8-hr shift. Depending on soil conditions and casing diameter and length, auger boring typically takes place at a rate of 1 to 12 m/hr (3 to 40 ft/hr).
Iseley, T. and Gokhale, S. (1997). “Trenchless installation of conduits beneath roadways.” NCHRP Synthesis 242. Transportation Research Board/National Research Council, Washington, D.C., 36.
the Driller (2002). <http://www.millerthedriller.com/auger/index.htm>