Horizontal auger boring has come a long way from its start in the coal mining industry in the early 1930s (Hartman, 1992). In 1941, Kandal Motors started manufacturing the first marketable Auger Boring Machine (ABM), making the technology commercially available (ASCE, 2004). Over time, the ABM was modified and improved to meet contractors needs, including splitting the ABM along with an increase in power and thrust to keep pace with the increasing drive length and need for a larger diameter jacking pipe. Early ABMs were not as dependent on accuracy and did not have mechanisms that allowed steering. Today, utility owners and engineers require that strict line and grade tolerances be maintained, and manufactures have made significant strides in making ABMs steerable to meet the growing demand. At its infancy, ABM usage was limited to mining in silts, sands, and gravels above groundwater, and rock with unconfined compressive strengths less than 4,000 psi (28 MPa). Developments over the past 20 years have extended the variability of ground conditions within which ABMs can operate.
There are two distinct conventional ABM methods: the Track Method, in which the casing is advanced in the jacking pit on a predetermined length of track, and the Cradle Method, in which there are limited operations in the pit and the casing is suspended from a crane and advanced forward by a hydraulic winch hooked to a jacking lug secured at the bore entrance.
Conventional auger boring
Conventional ABM methods can be coupled with specialised cutting heads, such as the large diameter boring attachment (LDBA) that can facilitate the installation of pipe sizes ranging from 4 to 156 inches (0.1 to 4 m) and is effective in a wide variety of ground conditions, ranging from dry sand to firm clay (optimal) to solid rock. Conventional auger boring methods can accommodate cobbles and boulders up to about 30 per cent of the casing diameter. Generally, auger boring is not compatible with cohesionless soils subject to groundwater head; however, short granular layers interbedded with cohesive soils forming a plug can be mined with this technique. The largest pipe installed in the US using conventional ABM methods to date was completed in Phoenix, Arizona, and was
520 ft (170 m) long and 84 inches
(2.1 m) in diameter.
Pilot tube method
Horizontal auger boring is also used in conjunction with the Pilot Tube Method (PTM), a multi-stage method of accurately installing product pipe by use of a guided pilot tube and followed by upsizing to install the product pipe (Akkerman, 2011). This method has been commonly referred to as “pilot tube microtunnelling” and the “guided boring method.” PTM can accommodate installations with pipe diameters up to 48 inches (1.2 m) drive length of up to approximately 400 ft (120 m). Pilot head steering is accomplished by aligning an angled pilot head for the desired course and thrusting forward. PTM can be used in soils where blow counts are greater than zero but less than 50, and are displaceable. Large cobbles and boulders can prevent the pilot tube from advancing, and may halt or modify the installation process. Recent developments such as lubricants for loose sands, water control reaming heads for wet sands, and testing of air hammers for use in rock will increase the range of subsurface conditions in which PTM can be used. The recommended drive length of PTM is up to about 300 ft (90 m), with a few recorded drives greater than 500 ft (150 m), but the drive length depends on subsurface conditions and pipe diameter (Akkerman, 2011).
Small boring
In 1996, Robbins Company developed Small Boring Unit (SBU), a specialised rock-cutting head used in conjunction with the ABM to efficiently cut rock with a UCS greater than 4,000 psi (28 MPa) (Long, 2006). The cutting head technology incorporated into the SBU had been in use for more than 50 years on hard-rock cutting tunnel boring machines. There are two kinds of SBU: SBU-A, in which the cutting head is powered by the ABM, and SBU-M, in which a separate motor powers the cutting head. SBUs are best suited for full-face rock. They have been used in mixed-face conditions and some in full-face soil application, but such use is not recommended if soils are not partially cemented. SBUs operate above the groundwater and work well in weathered rock or cemented cobbles; however, they are not suitable for use in soils. The longest drive to date 600 ft (183 m) for the SBU-A will likely remain the upper limit for the machine (Sivesind, 2011).
Conventional horizontal auger boring, the catalyst for all other auger methods, has undergone many changes over the past 75 years, from its early days in mining coal to completing 600 ft (183 m) crossings in rock with strict line and grade tolerances. Each boring method presented above has unique characteristics, but construction related risk is one thing all methods have in common. Risk can include steering variability, long drives and associated high frictional forces, obstructions, groundwater conditions, and operator experience. Any of these risks could result in an unsuccessful crossing and must be considered by designers before undertaking any trenchless project. More information on this topic is presented in “Auger Boring—A Historical Review of Techniques and Applications”
(Martin and Grolewski, 2011).
