Augers have been employed to excavate earth for centuries. First powered by humans and then domesticated animals, since the industrial age augers have been powered by virtually every known prime mover. Modern Auger Boring Machines (ABMs) produce thrust for pipe jacking as well as torque for the auger. Typically, in soft ground, the pipe is jacked forward, cutting the periphery, while the auger removes material from the face and pulls the cut material back to the jacking pit.
In the past 20 years, ABMs have been made much more powerful. ABM thrust was increased to keep pace with the increasing strength of jacking pipe. Improvements in materials properties resulted in longer life for bits, which allowed longer tunnels to be driven in a single pass. So, ABM power was increased to provide the torque needed to operate longer augers, and to cut larger diameter tunnels.
However, until recently the method was limited to loose soils, sands, gravels, consolidated materials and rock with UCS less than 25MPa. The strongest of these materials are generally cut using bits, with a variety of shapes, with tungsten carbide button inserts.
Large scale TBMs have been cutting ever-harder rock since the first successful use of rolling disc cutters in the early 1950s. Disc cutter technology has seen perpetual improvement since then. Disc cutter load capacity has increased through improvements in disc ring metallurgy, increased bearing capacity and improved lubricants. In addition, the application of disc cutters has improved as TBM manufacturers gained knowledge of the effects of cutter spacing, penetration to spacing ratios, gauge-cutter development, cutter housing design, muck flow, etc.
In an attempt to cut through rock, ABM owners have periodically experimented with creating their own disc cutterheads. Not surprisingly, the first ABM disc cutterheads were rather crude copies of TBM cutterheads and disc cutters. They were not thoroughly engineered devices, and met with only limited success.
For many years, there were no companies making a full range of underground excavation equipment (e.g. TBMs, Disc Cutters, ABMs, MTBMs, EPBMs, slurry machines, etc.). With ABMs, hard rock TBMs and disc cutters being made by different manufacturers, there was little technology transfer between the products.
In recent years there has been growing horizontal integration of underground equipment manufacturers and today there are a few companies making all of these products. As a result, the rate of technology transfer has increased, and auger boring is benefiting.
Recognising the need for ABMs to be able to cut harder rock efficiently, equipment manufacturers began to develop purpose-designed rock-cutting heads for auger boring machines. Commonly referred to as small boring units (SBUs), these rock-cutting heads make use of a half century of TBM hard-rock cutting technology. The modern SBU is essentially a miniature hard-rock TBM cutterhead and mainbearing, mounted within a simple shield. The auger shaft rotates the cutterhead and the cut rock is removed from the tunnel by the auger.
Small boring unit (SBU)
The SBU shield is typically welded onto the front of the steel casing pipe, inside of which the auger runs. The auger’s hexagonal shaft connects to the cutterhead’s hexagonal drive shaft. The cutterhead rotates with the auger. As the pipe is jacked forward, thrust force from the pipe is transferred to the SBU shield, then to the mainbearing and finally into the cutterhead. The pipe jacking force provides thrust to the rock cutting head.
As the head is rotated, the disc cutters break the rock, and the cut rock is pulled from the face by rotating scrapers on the head. The scrapers push the rock cuttings behind the cutterhead to the auger. The auger pulls the cuttings through the liner pipe to the jacking pit.
Rock-cutting heads for ABMs
Experienced hard rock TBM manufacturers understand the requirements for efficient rock cutting with disc cutters on large diameter machines. Adaptation of the disc cutter method to small boring units (SBUs) for use with ABMs presented certain unique design challenges.
Unlike when operating in soil, when boring through rock the jacked pipe cannot cut the periphery, or “gauge”. In rock, the disc cutters have to be employed to cut the gauge. To do this efficiently requires a great deal of knowledge about the application of rolling disc cutters on full-face rotary cutterheads.
The objective for efficient rock cutting is to break the rock into chips, rather than crushing it into fine dust. Crushing the rock unnecessarily requires more energy and results in unnecessary wear on cutters, cutterhead and augers. Inefficient rock cutting drives up the cost of equipment operation.
For a given distance of penetration of the cutter into the rock (per revolution of the cutterhead) there is a preferred distance of spacing between adjacent cutters. The spacing to penetration ratio must be optimised for efficient rock cutting to take place.
Cutter penetration is governed by several factors, including: rock strength, rock mass properties, cutter load, cutter diameter and cutter ring tip-width. There is literature available from the academic community regarding theoretical methods for estimating the penetration rate for rock cutterheads.
Most penetration estimating methods require as minimum inputs; unconfined compressive strength (UCS); Brazilian tensile strength (Bt); fracture spacing; fracture dip and strike; cutter diameter; cutter tip-width; cutter load and cutterhead speed (revolutions per minute). With additional information on hard-particle content of the rock, cutter materials and cutter cost, some estimating methods can also estimate the cutter cost per volume of rock excavated.
Most equipment manufacturers have developed their own proprietary algorithms, which are benchmarked against their internal performance records from previous projects. Having access to a large amounts of field data is imperative for the development of an accurate penetration-estimating algorithm, and an accurate penetration-estimating algorithm is an essential tool for designing an efficient, rock-cutting cutterhead.
After the rock has been cut, it is drawn from the face and pushed to the aft side of the rotating cutterhead to the auger for removal from the tunnel. The scrapers, or paddles, used to move the muck to the back of the head are often subjected to high levels of abrasive wear. They are generally made of special abrasion-resistant steels.
In the case of very long tunnels, or extremely abrasive rock, it may be necessary to change cutters before the tunnel drive is completed. It is possible to do so with the SBU. By making the gauge cutters retractable and altering the design of the shield it is possible to make a retractable boring unit. With this design the SBU can be withdrawn through the liner pipe and the boring unit serviced on the surface.
ABM thrust requirement
The aft end of the SBU shield is welded to the leading edge of the steel liner pipe. Thrust force is transmitted from the ABM pipe-jacking system to the steel liner pipe, and then into the SBU shield. From the shield, the thrust force is transmitted into the main bearing housing, through the bearing, and into the cutterhead and disc cutters.
The ABM pipe-jacking system provides thrust to overcome liner pipe friction as well as to load the disc cutters, as required to cut the rock. The thrust required for any specific cutterhead to efficiently break the rock is a function of the properties of the rock to be cut, the cutter ring diameter, cutter ring tip-width and the number of cutters on the head.
In most applications, the friction on the liner pipe far exceeds the thrust required for disc cutter loading. As a result, the thrust required to use a rock-boring SBU is seldom a problem for a modern ABM.
ABM torque requirement
The ABM power unit provides the torque for the auger as well as the torque to power the SBU cutterhead. The torque is transmitted from the ABM drive output into the auger string and from the auger string into the SBU cutterhead.
The ABM system provides the torque to overcome auger-to-liner pipe friction, to overcome friction between the rock cuttings and the liner pipe, as well as toque to the SBU cutterhead. The torque required to power a specific cutterhead is a function of the properties of the rock to be cut, the cutter ring diameter and tip-width, the number and radial location of cutters on the head, and the cutter penetration into the rock. On an ABM, the torque required to turn the auger is function of the auger diameter, pitch, volume of material in the auger, material size, presence of water, etc. On typical tunnels, the torque required to turn the auger is generally 2 to 3 times the torque required to turn the SBU cutterhead. Again, most modern ABMs have sufficient torque to power the SBU cutterhead.
The SBU cutterhead is fitted with a standard male hexagonal adaptor onto which is fitted the usual auger female hexagonal adaptor, to provide torsional power to the cutterhead.
Steering the SBU
The SBU is typically fitted with small, manually adjustable stabilisers pads. With the SBU shield welded to the steel liner pipe and the auger installed there is limited opportunity to steer the machine. It is imperative to adjust the stabiliser pads as required while the initial 15m to 20m of tunnel is bored, in order to get the pipe started on the correct path. With a good start, the machines will typically maintain direction. During the initial tunnel boring, it is usual to remove the auger from within the liner pipe several times to check the line and grade of the advancing SBU.
When proper alignment is established, the stabilisers are adjusted to prevent the natural tendency of the rock boring head to climb and move to the right. This tendency is due to the clockwise rotation of the head when viewed from the back (with the auger powering the cutterhead, it is not possible to have a bi-directional head).
Large diameters and long drives
As the tunnel and auger become very long, auger friction can become large. For very long tunnels, the SBU can be fitted with a separate cutterhead power unit, either electrical or hydraulic, so that all ABM torque can be employed to turn the auger.
With the use of independent cutterhead power on the SBU, it is possible to bore much larger diameter tunnels. It is also possible to remove the cut rock with an auger much smaller than the full tunnel diameter.
Many ABM rock drives have jacked reinforced concrete pipe (RCP), within which lies a smaller steel pipe, inside of which is the auger used for muck removal. The smaller auger diameter and independent cutterhead power allow for very long tunnel drives with minimal ABM auger power. For further reduction of auger friction, water or bentonite can be injected into the auger casing. At larger diameters, it is also possible to fit the SBU with articulation jacks that allow continuous steering.
Full size, full-face augers can be used with SBUs in sizes ranging from 610mm to 1520mm. The use of big RCP pipe with a small auger generally begins at diameters greater than 1220mm.
Conclusions
To date, SBUs have cut rock to 175MPa. Although SBUs were originally intended for drives up to 120m, they have been successfully employed on much longer drives. J & J Boring, in Virginia US, completed a 233m long, 1371mm diameter, drive through mica-schist with UCS to over 55MPa. It would appear that the ABM + SBU method could successfully complete drives of perhaps 250m-300m. Auger torque limits would likely preclude longer drives.
Since many contractors already have suitable ABMs and augers in their fleets, it is frequently only the incremental investment in an SBU that is the added cost for excavating a rock tunnel. When the geology is appropriate for the method, an ABM + SBU system can be a very efficient method of excavation and pipe installation, and provides a very cost effective solution for small bore, rock excavation.
