Auger boring through hard rock: overcoming the challenge

Feb 02, 2011

Hard rock is the true nemesis of the auger boring contractor, the evolution of auger boring know-how and its increased recognition as a more widely used and understood method of underground technology has introduced disc cutter head product tooling.

The disc cutter concept borrows on a smaller diameter platform than the cutting method that large tunnelling machines utilise to rotate, fracture, and remove rock chip spoils from the bore path. Manufacturers such as American Augers (AA) are developing and seeing proven field results in their version of the disc cutter head. One example of creating success with man, machine, and technology is an auger boring job in Pennsylvania, USA through an unyielding rock formation.

Cutting through
The contract in Doylestown, Pennsylvania called for the installation of a 37 metre long gas pipeline under a two lane roadway. Henkels and McCoy subcontracted Case Boring Corporation from Gasport, New York to install the pipeline. Confronting the span of the bore was a sandstone deposit that the disc cutter was more than capable of surmounting, as the engineering design is suitable for intrusive rock formations up to 25,000 psi.

Case Boring Corporation representative Mark Case said “The AA disc cutter head was the exact right choice for the job. It was effective and productive in both solid and very fissured rock, the type of rock that can give a boring contractor fits with other types of tooling.”

The Case Boring crew created a 15 metre long and 3 metre deep shored pit, operating on 30 feet of auger boring extension track, which would set the stage for either achieving success or create less than ideal conditions. Using a 1990 model of a 60-1,200 auger boring machine and a brand new 36 inch diameter disc cutter head, the Case Boring operator began pushing the product through the sedimentary landscape at a steady rotational speed. Maintaining consistent speed in a slow, but optimum range of 17 – 25 RPM allows disc cutter heads to work through the bore at a pace. This ensures a steady flow of the rock cuttings, and prevents any significant hindrance to the spoil return process. For this job, the spoils varied from a powdery dirt substance to finger nail clipping size remnants.

The RPM rate must be balanced to coincide with the thrust pressure being produced by the auger boring machine. Throughout the total distance bored, the auger boring crew had to keep a careful eye on the variable factors that influence the amount of thrust to apply including: psi level of the rock being cut, total size of the bore, rate of spoil return, diameter of the cutting head, and overall machine torque output and speed. The bore maintained a variable output of 220,000 ft-lbs. of thrust.
An excavator was also used when the excavator bucket was placed on the product casing in close proximity of the disc cutter to apply pressure and to aid in the stability of the cutter as the machine made the initial penetration of the rock wall. This practice is typical for most disc cutter head jobs because the critical nature of the cutters first contact with earthen embankment will create some vibration that if not supported or monitored could cause the line of the bore to be compromised. The excavator was withdrawn shortly after the entire disc cutter had disappeared into the formation, and the vibration ceased because the head and the product casing was now shrouded in the compact composition of the earth, and the operators of the bore had settled the machine into a ‘sweet spot’ that would allow them to steadily progress.
The bore
The bore was done at zero percent grade. If maintaining line and grade is required, the disc cutter is equipped with power assisted steering jacks, ensuring steering corrections can be made with ease. In total, it took one and a half days to complete the bore. A traditional auger bore, without the disc cutter, would have been difficult and time consuming.

AA Field Service Technician Jim Lee said “Using conventional product tooling this bore would have been possible, but we may have only been able to bore five to eight feet per day. I know from experience using a traditional rock cutting head in that type of sandstone would of required constant maintenance in replacing the carbide bullet tips every two or three feet.”

Mr Case agreed, “Without the disc cutter the bores would have taken three times longer to complete, involving considerable re-tooling and time spent pulling and reinserting the auger. The head turned very easily, greatly reducing wear and tear on our auger string and drive train.”
Auger boring for tough conditions
Auger boring and its associated equipment or tooling, like disc cutter heads, are typically less expensive and reduce downtime more so than the conventional practice of open trenching. In neighbourhoods, metropolitan zones, wetlands/waterways, and in infrastructure development areas auger boring creates less physical disruption and can save a considerable amount of expense on product installation, reduce restoration costs and provides a tremendous amount of goodwill to the community and its inhabitants.

Another down time limiting factor for disc cutter heads is that the large diameter of the head allows the head to retract from the face without moving the product casing. Having a retractable cutting head also allows for cutter change and service that can be accomplished outside of the heading.

Auger boring itself is a test of both man and machine, but when those two factors are confronted with tough ground formations that can stress human emotions and mechanical muscle, the real test is how utilising proven practices and today’s technology, like disc cutter heads, can prevail in complicated situations.

“The disc cutter head will allow us to make bores in a much more cost effective way than ever before. We will be able to entertain boring longer crossings than ever before due to the easy turning nature of the head and the unique steering advantages the head give us,” said Mr Case.
Advantages of a disk cutter
From the perspective of the contractor, the advantage to using a disc cutter head is that the unit is mounted directly to the product casing, which enhances cutter head stability and allows for longer cutter life. The individual cutters are manufactured with high strength steel that allows for increased working life because during use they perform a rolling motion that creates no friction. The disc cutter also benefits the operator by maximising performance in various geological formations and is designed to withstand the severe loading of mixed face conditions.

Disc cutter heads have the ability to be industry wide contributors because they are compatible to any make or model of auger boring machine and auger section that is fitted with a 4 inch or 5 inch hex.
>> This article is presented with permission of Trenchless International / Great Southern Press. <<

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