DrillMaster: New Methods for Gravity Sewers

If we in the infrastructure business — whether municipality, contractor, consultant or equipment supplier — are to learn to accomplish more with the scarcity of available funding, we must be open to alternative sewer-installation methods. For your consideration, three cost-reducing techniques are briefly presented in this report.

Pilot-Tube Microtunneling (PTMT)                                                                                             

Also called Guided-Auger Boring, the PTMT process, for use primarily in displaceable soils, uses an auger-boring machine, set in an entrance pit at the proper line and grade, to first install a pilot-tube. The pilot tube is made up of short sections of small-diameter pipe and has a front-mounted, slant-nose steering head with a lighted target.

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A camera-equipped theodolite is set up in the entrance pit at the proper line and grade. The theodolite — so to speak — looks down the pilot tube at the steering head’s target, determines the head’s position and  steering orientation, displays this image on a monitor and thus allows the operator to make steering corrections.

When the steering head reaches the exit pit, a casing section (sometimes fitted with an enlargement reamer) is connected to the last pilot-tube section in the entrance pit. As this and subsequent casing sections (which contain internal augers) are pushed forward, following the pilot-tube bore, spoil is conveyed to the entrance pit.

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Pilot-tube sections are recovered in the exit pit, as are the casing sections and augers, when the product is pushed into the bore behind the casings. Casing and auger sections can be contained in the product to simplify the process. Pipe diameters from 4 to 48 in. are typical with PTMT.   

Among pilot-tube-system manufacturers are Barbco, Akkerman and Herrenknecht (Bohrtec), the latter with a new machine designed for use in harder soils.

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Vacuum-Excavation Microtunneling

New from Vermeer is the AXIS Guided Boring System, essentially a laser-guided microtunneling system using vacuum-extraction of spoil. The system’s hydraulic power pack, vacuum system and spoil-storage tank are located above-ground. In the entrance pit is a rack with a thrust/pullback carriage and a gear box producing rotational force, along with a pipe laser set at proper line and grade.

Using the rack, a drill head with a flat-face cutter and containing a camera begins the bore. As the bore progresses, drill-casing segments (6.5 ft long) are added. Each segment contains a sight channel for the laser, vacuum channel for spoil removal and a driveshaft to power the cutter. The camera views the laser beam on the target, then displays the image on a monitor, allowing precise steering corrections.

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When the drill head reaches the exit pit, drill-casing sections can be extracted as the product is installed from the entrance pit by the pipe-jacking method. Or, the drill head can be removed, the product connected to the drill casing in the exit pit, and the product pulled into the bore. The AXIS system is designed to install 8-to-16-in. pipe and works to lengths of up to 400 ft.

Laser-Assisted Horizontal Drilling

The Ditch Witch OnGrade System combines the horizontal directional drill (HDD) with a laser-reference system and advanced electronics. First, an above-ground laser is set at the specified slope of the sewer. The HDD then surface-launches a conventional drill string and steers it down into an entrance pit, leveling off at the depth of the proposed sewer.

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The distance from the drill string’s beacon (the transmitter behind the cutter head) to the laser beam is calculated. This dimension becomes the benchmark-reference depth.

As the bore progresses, a grade checker uses a “grade pole,” on which is mounted a Ditch Witch 8500 Tracker (receiver) and a moveable laser receiver. The 8500 Tracker can calculate the distance between itself and the beacon, and between itself and the point at which the laser receiver intersects the laser beam. Added together, these two measurements give the laser-to-beacon depth; if it is other than the benchmark reference, then the operator steers to the benchmark depth.

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When the drill string reaches the exit pit, the bore can then be pre-reamed, or, the product immediately pulled back. The system — which accommodates fused HDPE, fused PVC, restrained-joint ductile iron and restrained-joint PVC — is designed to work to depths of 30 ft and has the capability to make radius bores.

Changing Times

The “new economy” is teaching us that we must be willing to look at our businesses with fresh eyes—to be open to new methods of doing our basic jobs. Our profitability, in fact, might hinge on such innovative thinking.

Richard Levings is underground products manager for the Charles Machine Works Inc./Ditch Witch and is a member of the Drillmaster Advisory Board. All reports are review by the Drillmaster Advisory Board: Levings; Ed Savage, Vermeer Corp.; Frank Canon, Baroid Industrial Drilling Products; Ron Lowe, Myers-Aplex, a Pentair Pump Co.; and Trevor Young, Tulsa Rig Iron.

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