Rock driller expedites difficult HDD rock bore project

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Lennestadt, Germany, Sept. 25, 2007 -- Although Elbrinxen may sound like a town from Middle Earth, it's actually a town close to Bad Pyrmont in Germany -- and the name of a difficult underground bore project planned nearby.

Initially the installation of a new gas pipe was planned over a length of 12 km from Lügde via Elbrinxen to Rischenau. On short notice, the responsible supply companies cut the distance back to 3/4 and the water and power lines at the same time. The contractors named for the task were E.ON Westfalen Weser AG, Herford and the Waterworks in Lügde.

"The co-ordination of planning and execution [resulted in] an extremely short planning period of 3 weeks and task completion within 3 months," according to Rudolf Meyer from the engineering office in Hameln, instructed with planning and supervision of construction work.

With this project, already existing 30 KV overhead service lines had to be dismantled. Installing power cables inside protection pipes -- either open trench, ploughed in or with bores -- are now part and parcel of the standard program of a civil engineering company. Snow, wind and ice breakages are therefore a thing of the past.

Water pipes with insufficient diameters and requiring sanitation were exchanged during the whole construction process. A gas pipe, OD 315 and 225, triggered the idea of a multiple bore path and completed the pipe bundle.

The construction measure here consisted of a 90-meter-long section, which was to be completed with the trenchless method because of the underground crossing of an adjoining public road, the Ilsen stream and, 30 m higher, a water feeding mill trench.

The open installation in the trench did not provide any great demands to the whole task. To be able to install a pipe bundle using the HDD method, consisting of two protection pipes, HDPE, OD 160 for the 30 KV medium high voltage cable; a HDPE -- OD 110 (protection pipe OD 160) as gas pipe; and a HDPE pipe, OD 160 as main transport pipe for the drinking water supply, a specialized bore company was required. The company was Meier Tiefbbau GmbH with many years of bore experience from Hessisch-Oldendorf. They successfully applied their Grundodrill 15 N (Manufacturer: Tracto-Technik, Lennestadt).

The comparative above-average performance, the auto-bore, the digital instrument display and other features distinguish the new Grundodrill 15 N. The compact construction of this rig allows work to be carried out in narrow streets and alleys, such as in this case, without causing any roadblocks.

The bore was first tackled with standard bore equipment. But after 5.6 m depth, the bore came to a stand-still. Surprisingly a lime bed was spread over the expected marl layers and the variegated sandstone below.

A rock drill head was required to bore through this rocky layer. This bore tool, developed for mineral oil applications, is driven via a mud motor (Moineau -- screwed principle) from drilling fluid with high pressure. These hard stone tested bore tools destroy even tougher rocks in the process and the bore spoils are extracted with the fluid.

The Grundorock mud motor produces the pilot bore with an outer diameter of 4 1/2", approx. 108 mm, in one working process and one work day. Of particular note with the Grundorock mud motor is the comparatively low drilling fluid consumption.

The 90-meter-long bore reached it's deepest point at 7.80 m below ground level. The expanded bores were then carried out in the usual manner: in single steps, from the target point backwards to the starting point. Also in single steps, the bore diameter was expanded to 205 mm, then 380 and finally to 450 mm. These working processes took 3 work days in total to complete.

Normally used as hole-openers, the rock driller's standard expanding tools were applied. In this case, though, the special backreamers from Tracto-Technik were to be put to the test, as it is more economical to use backreamers in rock formations than to use hole-openers whenever possible.

The armor plating of the TT backreamers can be easily exchanged in case of wear and tear and at an acceptable cost. Furthermore, the bore tools can easily be adapted to suit the form, hardness and aggressiveness of the relevant soil conditions.

In this case, everything went according to plan: The hole-opener was not required at all.

The bore process was supported by a drilling fluid, consisting of water and Bentonite, a natural swell capable clay mineral. The disposal, approx. 130 m³ was no problem whatsoever. It was transported in a barrel to be used on agricultural surfaces -- clay processing of meager sandy soils. Preparation and disposal was carried out efficiently, without any interruptions to work on site.

The collected pipe installation of the 4 HDPE pipes was carried out without any difficulty whatsoever within 1.5 hours.

The project described here, installing 4 HDPE-pipes in a pipe bundle, trenchless with the HDD rock drilling technology, is a truly demanding construction measure, but with professional planning, an informative contractor and an experienced company, it can be carried out successfully.

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