Front Steer: A Competitive Technology for Trenchless Pipe Laying up to OD 800

Worldwide, bigger cities are growing and in need of renewing or reconstructing sewers. The acceptance for long lasting jobsites for infrastructure, especially for pipe laying, which often disturb the environment and lead to massive traffic jams during rush hours, is continuously decreasing in these cities.

That’s why the authorities and planners have for a long time preferring alternative methods to accomplish building activities for sewer infrastructure. The trenchless technology is a well proven method for pipe laying to avoid environmental impact on the surrounding of such sewer constructing or renewing jobsites.

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To regulate and describe the different kind of trenchless pipe jacking methods, the German Water Association has published the DWA A125. In this publication — available in English language, too — the different kind of pipes, of methods, the requirements for planning and accomplishment, the special conditions for crossings of railways, highways and rivers are detailed described and defined.

The vast majority of sewers with smaller diameters and distances up to 150 m (500 ft) are mostly placed in lower depths. For these cases, slurry machines with all their additional equipment are not competitive against conventional open-trench construction methods.

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Therefore in the past two decades auger boring machines guided by pilot rods were developed to provide an easy to handle, fast to install and a price competitive method for these tasks. For 20 years Bohrtec has continuously developed and improved these machines to make them suitable for nearly every kind of soil.

Some first milestones in the development of these machines by Bohrtec include:

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·      Double wall pilot rods, which allow pilot drillings up to 150 m (500 ft)

·      Groundwater augers, which allow auger boring 3 m beyond groundwater level even in fine sand or running soil

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·      Back reamers, which allow plastic pipes to be installed with the accuracy as needed for sewers

·      Optical guided systems with hollow stem augers, which allow highest accuracy in vertical and horizontal direction.

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Nevertheless all these developments were based on and adapted to the pilot tube boring, i.e. their working range was limited to jobsites with displaceable soil. To widen this working range of auger boring machines even to non-displaceable soil conditions, Bohrtec started the development of guided drilling heads 10 years ago.

The first step in this development was the construction of a flap controlled drilling head without the need of pilot rods. Four hydraulic driven flaps were installed in the outer casing of this drilling head, which could be activated by the controller to correct the drilling direction, if needed.

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Flap Control Head

BAUMA 2004, Flap Control Head

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Although this development could successfully be tested on different jobsites, the possibilities to correct the drilling head in case of major deviations were limited. The reason for this limitation was the small size of the flaps compared to the outer surface of the steering pipe.

This knowledge led to the next milestone of development:

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Front Steer, which allows instant and direct reacting correction of the boring direction due to the activation of the whole drilling head surface.

As a guiding system, the well proven optical LED Target with Theodolite of the pilot guided auger boring systems was implemented in the Front Steer. So the Front Steer can be jacked in soil conditions of non-displaceable soil SPT > 35 even with single stones or soft rock up to 20 MPa with the highest accuracy in range of millimeters (10th of an inch) while using hollow stem augers, as well as in vertical as in horizontal direction.
Frontsteer_en_optische Gasse
Principle of the Front Steer navigated by theodolite

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To give even customers without hollow stem augers the possibility to use the Front Steer, alternative methods to determine vertical and horizontal deviation were developed by Bohrtec. An electronic inclinometer with highest precision and an electronic hydrostatic water level to determine the vertical position were implemented and their values are now displayed on the control unit. To determine the horizontal position, a simple measuring method was created that works like an inclinometer for the horizontal direction, i.e. deviations of the desired directions are immediately shown.

Frontsteer_en_ohneoptischeGasse
Principle of the Front Steer navigated by Inclinometer and electronic hydrostatic water level

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With the combination of these three systems an accuracy within the range of 2 cm (range of 1 in.) for the vertical position and an accuracy within the range of one decimeter (range of 4 in.) for driving lengths up to 120 m (500 ft) can be reached easily.

The main advantages of the Front Steer technology up to this milestone were:

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·      Application in non-displaceable soil (SPT>35) and soft rock up to 20 MPa

·      Adaptation to all existing auger boring systems and standard augers

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·      Adaptable drilling heads for every ground condition

·      Augers can be pulled to manually remove obstacles

·      Simple to operate

Although this system offered a very vast field of application in different ground conditions more and more customers, especially from countries and cities with difficult ground conditions, asked for making the system suitable even for hard rock.

Due to the modular design of the Front Steer, Bohrtec was able to adapt a down-the-hole hammer into this system. Front Steer drilling heads with 400- and 600-mm OD (16- and 24-in.) in combination with a down-the-hole hammer were tested in hard concrete, hard rock up to 100 MPa with a performance of 10 mm per minute (half an inch per minute).

Testing the abilities of a Front Steer in concrete with a strength of 60 MPa and further testing the abilities of the Front Steer in granitewith a strength up to 166 MPa and basalt with a strength up to 196MPa

Now almost the full range of every ground condition can be driven through with the auger boring systems of Bohrtec. For every soil and every pipe jacking task Bohrtec provides worldwide the best and competitive solution for small diameters up to 1,000 mm (40 in.).


Front Steer Heads, 400/600/800 mm (16″/24″/32″) all applicable with down the hole hammer as well

 
Summary

Due to the innovative developments of Bohrtec, the company is still the worldwide technology leader in guided auger drilling technology. Besides trenchless laying of supply and sewer lines also special applications like pipe roofs and drainage drillings can be easily performed with Bohrtec equipment.

Meanwhile jackings with lengths up to 150 m (500 ft) were driven successfully with high accuracy. The Bohrtec machines and systems allow the use of pipes up to OD 1,400 mm (55 in.).With the Front Steer Bohrtec has overcome the limitations of pilot rod systems and is now able to provide precise auger boring within the hardest soil conditions.

 
Case Studies

SpitalBülach, Switzerland

For a project in Swizerland the city of Bülach asked the company Huber Leitungsbau to drive five 100-m steel pipes OD 609 with an inclination of 1.5 percent for the supply of the Spital of Bülach (Hospital of Bülach).

As the soil conditions, gravel with bigger boulders, seemed to be quite difficult, Huber Leitungsbau decided to use the Front Steer technology with an 8-in. down-the-hole-hammer.

Due to the power of the combination of Front Steer with a down-the-hole-hammer the five drives could be driven within a short period of eight weeks.


Front Steer FS609 / 24″ on a job site in Switzerland

 
Bochum, Germany

For a large inner-city construction project named “Konrad-Adenauer-Straße,“ the company Batteux from Münster was assigned by the civil engineering department of the city of Bochum to perform three drillings DN 300 out of a shaft.

The particular features of this construction project were an extremely short construction period and especially the very small space for the jobsite set-up inside a barred drive lane, in order to be able to lead the inner-city traffic single-tracked in both directions.

Due to the ground/rock conditions onsite — FD/FZ 1-2 with rock strengths of up to 15 MPa —it was initially planned to use a microtunneling machine DN 300 with slurry material removal.

While installing the launch shaft, the rock on pipe level could be inspected more thoroughly and it turned out to be mudstone. So Batteux realized that it had to install a high-capacity separation system, if using its own microtunneling system with slurry material removal. But such a separation system could hardly be installed within the space limitations on the street track.

So Batteux decided to hire a subcontractor, Weitz from Leverkusen. With the drilling equipment of Bohrtec (auger boring machine BM 500, with hollow stem augers, measuring unit for optical guided jacking and Front Steer FS409), Weitz & Co. was able to offer to drive the three drillings within a shorter execution period as planned and — as a natural consequence of this — a much better price.


Circular Shaft 3.2 m in Bochum

The auger boring machine type BM 500 was set up in a circular shotcrete shaft.

Just as for pilot pipe jackings, a theodolite was installed in the starting shaft and aligned on the nominal axis in order to control the drilling. Finally, the Front Steer drilling head was inserted into the BM 500 and connected in just a few movements to the required control lines.

Despite challenging and changing ground conditions (strongly cohesive at first and rocky clay/mudstone with strengths of up to 15 MPa later),the first jacking length of 60 m (180 ft) was completed within one week including all set-up times with an accuracy less than 5 mm (0.2 in.).

Also, the other two drillings — 26 m (78 ft) and 10 m (30 ft) — were completed successfully within just a few days including all set-up times.

All parties, the owner (City of Bochum), the main contractor (Batteux) and the subcontractor (Weitz & Co.), got their benefit in using the innovative and well proven auger boring systems of Bohrtec.

Hagen, Ergster Weg, Germany

For the development of a new construction area at Ergster Weg West in Hagen Germany, a 280-m (918-ft) long sewer, pipe material is HDPE (PE 80) with an outer diameter of 225 mm (9 in.), was planned to be installed with a down grade of 1.88 percent.

The ground conditions were predicted by the planner to be mixed soil with mudstone (up to 35 MPa) and with a slight possibility of sandstone layers (up to 100 MPa).

The owner Wirtschaftsbetrieb Hagen, the contractor Ferdinand Altbrod GmbH & Co, KG, 59889 Eslohe, the first subcontractor Herbert SpiekermannBohrtechnik und Spezialtiefbau GmbH, 57392 Schmallenberg, and the second subcontractor Weitz & Co., 51373 Leverkusen, decided to implement this project by using Bohrtec auger boring equipment.

A Bohrtec auger boring machine BM 400 LS equipped with a 6 m (20 ft) long frame extension and an optical guided Front Steer system FS419 / 24″ with retractable overcut head were chosen to be suitable for this predicted ground conditions.

While performing the job with up to 10 m (30 ft) per day in the mudstone area with and auger with drilling head it showed up that the strength of the sandstone was significantly higher than expected. Strength tests of the sandstone performed after finishing the jobsite proved values up to 180 MPa.

Despite of the very high values of the sandstone Weitz & Co.was – of course with dramatically reduced performance — finally able to jack through this 12 m (39 ft) long part of sandstone successfully without the need of a down-the-hole-hammer.

Had this been know earlier, Weitz & Co.and Bohrtec, might have chosen a down-the-hole-hammer as best for this job. But now, all parties know that the Bohrtec auger drilling machines can even be used in harder soil conditions than expected.


Germany, Ergster Weg Hagen, Front Steer 400 mm

This article was submitted by Bohrtec, based in Germany.

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