Milestone in guided auger boring

Nov 26, 2009

A novel steering technology from Bohrtec, extending the range of applications for guided auger boring techniques, has been used for the first time on two pipe eating projects in Germany.

Despite the fast-paced development of guided auger boring technology using pilot rods in the past ten years, these techniques can only be used in displaceable grounds. Therefore the conventional ground displacing pilot drilling technology cannot be used in dense material (SPT values of standard penetration tests > 35) and in rock.
With the development of a novel steering technology, the patented Front Steer, Bohrtec has for the first time overcome the application restrictions and enabled applications in very dense strata with SPT values > 35 and in moderately strong rock with strengths of up to 10 MPa. This is achieved with the help of a guided auger boring system that excavates the ground as it advances.

It should also be noted that this significant extension of application possibilities does not affect the economic advantages of the guided auger boring using pilot rods because the guided auger boring system with the Front Steer retains the advantages of quick and easy set-up of the equipment onsite and simple operation.
Guided auger boring with the Front Steer
Just like the conventional ground displacing methods – the Front Steer system uses the proven optical guidance system consisting of theodolite with CCTV camera, monitor and LED target board. The ground/rock is continuously excavated by the cutting head and then transported to the starting shaft by augers with a hollow centre for the optical path. The steering pipe uses the ground reaction force for steering. The machine operator can perform steering movements remotely by tilting the steering pipe in either manual or automatic mode. The control panel shows the respective steering position of the head.
Pipe eating in Wilmersdorf, Berlin
Although the Front Steer technology was developed for non-displaceable ground and weak rock, the system had a very special challenge on the first project in Berlin. In the course of the Redevelopment Area Forckenbecken construction project in the Wilmersdorf district of Berlin, an existing stoneware DN 175 sewer under Misdroyer Street needed to be replaced by a new DN 300 sewer on the same alignment. The planning of the Berliner Wasserbetriebe (Berlin Water Service) included online replacement by trenchless construction using the pipe eating method because the 60 metre long line passed below six mature trees with trunk diameters of 300 to 350 mm.
Frisch & Faust – experienced with Bohrtec pilot drilling systems – found this was a perfect occasion to use the new guided auger boring method with Front Steer. The BM 500 optical measuring system and the appropriate steel casings with hollow augers were already at hand. Therefore, only the Front Steer and the operation and control panel had to be provided in addition to the existing equipment.
The soil layers classified as LBM 2 were rated on site as LBM 3. The six house connections on this length of sewer were diverted in advance. It was necessary to remove the house connection branches before beginning the drilling works as they were fixed to the old sewer with three metal clamps. Due to water pipes, gas pipes and electrical lines across the starting shaft the pit was supported with wood sheeting with a concrete thrust wall. One week before beginning the drilling works the existing sewer was grouted up.
After completion of all preliminary works by Frisch & Faust, the Bohrtec team arrived with the Front Steer equipment and in only a few hours successfully drilled the first metres. From the beginning the control of the new steering head was excellent and even the different soil conditions – with a sand and gravel bed below the old pipe with stone surround and soils of the classes LBM 2/LBM 3 as well as the grout in the old line – did not lead to any problems with regard to the soil clearing or the steering.
Following a short induction period with the new steering system John Adams, an experienced machine operator with Frisch & Faust, was able to steer the head independently due to the simple operational handling. The Frisch & Faust team, well-trained in many conventional pilot drillings, achieved a drilling performance of about four metres per hour including all coupling and other activities on their first day. At the end of day 25 metres of 419 mm diameter steel casings had been driven successfully, although the drilling works only began late in the morning.
After only two days, 54 metres had been driven without incident. Although the installation of the steel casings could have been completed on the second day, the works were stopped short of reaching the target shaft in order to demonstrate the Front Steer driving into the target shaft and to explain the operation to representatives of the Berliner Wasserbetriebe.

After reaching the target shaft, the steering head was uncoupled and the team started to push in the DN 300 clay jacking pipes. It took only four days to complete the 60 metre drilling length to the satisfaction of all parties and the pipe eating premiere with the new Front Steer proved to be a complete success.
Success inspires alternative for Berliner Strasse/Treskow Allee
Inspired by the successful use of the Front Steer on the Misdroyer Street project, Frisch & Faust prepared an alternative proposal for the Berliner Strasse/ Treskow Allee project. An existing DN 200 clay sewer pipe had been in operation for several years but because of further development an additional connection to the main sewer became necessary. One section of this replacement sewer, which passed beneath the rails of a tramway, was to be performed using Trenchless Technology, while open trench was intended for the installation of the rest of the pipe run. This original plan envisaged leaving the abandoned and backfilled sewer in the ground since laying the new pipe on the same alignment would not have been possible without disturbing the rails.
As an alternative, Frisch & Faust proposed the online replacement of the existing sewer by pipe eating with the Front Steer system. The advantage of this proposal was the fact that the risk of meeting unknown obstacles was minimised by following the existing line.

After approval from Berliner Wasserbetriebe the preliminary works were completed and the Bohrtec Front Steer was used a second time for the online replacement of an existing sewer. Despite a smooth beginning to the drilling works, after five metres there was a dramatic increase of the pushing force and the cutting wheel torque. It was presumed that the machine had struck a sheet pile because according to the construction plan there had been some sheet piles at this position of the old line which should have been cut down to a depth of about three metres, but this was obviously not the case.

Despite the risk of drilling head damage, it was decided to continue tunnelling because the cost of disturbing the rails and the consequential cost for the rail replacement bus service would have been considerably higher. The sheet pile remains were passed despite much wear and tear of the cutting tools and the full length of 28 metres was completed successfully.

The Front Steer proved to be a fortunate choice, especially with regards to the sheet pile. If a conventional ground displacing drilling system had been used – as originally planned – the drilling would have been stopped when the sheet pile was encountered, thus disturbing the rails and requiring a special bus service.
Conclusion
Although the featured projects, achieving trenchless pipe replacement with the pipe eating method to remove the old line, do not really represent the planned standard use of the new Front Steer system, it has proven effective for guided auger boring even under these extreme conditions. With the development of the Front Steer system, Bohrtec has 'crossed the Rubicon' by extending the range of application of the well proven and economical guided auger boring techniques successfully used for small diameters and short lengths into non-displaceable ground (SPT > 35) and rock with strengths of up to 10 MPa.

Contact

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52477 Alsdorf

Phone:

+49 (0) 2404 9881 - 0

Fax:

+49 (0) 2404 9881 - 10

E-Mail:

info@bohrtec.de

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