„Front Steer“ – Premiere in Berlin

New Milestone in Guided Auger Boring

A novel steering technology from Bohrtec to extend the range of application of guided auger boring techniques was used for the first time in two pipe eating projects.

Despite the fast-paced development of the technology of guided auger boring using pilot rods in the past 10 years, these techniques can only be used in displaceable grounds. Therefore the conventional ground displacing pilot tube microtunneling technology cannot be used in dense material (SPT values of standard penetration tests > 35) and obviously in rock.

With the development of a novel steering technology, called Front Steer (patented), Bohrtec have 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 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 on site and simple operating.

Guided auger boring with the Front Steer

Just like the conventional ground displacing methods – the Front Steer system uses the well proven optical guidance system consisting of theodolite with CCTV camera, monitor and LED target board. As shown in figure 1 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 and the control panel shows the respective steering position of the head.

60 m Pipe Eating in Berlin Wilmersdorf

Although the Front Steer technology was developed for non-displaceable grounds 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 Berlin district of Wilmersdorf an existing sewer under Misdroyer Street (stoneware DN 175) needed to be replaced by a new DN 300 sewer on the same alignment. From the beginning the planning of the “Berliner Wasserbetriebe” (Berlin Water Service) included on-line replacement by trenchless construction using the pipe eating method because the 60 m long line passed below 6 mature trees with trunk diameters of 300 to 350mm. 

When the Berlin company Firma Frisch & Faust – who already have different Bohrtec pilot drilling systems– received information about this project, they found this was a perfect occasion to use the new guided auger boring method with “Front Steer” which excavates the ground as it advances. The BM 500, optical measuring system and the appropriate steel casings with hollow augers were already at hand. 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 6 house connections on this length of sewer were diverted in advance. When doing this it was found to be necessary to remove the house connection branches before beginning the drilling works because they were all 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 of the drilling works the existing sewer was grouted up.
After completion of all preliminary works by the company Frisch & Faust, the Bohrtec team arrived with the Front Steer equipment and after a few hours had already successfully drilled the first meters. From the beginning on the control of the new steering head was excellent and even the different soil conditions – sand / 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.

Already after a short induction period with the new steering system Mr. John Adams – an experienced machine operator with Frisch & Faust – was able to steer the head independently and without any problems due to the simple operational handling. The team of Frisch & Faust, well-trained in many conventional pilot drillings achieved a drilling performance of about 4 meters per hour including all coupling and other activities on their first day. At the end of day 25 m of Ø419mm steel casings had already been driven successfully, although the drilling works only begun late in the morning.

After two days 54 meters were driven without special incidents. Although the installation of the steel casings could have been completed on this second day the works were stopped short before reaching the target shaft in order to be able to demonstrate the Front Steer driving into the target shaft and to explain the operation to representatives of the “Berliner Wasserbetriebe” the next morning. 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 4 days to complete the 60 meters drilling length to the satisfaction of all parties involved and the Pipe Eating Premiere with the new Front Steer proved to be a complete success.

Specific proposal for the construction project 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 main sewer became necessary. One section of this replacement sewer which passed beneath the rails of a tramway was planned to be performed in trenchless construction 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 proposal Frisch & Faust proposed the on-line replacement of the existing sewer by Pipe Eating with the Front Steer system. The essential advantage of this proposal was the fact, that the risk of meeting unknown obstacles was minimized by following the existing line.

After approval from „Berliner Wasserbetriebe“ the preliminary works were done and the Bohrtec Front Steer was used a second time for the on-line replacement of an existing sewer. After a smooth beginning of the drilling works – the steering of the Front Steer as well as the excavation were as smooth as on the first Pipe Eating construction site – after 5 meters there was a dramatic increase of the pushing force and the cutting wheel torque. It was presumed that a sheet pile had been struck since 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 3 meters, 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 distinctly higher. The sheet pile remains were passed despite much wear and tear of the cutting tools and the full length of 28 m was completed successfully. The choice of the Front Steer proved to be a lucky choice especially with regard to the sheet pile. If a conventional ground displacing drilling system had been used  – as originally planned – the drilling would have had to be stopped when the sheet pile was encountered, thus inevitably requiring the rails to be disturbed and the provision of a special bus service.

Although the described projects of trenchless pipe replacement using 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 proved to be 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.

By Dr.-Ing. Gregor Nieder