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Sewer service connections created using the auger boring method with GRUNDOBORE400
03.04.18 - Imagine one of the busiest streets in the centre of Prague had to be fully or partly closed off for an extended period of time for car and tram traffic. The local residents and business people would be exposed to construction noise and dirt for months. This would be almost unthinkable on Prague's high street 'Narodni Trida' ('Avenue of the Nation'), built in 1781, with its historic buildings worthy of protecting. If for no other reason, this is why the authorities of the city of Prague decided against an open construction for establishing four sewer service connections and chose to make use of the auger boring technique for installing new vitrified clay pipes ND 200 instead. The spatial conditions locally were too close and the negative impact on the surroundings of the site environment would have been too great, particularly in the case of an open construction for which a 7 m deep pit would have to be dug. The GRUNDOBORE400 auger boring unit from TRACTO-TECHNIK was used for installing the vitrified clay pipes used for the gravity line on the outskirts of the historic city centre of Prague. Not only the special structural conditions on site and the necessity to adhere closely to the course of the slope made heavy demands on the construction company D & Z spol s.r.o, but another challenge awaited the employees of the Prague-based construction company 7 m beneath the surface.
Difficult ground conditions
The city centre is spread over an area of almost 130 ha; this is where the river Vtlava must formerly have had its bed. The workers came across its legacies in the form of up to 15 cm large rocks in the otherwise sandy ground during the non-directional auger bore. "The geological conditions we found were difficult. The sand, which had hardly any stability, was ravaged with larger rocks", remembers Daniel Grunert from TRACTO-TECHNIK, who trained the workers of D & Z on site in the technology and handling of the GRUNDOBORE400. The rocks would have wedged in the auger drill time and again. To eliminate this problem, the conveyor auger had to be rotated alternately in one direction and then the other, until the wedged rock came loose and could then be transported back to the launch pit. Under certain circumstances, the process also had to be repeated en route to the launch pit. Nevertheless, the work progressed considerably faster than with the other methods usually applied by D & Z.
Auger boring instead of tunnel construction
Until they started using the auger boring method, D & Z applied other tunneling methods which were very time-consuming, making an avarage progress of 20 to 25 cm a day, with peaks of 50 cm at best. "The original method would have been too time-consuming and tedious", explains Martin Vesely from INTERGLOBAL, the long-term Czech sales partner of TRACTO-TECHNIK. "Many cubic metres of earth would have had to be moved and an extremely complex construction technlogy with many additional employees would have been needed to carry out the job."
D & Z became aware of the auger boring machine GRUNDOBORE400, which allows pipes to be installed at an extremely high positioning accuracy during the visit to a trade fair. The GRUNDOBORE400 works with exreme efficiency at a maximum feed rate of 2.8 m/min and a thrust of up to 400 kN or maximum pullback of 315 kN. The auger boring machine is suitable for the installation of sewage and waste water service connections right up to collectors for protective and product pipes up to OD 404. The managers of D & Z were quickly convinced of the benefits of the auger boring method. The installation of the new sewage pipelines in the historic centre of Prague became necessary as the dimensions of the old service connection pipes were no longer sufficient. The service connection pipes lead into an existing walk-in brick collector with an oval profile of 2100 x 1300 mm.
Intensive planning, Thorough performance
The starting point for the work in the city centre of Prague was an approx. 7.7 m deep and 1.3 m x 2.1 m wide launch pit, which took almost three weeks to excavate. The assembly and setup of the auger boring machine took another day, meaning the drilling work could finally start on the 19th of May 2017. This was preceded by an intensive planning phase taking approximately six months, during which the client, construction company, machine manufacturer and salesperson worked together intensely.
The OD 273 protective pipe which was welded together from 1 m long sections was driven from the launch pit at an inclination of 8 to 10% in the direction of the collector, no further than 9 m away. Owing to the problematic ground conditions the bore could only be performed at a relatively slow pace and with extreme care to maintain the demanded inclination. For this purpose, the double-ended clamping device as part of the pressure bridge of the GRUNDOBORE400 was the most importent asset for particularly straight pressing in of the pipes, thus providing high directional stability.
Non-Directional Auger boring
In the case of Narodni Trida, one bore took a day, this being due to the challenging geology on site For the auger boring in Prague, protective steel pipes were initially driven through the ground. For this purpose a bore head is attached to top of the inner bore auger at the open end of the protective pipe, which breaks down the soil without displacement. At the same time the loosened soil is transported back to the launch pit in the inside of the pipe with a conveyor auger, thus paving the way for the steel pipe.
While the product pipe, in this case an ND 200 pipe made of vitrified clay, was being pressed in, the steel pipes were pushed from the product pipes into the collector, where the one metre long pieces, which had formerly been welded together in the launch pit, were flame-cut and retrieved in sections. Because a linear course of the bore path was not specified beforehand, it was possible to press the casing pipes in directly, a pilot bore for determining the direction was not required. This procedurally less complex and also more cost-efficient method was possible because of the sufficiently great inclination and the adequate dimensions of the collector, positioned at a distance of no more than 9 m. In contrast to the non-directional method chosen in Prague, for a directional pilot bore, i.e. in the case of a precisely defined linear route, initially a pilot rod with pilot bore head is driven precisely and with positional accuracy from the pit, displacing the soil into the surroundings on its way. After these preparations, an auger boring operation with steel casing or retrievable pipes, guided by the accurately installed pilot bore string, can be carried out.
The actual boring and installation of the vitrified clay pipes was completed successfully with one pipe per day; in total this step took three working days per bore, without any undue negative impact on traffic and pedestrians. An excessive burden on local residents with noise and dirt was also avoided. In addition, an open-cut installation method causes higher costs because the damaged surface of the road has to be restored completely when the work is done, an individual operation which is omitted if the auger boring method is applied. So it comes as little surprise that all parties involved in the project reached a favourable conclusion. Martin Vesely: "Despite all the challenges, we managed to accurately realise the bores with the required inclination and in a comparatively short time. The solution proved to be economical, effective and gentle at the same time." A good reason for D & Z to stick to the GRUNDOBORE400 for other installation projects in the future.
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