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Multiple crossings beneath the river Stockacher Aach
01.07.16 - In Stockach, close to the Lake Constance in the District of Constance, Germany, the installation of a new power supply line demanded several crossings beneath the Stockacher Aach, a river flowing from the west of Upper Swabia. To do so, four HDD bores were required which were made using a GRUNDODRILL 25N and a GRUNDODRILL 18ACS drill rig.
New supply pipes for new control centre
The Municipal Works of Stockach operates a power supply system with approximately 4,900 metering points in the centre of the city of Stockach. Roughly 8,000 citizens and 650 customers from commerce and industry are connected to this power supply system. To guarantee a safe and reliable power supply, the Municipal Works has to invest in ongoing maintenance and development of the complex infrastructure, year after year. The installation of a second control unit in the autumn of 2015 was a further significant milestone for the power supply of Stockach.
Until then, the power supply network was connected to the relay station of the BW networks via a control unit. “This control unit was the central supply unit for the complete city centre and provided a reliable service”, explained Tobias Graf, leader of the power supply team. The steady growth of Stockach as a flourishing residential and employment location requires the adaptation of the power supply to the increasing demand. With the installation of a second control unit, the Municipal Works of Stockach has fortified the network, upgrading it for the future.
The new control unit has been built in Stockach in the area of a paper mill. Utilising a 1,500 metre long feed line, it is directly connected to the relay station of the network operator BW networks and now provides a parallel power source for Stockach city centre via two control units. In the long run, this offers the greatest possible safety and reliability of provision to all customers. In addition, the Municipal Works of Stockach provides a higher performance level.
Power Line Route
Over the course of the power line route some sections were completed in open cut and others using HDD (Horizontal Directional Drilling) techniques. The HDD sections were very complex in some parts because it was usually necessary to cross diagonally beneath the river. The construction had to pass beneath part-man-made river banks, weir steps and very environmentally sensitive strips of river bank. The northern side of the river with its wide-looping course forms the border for the industrial and residential building development. The industrial area has also been expanded with artificial fill up to the edge of the northern river bank. Even though the Stockacher Aach should not be mistaken for the larger river, the Radolfzeller Aach situated further to the west and originating from the largest natural spring in Germany, it is still subject to strong seasonal fluctuations of water flow, not unlike its larger neighbour. Short violent water flows quickly alternate with low water time periods. These severe changes of the water flow can occur within a period of just a very few hours. Given the proximity to the buildings, this may lead to drastically changing situations in a flash.
The Stockacher Aach, along with its little tributary stream within the urban area, the Mahlspürer Aach, flows through coarse crushed terrace rock from the Würm glaciation period. This crushed rock is very coarse and it is usually classified as boulder material, even though the ‘particle’ size may even go beyond this. The origin and composition of this coarse rubble comprises almost the complete spectrum of central to north alpine rock formations. This means that the rocks often have a compressive strength of between 160 and 340 MPa. In particular, the rubble from the Central Alps, of which there is a significant proportion, along with granite, gneiss, amphibolite, diorite, ophiolite, diabase and chert, demonstrates the highest compressive strength values.
Between the rubble and the smallest blocks of this coarse crushed rock there is also found different sand types. Larger chunks made of molasse sandstone were also found in the bore path. The crushed terrace rock from the Würm glaciation period is highly permeable making it a great challenge when utilising drilling technology. Even with superior rock boring equipment, the drilling control in this coarse crushed rock is no simple task and the high permeability of this Ice Age rubble demands very elaborate borehole support. Bentonite alone is unable to achieve the required stability for the large upsizing diameters, therefore eco-friendly polymer additives are used to support the borehole.
HDD operations beneath and along the river Stockacher Aach
The renowned HDD technology contractor Maier Bau was granted the contract to undertake a total of four ‘double’ HDD operations (two parallel bores each with a space of approximately 1.5 m in between each set) beneath the Stockacher Aach and along its bank. The drilling operations comprised several sections for the installation of the required pipe including:
two double fluid-assisted bores of each about 150 m length, pulling a bundle of 3 x 140 mm OD pipe and 1 x 50 mm OD pipe, with on a parallel course to the first bore and with a space of 1.5 m in between, a second bundle with 2 x 160 mm OD pipe and 1 x 50 mm OD pipe
two further double fluid-assisted bores in each about 140 m length, pulling a bundle of 2 x 160 mm OD pipe and 1 x 50 mm OD pipe, again parallel to another bundle of 3 x 140 mm o.d. pipe with 1 x 50 mm OD pipe. The total bore length over all the installations exceeded 500 metres.
To achieve the installations, two drilling rigs were used. The first was a GRUNDODRILL 25 N drill rig (which was used for three of the bores) and secondly a GRUNDODRILL 18 ACS drill rig (which was used for one bore), both machines were.
For the pilot bores, which required approximately 6 to 10 working hours each to complete, hard drill heads manufactured by Tracto-Technik were put to use. The minimum cover beneath the riverbed was predefined at 1.5 metres. In the event a cover of at least 2 metres was finally achieved. Upsizing of the pilot bore holes was carried out in three steps. Step one with a cone reamer of 250 mm diameter, step two required a cone reamer of 350 m diameter and, finally a cleaning run with a 380 mm diameter smoothing reamer. Each upsizing step took about 6 hours in the coarse rubble material. The pipes were pulled in as bundles as described above, with the larger pipes designed to accommodate the single conductor lines of the new medium voltage cable and glass fibre control and data lines being pulled into the accompanying 50 mm diameter pipe. With the exception of sandstone obstacles close to the weir steps of the Stockacher Aach which somewhat slowed down the drilling routine, all drilling jobs were carried out successfully and on schedule, in spite of the really difficult geology of the site. After four weeks of drilling, all four bore paths were passed over to the land surveyor crew of the municipal works for initial measurement.
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