Tuesday, August 16, 2022


District heating offers a very high level of service security and, if produced in a sustainable manner, is also one of the most environmentally-friendly and economical energy sources. For the user, it is inexpensive and convenient. The fact that district heating is still not available everywhere is often due to the costs of developing areas for district heating networks, in addition to various technical aspects. The use of trenchless technology can provide a remedy here in an economically and ecologically-friendly way. As for example in the Bavarian town of Eggenfelden, Germany where a district heating pipeline was installed beneath a reservoir in only 2 days.

Lake Mertsee, on the north-eastern outskirts of Eggenfelden in the lower Bavarian district of Rottal-Inn, is a reservoir on the river of the same name, Mertsee, which was built in 1961 for flood protection and as a local recreation area. The dam, built as an earth embankment, essentially consists of an artificial banked area and rock fill. With a water surface of up to 23 hectares and a height of 15 m above the base of the foundation, Lake Mertsee has a water storage volume of around 1 million m³.

In 2021 a small group of smart minds founded the ‘Energy Corporation Karl-RolleStrasse e.G.’ among other things, for the ‘construction and operation of plants for the generation and supply of heat to members as well as the operation of a heat supply network’. For a few years now, an idea has been spreading around the city to build and operate a sustainable combined heat and power plant that, in addition to the planned residential blocks and various buildings for the Karl Rolle business park in the north of Eggenfelden, would also heat the outdoor swimming pool located there. The latter was to guarantee that the developed concept would work, as a heat consumer for the summer ahead was badly needed, so the open-air swimming pool seemed to fit into this perfectly.

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Construction work started in November 2021, the heat storage tank was installed in early March 2022 and the construction of the district heating pipes began in mid-March. The first section was also the most urgent with the connection line to the outdoor pool, as the pool water is to be heated with ‘the new district heating’ as early as the 2022 swimming season. The shortest, most sensible approach for the protection of the environment and nature, but also most economically favourable route for this district heating connection line to the open-air swimming pool, was clearly across ‘Lake Mertsee’. ‘Directly across’ the reservoir naturally meant, from a technical point of view, ‘directly underneath’ and therefore applying the trenchless horizontal flush drilling method. A conventional open trench construction was not even up for discussion.

With its many years of experience and the high standards the company sets itself, the TPP (Trenchless Pipe Pulling) GmbH & Co KG team is a proven specialist for horizontal drilling technology. When things get tricky, everyone can rely on their professional know-how and their experience of working hand-in-hand. Last but not least, the team also trust their own technical equipment and their reliable and precise drilling rigs. For the planned length of the Lake bore, the expected height difference and the well-known complicated structured subsoil, the team chose to bring a TRACTO HDD rig, the GRUNDODRILL 18N with TD82 drilling rods and maximum torque of 10,000 Nm from their own fleet.

Also, the bore diameter and the diameters of the necessary expanding heads for the 280 mm o.d. SDR 17 PE protection pipe to be pulled in, helped to influence the decision to choose the ‘King of Rock’ machine. The usual work and preparations for the jobsite set-up took place early March 2022 with construction of the starting and target pits on the dam crest at the level of the combined heat and power plant in the Karl-Rolle-Strasse, respectively at the outdoor swimming pool on the other side of Lake Mertsee. Also required was the selection of the appropriate drilling tools and expanders, as well as production of the drilling fluid in a ratio of 35 kg/m3.

Everything was in place for the start of the pilot bore on 16 March 2022. According to the planning, the total distance from the starting point on the crest of the dam level to Karl-Rolle Strasse 43 to the end of the crossing on the opposite bank at the open-air swimming pool site, was measured at 180 m. The bore head had a diameter of 190 mm. Drilling started. The GRUNDODRILL 18N worked its way through the embankment to at least 4 m below the lake’s water level with a total height difference of 15 m in relation to the starting pit. Despite difficult soil conditions, the pilot bore progressed at a good speed with a torque of approximately 6,000 Nm and an average push/pulling force of up to 75 kN; (the machine has 180 kN available) the drilling fluid requirement was just about 90 l/min.

After precisely 180 m the bore head reached the target pit on the opposite bank of the dam as planned. Then it was time to retrieve the bore head and replace it with a stepped reamer of 350 mm diameter for the first expanding process. A stepped reamer with 440 mm diameter was used for the second expanding process and the connection to the protection pipe (which comprised 12 m segments welded together) to simultaneously pull in the pipe string. This then demanded the full power of the GRUNDODRILL 18N, with a maximum torque of 10,000 Nm and 55 kN pulling force the culvert was successfully completed under Lake Mertsee. The insertion of the district heating pipe which comprised twin 75/75, 202 mm diameter pipe inserted into the protection pipe seemed almost a minor issue after the actual culvert construction.

Crossing underneath the Lake Mertsee, for which the TPP team and the ‘King of Rock’ did a really great job, has fulfilled the expectations of the energy co-operative – the open-air swimming pool will open on time for the start of the season and both the heating plant and the construction of this district heating connection fully meet the sustainability criteria. The new combined heat and power plant will save about 83 tonnes of CO2 emissions alone and use local wood chips for its operation. In contrast to the construction of the pipeline with the open trench method, which would have been unreasonably long and complicated, the crossing of Lake Mertsee has resulted in significantly lower costs and also minimal fine dust and noise emissions.

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