Background

What is today called Lausitz Energie Bergbau AG operates the Schwarze Pumpe brown coal power station with a nominal capacity of 1600 MW near Spremberg. The brown coal for the station is excavated in the nearby opencast mine in Nochten and elsewhere. As the brown coal stratum being excavated lies below the natural groundwater table, the water table in the area surrounding the mine needs to be lowered. Two parallel underground GRP pipelines are used to transport the groundwater and mine water from the mine to the mine water processing plant and then to the Schwarze Pumpe power station and other sites as cooling water.

Nochten Water Pipeline 1 (DN 1200) was commissioned in 1998, while Nochten Water Pipeline 2 (DN 1100) was taken into service in 2012. The two pipelines crossed the Spree river on a pipe bridge close to the town of Spreewitz. The water transported from the mine is used as cooling and process water by the Schwarze Pumpe power station and the adjacent industrial company, and as drinking water in neighbouring distrcts.

Problem

Several floods in 2010, 2012 and 2013 in the area around the pipe bridge near Spreewitz resulted in a pipe rupture in a GRP bend in Nochten Water Pipeline 1. Thorough repairs on the existing GRP pipelines would be very time-consuming and have an excessively high impact on mining of the brown coal, which can only operate without the pipelines for a few days. On the other hand, continuous temporary repairs also do not represent a safe solution, particularly as it is likely that the Nochten mine will need to be drained for about another 30 years according to current estimates.

It is also important to consider that both the site of the damage and the entire pipe bridge are situated in a landscape conservation area in the Spree flood plain. Inspections of the pipe bridge also revealed that the bridge piers have been slightly lifted. The resulting deformation of the supports has led, among other things, to deformation of the pipeline and presumably also to excessive stress on the GRP piping. The complex repair of the damaged section would not rule out the risk of further damage to other sections around the pipe bridge.

Alternative solutions

To secure the continued safe operation of the two pipelines, it was therefore necessary to devise an economical and permanent solution that would impact mine operations as little as possible and represent a technically sound solution taking the expected service life into consideration. To this end, 4 different possible approaches to resolving the problem were investigated in an option analysis before the engineering work began. The following were investigated: partial/complete reconstruction of the pipe bridge with related makeshift arrangements for the pipeline and the new construction of a culvert using microtunnelling or the HDD method. The necessary soil investigations were carried out in the run-up and the further planning was then based on them.

AGRU Kunststofftechnik GmbH

Design and planning

A new installation using the horizontal directional drilling method with pipes made from PE 100-RC was the most economic and easiest solution to implement. Pipes made from PE 100-RC are permanently resistant to corrosion from the mine water transported here. Given their high flexibility, wash borings with small installation radii are also possible. Any ground subsidence that occurs can be absorbed by the permissible deformation of the pipes without the danger of damage as a result of cracks or breaks. AGRULINE pipes made of PE 100-RC meet all the requirements and are tested and approved in accordance with PAS 1075 for trenchless laying techniques.

The benefits of the horizontal directional drilling method in this particular instance include the relatively minor impact on nature and the environment and the least amount of underground construction work requiring drainage. A speedy approval process and a calculable geological risk swung the decision in favour of this method. The decision to build a completely underground pipeline made it possible to decommission the pipe bridge. This also reduced the risk of external influences such as vandalism and impacts from temperature fluctuations. Since it was possible to continue to use the existing pipeline during the construction work, it was not necessary to implement a temporary makeshift solution, which meant no work was carried out in the immediate area of the water to complete the construction. There was sufficient space available on both sides for connection.

The disadvantages of the construction method selected lay in the longer length of the pipelines and the amount of drainage needed during installation of the connection pipes from the wash boring up to the existing pipeline.

The invitation to tender was divided into three stages:

• Construction of the wash boring for both pipelines, OD 1400 mm and OD 1200 mm;
• Construction of the connection to the existing DN 1200 mm Nochten Water Pipeline 1 on both sides;
• Construction of the connection to the existing DN 1100 Nochten Water Pipeline 2 on both sides.