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Collector renewed in the Minderbach valley by means of pipe bursting

08.04.19 - By Dipl.-Geogr. Gerhard Renz, Isas GmbH, Site Manager, Germany
Photos: Isas GmbH

The nature conservation authorities agree: small intervention with a big effect! The collector of AZV Nagold was renewed continuously over a length of 1,200 m by means of burstlining, the cross section was expanded from DN 300 to DN 400. The minimally invasive intervention in the sensitive eco system of the Mindersbach Valley was fortunately also the best economic solution.
The Mindersbach Valley nature reserve is located in the Northern Black Forest, between Mindersbach and Ebhausen. The wastewater duct of AZV Nagold that runs through the Mindersbach Valley was badly damaged. Cracks, broken fragments, ruptured pipes, root intrusions, pronounced deformations and massive clear water infiltrations made necessary a renewal of the existing stoneware sewer DN 300. A DN 400 mm duct was needed hydraulically.

Most of the sewer pipe sections were located entirely in meadow area and woodlands, often in rough terrain. It fell into decline. The height difference of the rehabilitation section was approx. 70 m. Individual sewer pipe sections affected the mainly parallel service road. Driving on the service road with heavy machinery was prohibited. "Normal construction traffic" was allowed. The local conditions, the damage and the groundwater situation in the area of the wastewater pipes to be rehabilitated suggested a renewal by means of burstlining taking into consideration the improvement of the hydraulic capacity of the collector.

In order to ensure accessebility to the construction site with loads of up to 40 t, a few temporary construction roads were enough. Insertion lengths of over 260 m at one time had to be realised. The expansion from DN 300 mm
to DN 400 mm meant a big challenge for the machine technology, the material and the construction site parties on site. The planning work was done by the Isas engineering firm. Three excavator prospecting works were carried out in the run-up to the planning work to explore the underground conditions in the rehabilitation area. A sign of relief! The existing soil was classified as displaceable by the geologists.
 

LITTLE SCOPE

The nature conservation requirements (§ 5 No. 4 NSG-VO "Mindersbach Valley") as well as the Federal Act for the Preservation of Nature also had to be observed. A site visit with the responsible district administration was carried out for the detailed implementation of the nature conservation issues for the measure. The need for an ecological report defined during the appointment happened at the same time as the design planning by a specialist office for forest and landscape ecology.
 
The receiving water control also had to be suitable. The pipelines for the wastewater guidance were installed parallel to the creation of the construction roads. In order to meet the nature protection requirements and prevent the escape of wastewater through leaking hose pipes, the sewage transfer pipeline was realised with welded PE pipes. The receiving water control had to be designed for average rainfall. In the case of heavier rainfall, as a result of which wastewater production of up to 150 l/sec was possible, the reduction was done into the neighbouring receiving water, like in the condition before the rehabilitation.
 

STATIC AND DYNAMIC

And then powerful machines were needed and thus concrete thrust bearings with two layers of steel in the machine pits. On the one hand, there was "pulling", i. e. "static" pipe bursting: Renewal sections up to 260 m were realised in five installation procedures. Overall, 18 of 24 sewer pipe sections were expanded at a full 190 t tension force and with a bursting head with DN 560 mm.
 
For the remaining five sewer pipe sections it was better done "dynamically". Owing to a shorter burst section and angular deflections to the other sewer pipe sections, a press bursting method was used. This is more economical. Substantial pressure is built up here with a 80 t thruster behind the bursting head and a 40 t carriage.
 

GOOD WORK OF ALL INVOLVED

In order to only receive tenders from reliable and high-performance companies during the official tendering stage, certificates and documentation are required, e. g. quality labels and welding certificates. Rettberg GmbH from Göttingen was able to provide this documentation. With this company one had an experienced specialist on board; the realisation of the 900,000 euro construction measure was a challenge, whose implementation was perfect – one step after the other. After five weeks the necessary preliminary work was completed: the creation of the construction road, the construction of the receiving water section as well as the first construction pits.
 
Then it was time for the welding. 12 m jacking pipe rods made from PP-HM DA 450 were connected at the surface beside the construction road or the residential street by means of heated coil butt welding. A high standard was placed on the pipes with the necessary tensile strength and scratch resistance. Pipes made from special highly rigid polypropylene by Schöngen KG, Salzgitter (Concept HL pipes) were used and proved to be appropriate.
 
At the same time, the press-burstlining method was realised from pit to shaft with 1 m short pipes with snap-in connection by the subcontractor Alfes + Sohn from Wenden. Also here a specialist in the area was acquired for the measure.
 
Inspection and maintenance work can be easily performed across long distances using the sewer cleaning and TV inspection systems available on the market today. Most of the existing shafts were cleaned and filled after the burstlining. With regard to the external water situation or the temporary position of the wastewater pipes in the groundwater, the possibilities of potential external water penetration in shafts was also reduced.
 
The job was realised in autumn/winter 2017/2018. Peter Haselmaier, Technical Manager of AZV Nagold, is impressed: "The planning firm Isas and the executing company Rettberg performed outstanding work. The nature conservation authorities are satisfied. There are no complaints from the affected residents. The construction schedule and costs were observed. In brief: a well-rounded project".
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