Inspection of a culvert pipeline after 44 years of operation

Sep 30, 2019

The Mosel culvert at the “Deutsches Eck” in Koblenz

Culvert cross-section: top left four drinking water pipelines, below left the DN 800 wastewater pipeline, below right the empty pipes for the Bundespost and the DN 1250 wastewater pipeline. [Source: EADIPS®]


The city of Koblenz lies at the confluence of the Rhine and the Mosel. A landmark of the city is the Deutsches Eck with the equestrian statue of the first German Kaiser, Wilhelm I, at the point where the Mosel flows into the Rhine. Because of this difficult topographical location of the city of Koblenz, difficulties arise both in the planning and construction and in the operation of wastewater facilities. Hence sewage disposal represents a particularly major challenge for the Koblenz municipal drainage services (SEK).

At the beginning of the 1970s, in the immediate vicinity of the “Deutsches Eck”, one of the most important sewage structures in the city area was built: the Mosel culvert [1]. This takes about 70 % of the Koblenz wastewater beneath the bed of the Mosel to Koblenz-Lützel and from there onwards via a wastewater pumping station to the central treatment plant after Koblenz-Wallersheim.

In the planning of the project, in addition to the demands for wastewater disposal, the requirements of the former municipal waterworks, the current power supplier Energieversorgung Mittelrhein AG and the former Deutsche Bundespost were also taken into account.

Therefore the Mosel culvert was planned as a package, consisting of two DN 800 and DN 1250 wastewater pipes, four NW 450 (PE) drinking water pipelines and six NW 125 (PE) cable conduits. Particular attention was paid to the material for the wastewater pipes. It was to meet the following criteria:

  • resistance to domestic sewage 
  • absolute tightness of the pipe joints 
  • flexibility of the joints and the material, in order to take up subsidence in the area of the river 
  • resistance to abrasion 
  • possibility of installation using the pull-in process without disrupting shipping

Securing the pipes with steel frames and lowering them to the riverbed. [Source: EADIPS®]

According to [1], vitrified clay and concrete could not be considered “because of their weight”. PE and PVC pipes were excluded “because, for the nominal sizes in question, expensive sheathing would be necessary against deformation”. Ductile cast iron seemed to those responsible to be the most appropriate for the criteria required; above all at extremely demanding steep sections in the Koblenz city area, pipes in ductile cast iron had already proved successful.

In order to increase the abrasion resistance and strength of the ductile wastewater pipes even further, in each case an internal coating was provided: The DN 800 pipes were given a 6.4 mm thick lining of special mortar and the DN 1250 pipes a lining with a 1 mm thick coating of epoxy resin because, for technical production reasons, a mortar lining could not be applied. The length of the culvert to be constructed from outlet structure to inlet structure is 294 m.

In order not to obstruct shipping in the area of the work, the assembly was divided into three sections: two of 119 m in each case and one of 56 m. These were assembled on a kind of building slip (as can be found in a shipyard), parallel to the landside route of the culvert. The pipes were secured with 4 m wide steel frames and lowered with the help of barges into the already prepared channel in the river bed. Pressure testing has already been done with part sections on land.

In addition to the culvert pipelines (two DN 800 and DN 1250 wastewater pipelines, four NW 450 drinking water pipelines and six NW 125 cable conduits) the culvert upper head and the culvert lower head (Schartwiesenweg pumping station) with its four rainwater pumps and four wastewater pumps also belong to the complex components of the culvert.

The culvert and its structures. [Source: EADIPS®]

The reasons for the culvert inspection

The legal basis for the inspection of the Mosel culvert was and is Annex 3 (§ 4) according to the Rhineland- Palatinate State regulation on the self-monitoring of wastewater plant, EÜVOA/SÜVOA, [2]. According to this

  • “The correct working condition of sewers and wastewater pipelines is to be checked at least every ten years by means of optical inspections.
  • In water and mineral spa protection areas, shorter periods apply in accordance with the generally recognised rules of the trade. 
  • For new or as-new sewers and wastewater pipelines, the first two repeat inspections after commissioning are to be carried out after 15 years in each case.”

Apart from the necessary evidence of the tightness of the culvert pipelines to be provided for the supervisory authority, the concrete constructions in the upper head of the culvert were found to be in a critical state. The frequency of maintenance for the pumps in the Schartwiesenweg pumping station was seriously increased because of a build-up of wet-wipes. Clean-up measures here were therefore quite clearly urgent.

But as regards the overall construction of the culvert, the two culvert pipelines, DN 800 and DN 1250, were naturally also under discussion. Here the Koblenz municipal drainage services were faced with one quite decisive question with far-reaching consequences: Is the investment for renovating the adjoining structures of the upper and lower culvert heads including the pumping station still economical at all in view of the lack of knowledge about the condition of the two culvert pipelines which have been in operation for 44 years? And then: If the culvert should prove to be unfit for further use or no longer capable of renovation, definitive and quite different planning considerations would have to be explored. Therefore it was also necessary to evaluate the condition of the two culvert pipelines and to do this in the context of an inspection.

Unknown parameters: residual wall thickness s and the interplay of buoyancy and supporting forces. [Source: EADIPS®]

SLOFECTM functioning principle. [Source: EADIPS®]

The prerequisites for inspecting the culvert pipelines

As the EÜVOA/SÜVOA regulations for the monitoring of sewers only require an optical inspection, the initial deliberations looked at a mutual cleaning of the culvert pipelines followed by conventional video inspection. To do this, a pipeline must always remain in operation in order to ensure dry weather flow. However, this type of inspection necessitates complete emptying of each pipeline in turn, which did not appear practicable.

A further prerequisite for optical inspection is a guarantee of buoyancy control. A regular sounding in 2014 did in fact show that there was a sufficiently high covering of the culvert pipelines, meaning that buoyancy control was ensured. Nevertheless, the officers of the municipal drainage service considered the risk of exposing the two pipelines to the interplay of buoyancy and supporting forces without knowledge of the remaining wall thicknesses after more than 40 years of continuous operation as being too high. Hence, optical inspection was evaluated as being too uncertain and risky.

Limiting conditions for the inspection process

Very time-consuming: intensive pipe cleaning by pig (for each pipe the team worked 24-hoursa- day for four days). [Source: EADIPS®]

Therefore, as an optical inspection was obsolete, it was necessary to look for and find another solution. In order to gather and determine all facets of the problem as to what the inspection process needed to deliver, the officers of the Koblenz municipal drainage service (SEK) thought it wise to define the major limiting conditions for this first of all.

The inspection process should:

  • function with the culvert pipelines complete full 
  • supply information on serviceability 
  • provide data on the remaining working life 
  • provide data on the condition of the culvert pipelines 
  • be able to be performed in an acceptable and calculable period
Requirements for the inspection system

Thus the sewerage department of the Koblenz municipal drainage service had defined the essential framework conditions for the inspection process. On top of this, an inspection process with particular features needed to be found. Together with the engineering company, Ingenieurgesellschaft für Wasser-, Abwasser- und Energiewirtschaft mbH Tuttahs & Meyer from Andernach, which was commissioned for the planning and tendering of the project, they then specified the requirements for the inspection system to be selected, which almost exclusively related to the accessibility of the object to be inspected:  

  • The culvert pipelines are not designed for inspection by pig. There are no pig traps. The insertion of an inspection device has to be done at the lower head of the culvert. 
  • Access to the culvert pipelines is only possible from one end. The inspection system must allow bi-directional operation. 
  • The inside surface of the culvert pipelines is coated. The inspection system must be able to be used for checking the pipe wall through the coating. 
  • Thorough cleaning before the inspection is not possible. The surface may still present residues or deposits. Inspection must also be possible in the presence of low-level residues or deposits. 
  • The pipeline needs to be inspected in the filled state, whereby complete filling without air bubbles cannot be guaranteed. The inspection system must be able to be used regardless of the coupling medium under water and under atmospheric conditions.
The approach for the inspection

As a solution for the complex inspection of the two culvert pipelines, Ingenieurgesellschaft Tuttahs & Meyer proposed an inspection system which examines the metal piping system for corrosion by means of an eddy current using SLOFECTM (Saturation LOw Frequency Eddy Current) technology. Alongside other systems considered, it was only this one which proved to be practicable for this particular purpose.

The PLS type SLOFEC® internal pipe scanner. [Source: EADIPS®]

Lowering the SLOFEC® scanner into the lower head of the culvert. [Source: EADIPS®]

SLOFECTM is based on the technique of the eddy current process in which the area of the component being recorded is premagnetised and tested with relatively low-frequency eddy current signals. The magnetic field apply is disrupted by local instance of inhomogeneity in the pipe wall (e.g. the consequences of corrosion). And it is precisely this disruption which is displayed on the basis of the eddy current field induced at the same time. The eddy current probes applied, which are located between the poles of the magnets, are seamlessly joined together, thereby producing a close-meshed measuring range of the modular-based system.

The engineers from Andernach convinced the officers from the Koblenz municipal drainage service of the application and success of this inspection technology which, until then, had only been used in industrial plant and so was being applied for the inspection of a wastewater culvert for the first time in Germany. The public call for tenders was then carried out for separate lots: culvert cleaning and culvert inspection.


The contract for the cleaning was won by Norand Industrieservice GmbH from Löbnitz, the one for the inspection went to 8SEAS consulting engineers – water + energy from Nackenheim. 8SEAS commissioned the experienced company KontrollTechnik GmbH from Schwarmstedt, which specialises in inspection processes, to perform and supervise the work. This company had developed various types of SLOFEC® internal pipe scanners for inspecting underground pipelines in industrial plants.

Pipe cleaning

As the inspection system requires absolutely clean pipes, the first stage consisted of the intensive cleaning of the two culvert pipelines. Norand Industrieservice GmbH from Löbnitz needed four working days of 24-hour operations per pipeline in order to pull through the correspondingly large cleaning pig with a winch and hydraulics support.

The problem with wet-wipes is also worth mentioning here: during the pigging process, the wetwipes were adhered to large surface of the pipe wall; only by using flushing nozzles could they be released from the pipe wall and then be sucked away. After complete cleaning the pipes were filled with clean water.

3D positional measurement

After the very time-consuming cleaning of the culvert pipelines, its XYZ position beneath the river had to be determined. To do this, 8SEAS consulting engineers under commission by the Koblenz municipal drainage service used 3D gyroscopic measurements with Ductrunner measurement technology.

The sensors required for measuring the three-dimensional positional data, the energy supply and the data storage unit are integrated into the measurement probe. The sensors consist of gyrocompasses and accelerometers. At each measurement point, the changes in the direction and the speed are recorded. As this technology is not dependent on GPS technology it is particularly suited for use underground.


The non-destructive inspection of the pipe walls was then carried out with the PLS type SLOFEC® internal pipe scanner. It started in the Schartwiesenweg pumping station in the lower head of the culvert, travelled through the culvert pipelines (DN 800 cement mortar lining, DN 1250 epoxy lining) and ended at the upper head of the culvert.

The wired scanner was lowered into the culvert opening at a depth of approx. 15 m and then inserted into the culvert. The SLOFEC® scanner did not have its own drive system for this inspection but was winched through the culvert. For data recording purposes it was positioned in an axial direction by the winch, with the sensor head “pressed” against the pipe wall and moved circumferentially; the corresponding inspection section was 150 mm. After completion of a full circumference scanning by the rotating sensor unit, the scanner was pulled forward by a further 150 mm the axial direction and a new measurement started until the complete data recording across both culvert pipelines was finished.

Positioning the scanners at the culvert opening. [Source: EADIPS®]

Representation of measurement data and results for the DN 800 wastewater culvert. [Source: EADIPS®]


The data recording and the analysis of the inspection delivered some surprising results: Even after 44 years of continuous operation, strong currents and flooding, the ductile cast iron culvert pipelines only showed slight corrosion along the whole of their outside: at the “foreshore”, most areas were detected as having only slight local inhomogeneities (weak corrosive attack); it was only at the start of the inspection run, close to the pumping station (lower culvert head) that areas of more severe inhomogeneity had formed. On the Mosel riverbed no damage at all was found on the inside and outside of the pipes, in other words the pipe wall is unchanged after 44 years of operation!

From the evaluation and analysis of the data obtained there was (and is) no need for renovation of the culvert as any significant impairment to wall thickness by corrosion and/or other ageing damage was able to be definitively excluded, even in the lower head areas of the culvert.


The first testing system to be used in Germany for the inspection of a wastewater culvert, looking for corrosion using SLOFECTM (Saturation LOw Frequency Eddy Current) technology had its premiere in Koblenz with great success. With the results showing the good condition of the culvert pipelines, the way was opened for the investment to renovate the dilapidated concrete structures of the adjoining culvert upper and lower heads and thus further long-term use of the culvert structure as a whole.

The renovation of the concrete constructions in the upper head of the culvert was finished in 2017 and the “Schartwiesenweg” pumping station in the lower head of the culvert is planned for renovation as from 2020.

Based on the experiences with the pipelines of the Koblenz Mosel culvert, for future inspections of this kind there is still a need for improvement on two points:

  • because of the soiling matrix, an addition process should be used in support of cleaning by pig, such as the pulsed flushing process
  •  the inspection tool should be better protected against solids (the wet-wipes problem) for use in the field of sewage

The success of this culvert inspection is built not only on the sophisticated technology of the process but also on the boldness of all concerned, according to Ernst Ferstl, Austrian lecturer, writer and aphorist: “beating a new path means managing without signposts”.


[1] Weber, H.: Abwasser-Düker aus duktilen Gußrohren NW 1250 und NW 800 durch die Mosel. fgr Fachgemeinschaft Gußeiserne Rohre Jahresheft 6 (1972), p. 18-20

[2] EÜVOA / SÜVOA Landesverordnung über die Eigenüberwachung von Abwasseranlagen dated 27 August 1999, GVBI. p. 211, last modified on 14 July 2015, GVBI. p. 127, 160. Ministry for the Environment, Energy, Nutrition and Forestry. PDF document from Wasserwirtschaftsverwaltung Rheinland-Pfalz

Hans-Jörg Schulz
Eigenbetrieb Stadtentwässerung
Kammertsweg 82
D-56070 Koblenz
Phone: +49(0)261 129-4039
Wilhelm Kelb
Kontrolltechnik GmbH
Im Laab 23
D-29690 Schwarmstedt
Phone: +49(0)5071 981511


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