The city of Paris has access to a distribution network of potable water, comprising about 1,800 km of pipelines. The pipes transport water for more than two million residents and nearly two million people who work in the city each day. To meet the demand, an average of 680,000 cubic metres of water flow through the pipes daily.

Before being distributed, the water is stored in five principal reservoirs with a combined capacity of 1.4 million cubic metres, which is about a two-day supply. The network needs a very particular kind of treatment in order to avoid leaking and breakage. Such is the case with the old 800 mm diameter grey cast iron pipelines that exhibited weaknesses. Consequently, a 1,260 metre length of pipeline, partly under La Place d’Italie and then running alongside métro line 6 to Denfer-Rochereaux, required replacing.

Non-invasive analysis

In November 2007, a longitudinal break alerted authorities to the deteriorated state of this section of pipe. The reaction was quick and the water company, Lyonnaise des Eaux, implemented its ‘scanner’ system in order to test the state of the iron without cutting out any sections. Though the pipe’s interior had remained in serviceable condition, the exterior was corroded and the overall cast iron structure had weakened over the years.

Normally such a break would only warrant repair on a small section of the pipe. However, it was decided to err on the side of caution and commit to an overall repair. Axéo won the contract, allowing the company to be the sole contractor.

To complete the project, the company encouraged four of its employees to undergo specialised training to obtain an operating license for the lifting equipment. The crane was required to lower the pipes into the underground bore path. In the bore path an indispensable auxiliary machine manipulated the cast iron pipes through the underground tunnel via Pont-à-Mousson, transporting them at a rate of ten pipes every two days. These pipe segments are seven metres in length, but often have to be cut in half in order to pass through the bends of the tunnel. Before deciding upon cast iron steel was considered, but dismissed due to time constraints with welding, as well as the surge in price of raw materials. Discretion and efficiency

As part of the contract, Axéo agreed to minimise as much as possible the inconvenience to residents living along the banks of the river. So, along the entire route, only five shafts were dug, with only three remaining open at any one time. This also limited accidental damage to the concessionary pipeline networks that cram the Parisian underground.

In order to quickly and efficiently complete the project, the company anticipated three project stages: the extraction of the old pipe; the installation of the new pipe; and then the last stage of remediation. The removal is relatively quick and easy work. The grey cast iron segments of pipe are no more than four metres long, so it is enough to cut through the sealing ring and take out the pipe sections.

The installation phase is a little more complex and demanding. Once the completely new pieces of pipe are lowered down one of the shafts into the tunnel, they are transported by a vehicle with two guides that direct its motion – so as to avoid shocks to the sidewalls of the tunnel – and then pulled by a winch. Once in place, each segment is raised to its permanent position by jacks, fitted into the previous piece and put on support blocks.

The final phase has to put into place all the elements that will determine the durability of the new network. Each pipe is supported by two small custom-built columns, and each pipe has a ring around it to prevent it from raising or moving away from the tunnel wall. A knob is then fixed at every second segment to stop the pipe moving closer to the sidewall. This device should prevent movement in any direction. Finally, concrete anchorage is put into place at the level of the pipe’s bends to ensure their stability. Thorough testing

The pipes of this new network will have to withstand a service pressure of six to seven bars. To ensure the structure is water tight, tests are carried out at twelve bars over an hour. The disinfection phase of water analysis follows and then water is permitted to flow through the pipes for distribution.

Even though there are only five shafts along the entire 1,260 metre route, these still remain sites that must be reconstructed once all operations have been completed. In order to facilitate operations, the company has the authorisation to rebuild the roof of the tunnel with concrete. This process, which could be seen at La Place d’Italie, utilises wood shoring that will hold up the new tunnel roof.

The last remarkable aspect of this network is erecting a floodgate, weighing almost seven tonnes, at the third shaft. The pipeline replacement project includes very specific challenges, a great way for Axéo to demonstrate its savoir-faire.