Conjet Robot plays its part renovating Europe’s first sunken-tube tunnel

Published 22/2, 2018 at 15:31

The underwater Maastunnel in Rotterdam in the Netherlands was built using the sunken-tube or immersed-tube method and completed in 1942. This method of construction involved separate parts of the tunnel being built in a dry dock, and then floated into place and sunk into a trench dug in the river bottom. The Maastunnel was the world's first rectangular shaped underwater tunnel built in this way. After many years’ service, the tunnel is undergoing badly needed renovation work, with a new Conjet Robot 557 being used to renovate the top layer of the concrete floor.

Over time, working stresses and humidity have taken their toll on the Maastunnel. Some years ago, tunnel experts discovered that the concrete structure of the tunnel had decayed. Added to this, in 2019 the Maastunnel must comply with the latest EU laws and regulation for safety in tunnels. This led to a complete tunnel renovation being planned as the best solution to meet modern requirements of tunnel safety and efficiency. 

On 3 July 2017, the large scale renovation and restoration of the Maastunnel began. The job was performed in two parts: initially the southbound side was closed for renovation while the northbound side remained open. This was done in order so as not to limit traffic congestion in the Rotterdam city centre, as well as not to affect emergency and rescue services. 

The full scale renovation of both northbound and southbound sides of the tunnel will take around two years. The works will include concrete replacement, new signal and ventilation systems. As the underwater tunnel construction and architecture possess historical significance, much of the old infrastructure, such as the huge, spectacular cast iron ventilation system, will also be restored and preserved as city monuments.


Hydrodemolition challenge

One major assignment of the project is to renovate the 1m thick concrete floor of the tunnel, which lies 1.5m below the roadway. The top layer, with a thickness between 120 and 150mm, has to be removed with hydrodemolition and replaced by new concrete, with the existing rebar also being preserved.

The first stage of the hydrodemolition process started at the beginning of September 2017 and was finished 10 weeks later. In total, some 550m3 of concrete has been removed from the 570m long and 6.8m wide concrete floor of the tunnel.  As soon as the concrete top layer had been removed and the remaining rebar had been cleaned from rust, new concrete was immediately poured in place.

The concrete floor demolition of the tunnel floor was assigned to the Dutch concrete and renovation contractor, Hompert-Renes B.V. a specialist hydrodemolition company. This method was chosen as the most viable and efficient demolition method due to the requirement for low levels of noise and dust emissions.  Additionally, and in order to avoid cracks in the tunnel structure, the original rebar will automatically be cleaned from rust by the hydrodemolition process. 

”When it comes to the actual hydrodemolition, the work is quite ordinary since we are just removing concrete from the floor using two standard hydrodemolition robots,” says Jurjen Volmer, owner of his own hydrodemolition company, Volmer Industrial Services, which is a subcontractor to Hompert-Renes. Jurjen Volmer has four operators operating the two hydrodemolition robots (Conjet 557) 24hrs per day, Sundays to Thursdays. The remaining days are used for removal, cleaning and to refill the sections with new concrete.

”However, since this is a submerged tunnel in a river with a slurry bottom, we can’t remove the concrete straight on from one side to the other. To safeguard the stability of the tunnel we have do this in 94 sections, each section 6m long and 6.8m wide,” says Jurjen Volmer. He continues: "We demolish and remove the concrete in one section, then move on 30m forward to do another one, going back and forth like that to the other side of the tunnel. The water pressure, at which the Conjet robot works, is 1000 bar and it uses 280l of water per minute.”

While the actual demolition is quite simple with the standard Conjet 557 robot, the logistics of the operation are quite complicated. This is due to having to reach the 94 sections of the tunnel by specially constructed bridges, going forward and back again, and removing the demolished concrete out of the tunnel. Hydrodemolition has proven to be the best suited way of effectively dealing with the tunnel renovation, and the use of the Conjet robot has certainly made the demolition parts quite straight forward.

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