Letterbox cofferdam as connecting element

News item
Brievenbuskuip nieuwe zeesluis

The name says it all: the letterbox cofferdam is narrow and deep. OpenIJ hired a special crane to excavate it. The cofferdam is needed to connect the lock gate chamber with the lock sill of the outer head. Planning Engineer Rick Kleiterp explained what it is all about.

‘The enormous lock gates of the new sea lock will soon be running on wheels on the lock sill into and out of the lock gate chamber. But the working method used by the OpenIJ builders for the construction of the lock gate chamber was different from that for the lock sill. The lock gate chamber was built above ground, i.e. at ground level. The structure was then sunk into place by dredging away the soil underneath, the so-called caisson method. But the lock sill is being constructed below sea level in a deep, excavated cofferdam, directly at the required level. To combine these two building methods, a connecting construction element is needed,’ explained Rick Kleiterp, Planning Engineer with OpenIJ. ‘To realise that, we built a narrow cofferdam of 32 metres long, 3.5 metres wide and 24 metres deep between the gate chamber and the lock sill and because this cofferdam is so deep and narrow we call it the letterbox cofferdam.’

Low-vibration method

‘Most of the work mentioned above was carried out during the construction of the cofferdams for the lock sill and the lock gate chamber at the end of 2016,’ Kleiterp continued. ‘In order to prevent damage to the existing lock, we had to use a low-vibration construction method and lowered the sheet piles in a cement-bentonite (CB) support fluid. First we excavated a trench which we filled with the CB support fluid. Then the sheet piles were lowered after which the CB support fluid hardened. After the installation of the sheet piles, we had to wait until the end of 2018 when the lock gate chamber had been sunk to its required depth. As soon as the sinking operation had been completed we could start with the construction of the connecting element and excavate the letterbox cofferdam.’

Smooth excavation with telescopic crane

‘When excavating the soil between two sheet pile walls, the pressure of the soil on the outside tries to push the walls in,’ Kleiterp explained. ‘That is why we first installed a strut frame. A strut frame is a horizontal steel structure – a supporting structure with more or less the same function as temporary struts used for building a roof. After the installation of the strut frame, the cofferdam was filled with water and excavated to a depth of up to 23.5 metres below NAP. The crane stood just in front of the cofferdam and the strut frame was under water. In order to carry out the excavation in a controlled way and still keep up the pace we could have used a crane with a hydraulic digging arm. But excavating the narrow cofferdam and manoeuvring the digging arm through the strut frame would have been quite difficult. A duty cycle crane with a grab would be more convenient, because the grab can move vertically up and down. That's why we hired a special telescopic crane. It combines the best of both worlds: a crane with a hydraulic telescopic arm that can go straight down. That turned out to be a good decision and the excavation went very smoothly.’

Eliminating risks

‘In the subsequent process of excavating, divers will have to go down for welding work,' Kleiterp continued. ‘As early as in the preliminary phase, we realised that the use of the cement-bentonite support fluid mentioned earlier would leave a hardened cement crust clinging to the sheet piling. Chunks of hardened cement could come loose unpredictably – with risks for the divers. That is why we had decided to first clean the sheet pile walls completely with high-pressure water jets. Cleaning would be done partly from above and partly with spray lances through holes drilled in the cement crust. What remained would be cleaned by divers. Although we had foreseen that the cleaning work would be necessary, it turned out to be a time-consuming job. But safety comes first and cannot be compromised.’

Making the connecting element

‘The walls have now been cleaned and we are going to pour underwater concrete to form a 1.8 metre thick floor. This is necessary for the next phase. We must make the cofferdam dry to be able to start with the construction of the passage between the lock sill and the lock gate chamber. But the divers first have to do the necessary welding work. After that we will fill the cofferdam with sand and pump out the remaining water to be able to work at ground level again. Then – little by little – we will remove the sand. We will also install two more strut frames at different levels. According to our planning schedule, the letterbox will be made dry somewhere this summer. Then the work will start to connect the lock sill with the lock gate chamber. The connection will be completed in the autumn. At completion, a person standing on the floor of the cofferdam will have a clear view of 150 meters from the most northerly part of the lock sill to the most southerly part of the lock gate chamber.’