Construction of the new lock in Terneuzen using pneumatic caissons

Abstract

The lock complex at Terneuzen is the main entrance for ships coming from the Westerscheld and navigating towards the cities of Terneuzen and Ghent. The construction of a new large lock at the lock complex of Terneuzen is planned to increase its capacity. This lock will be built between the existing locks. The ground level is situated at NAP +6 meter. The minimum normative sea water level occurring twice a year is NAP -2.85 meter. The new lock will have larger dimensions than the current locks. It is assumed that a ship with a draft of 13.1 meter must be able to use the lock tide independently. Because of the large allowable draft the lock must be founded to a depth of NAP -17.26 m. It is becoming increasingly difficult to construct the new lock with traditional building methods like combi– or diaphragm walls to such extent. In this master thesis the use of pneumatic caissons as alternative construction method for the new lock in Terneuzen has been elaborated. Pneumatic caissons have been used for many times as construction method. However, pneumatic caissons have never been used with the dimensions required for the new lock head. (l = 132m, w = 45m & h = 33m) The pneumatic caisson method involves the construction of a relative rigid concrete box at ground level that is lowered into the ground by excavating the soil underneath it. Under the bottom slab of a pneumatic caisson an air-pressurized space, called the working chamber, is present which is made up of tapering walls around the perimeter of the caisson base slab. These tapering walls are called the cutting edges of the caisson. The working chamber is kept dry by the presence of air pressure. Due to the air pressure, groundwater is not able to enter the working chamber. The deeper the caisson is located below the water table, the higher the air pressure should be. In this master thesis two construction alternatives for the lock heads with help of pneumatic caissons are elaborated: 1. The construction of one large pneumatic caisson with a length of 132 meter, width of 45 meter and a height of 33 meter containing the complete lock head including the gate chamber and gate recess. The working chamber is divided into 14 compartments. 2. The construction of two (compartmentalized–) pneumatic caissons which can be subsided independently. One caisson with a length of 10 meter, width of 45 meter and a height of 33 meter is covering the gate recess and the other caisson with a length of 67 meter, width of 45 meter and a height of 33 meter is covering the gate chamber. Between the pneumatic caisson an immersible caisson is located. This caisson acts as foundation for the guiding rail and moving equipment and prevents piping below the lock gates. With help of SCIA Engineer, a software package for structural calculations, the structural feasibility of the caisson is analysed and worked out. Due to the lack of bending and torsional stiffness in the first construction alternative the occurring forces and moments are up to 2 times larger in comparison to the second construction alternative. The use of pneumatic caissons to construct the lock head has some advantages over the use of a traditional building pit. The building time of 22 months is 8 months shorter in duration compared with the building pit and the building costs are with respectively Euro 48.300.000 and Euro 37.700.000 much lower than the building costs of a building pit which is estimated at Euro 52.600.000. Moreover the pneumatic caissons can be constructed in controlled conditions above the surface level. Despite of the advantages the construction method has a large number of disadvantages. Different conclusions can be drawn up. Some conclusions are in favor of the construction with pneumatic caisson. On the other hand the construction method has some important disadvantages that can not be neglected. A considerably large working space is required for the sedimentation basin and bentonite de-sanding installation during subsidence of the caisson(s). The possibility of rotation of the caisson during subsidence and working under an overpressure increase the associated risks. It should be taken into account that about 50% of the surface area of the bottom slab must be reinforced against shear. Also a large amount of bending reinforcement (101 kg=m3–219 kg=m3) is required. Moreover there is less practical experience with the construction of pneumatic caissons. A comparison between the options on feasibility, safety, risk, required materials, building time and costs shows that the reference design, the use of a traditional building pit to construct the lock head, is the best conceivable option.Hydraulic EngineeringCivil Engineering and Geoscience