Adapted drainage technology for renovation of quay walls in Antwerp
The existing quay walls located on the right bank of the Scheldt in Antwerp are being renovated. The company THV Scheldekaaien has worked with G. Smeyers for the dewatering works from the tender stage onwards. G. Smeyers carried out the dewatering study through a 3D computer simulation and is currently working on the site, where the well pointing for the excavation of the anchor wall is done with vertical filters and piston pumps on two levels.
On 25 November 2013, Waterwegen en Zeekanaal nv issued a tender for the renovation of the existing quay wall on the right bank of the Scheldt. The works are located at the d’Herbouvillekaai and Ledeganckkaai in Antwerp. THV Scheldekaaien, a temporary trade association of the companies Artes Roegiers, Herbosch-Kiere and Hye, has expressed from the tender stage, its wish to work with G. Smeyers from Zandhoven for the execution of the dewatering work. An extensive dewatering study based on a 3D computer simulation had to be enclosed with the tender documents. “Since we have our own specialised study department, the preparation of such a study was not a problem for us”, says G. Smeyers.
The union of THV Scheldekaaien and G. Smeyers nv proved to be successful; after a brief revision of our offer, the assignment of the first subcontract was entrusted to THV Scheldekaaien.
Renovation of 19th-century quay walls
The existing quay walls were erected at the end of the 19th century. This happened in two parts; the northern part measuring approximately 3.5 kilometers was built between 1878 and 1886, the southern part (also known as Zuiderkaaien) measuring approximately 2 kilometers was built between 1897 and 1903. The quay walls to be stabilised in the wharf zone are located on the exact border between the northern and southern parts.
The quay walls were built using the same technology in both zones. The foundations were formed by separately pneumatically sunk steel caissons. The workrooms at the bottom of the caissons were filled with concrete after sinking. A separate wall was installed on each foundation caisson up to the low-water line under the shelter of a drying tank. The joints between the individual sections under the layer line were filled with concrete. Above the low-water line, the brickwork was carried out continuously over the adjacent sections.
The stability of the quay walls soon proved to be insufficient. As of the completion of the quay wall, subsidence and shifts of the quay wall were observed.
The renovation of the quay wall was split into five sections of approximately 100 meters in length. Phases 3 and 4 are now being implemented. First, a tubular pile wall is driven into the Scheldt for the existing quay wall. The construction pit is further being closed off by driving pile-in piles on both ends of the construction pit as well as on the land side of it. Both the combi wall and the sheet piling are driven deep into Boom clay. A watertight construction pit is thus formed. Within this tub, the existing quay wall is partly demolished “in the wet” and the tub is emptied with an open drain.
Dewatering with vacuum deep wells
The rear quay wall serves as an anchor wall. To this end, it must be dug free on the landward side at 10 meters deep below ground level. A dense net of leg anchors must be installed between the combi wall and the anchor wall, which in turn must be tensioned along the land side of the anchor wall. To be able to carry out this excavation “dry”, heavy dewatering work had to be carried out on the land side of the anchor wall. The groundwater had to be lowered from the +4.00 to a -3.00 level.
Given the depth of the groundwater reduction that had to be carried out, it was decided to dewater with vacuum deep wells. The result of the dewatering was not perfect; due to the presence of poorly permeable interference layers, a significant amount of residual water kept on flowing to the excavation.
Adjustment of implementation technology
From the implementation of phase 2 onwards, after consultation with THV Scheldekaaien, G. Smeyers has opted to adjust its dewatering method and to switch from dewatering with vacuum deep wells using underwater pumps to a stepped drain with vertical filters and piston pumps on two levels, namely a first filter row at level +4.00 TAW and a two filter row at level +1.00 TAW.
In this second phase, the filters are placed at a short distance of 2.5 meters and the drill holes are completely filled with draining sand. This method of implementation has proven particularly effective. The construction pit can now be excavated without residual water flowing into the excavation over interference layers.
By adapting the technique, the contractor’s work can be carried out in much more comfortable conditions. “The suggested change in the dewatering technique was particularly effective and we implemented it without reviewing the budget. Our client really appreciated that”, says G. Smeyers. “Customers who put their trust in our company right from the tender phase can count on us for the whole execution of the work, regardless of the problems that need to be solved”.
The change we suggested in the dewatering technique was particularly effective and we implemented it for our client without reviewing the budget.
20181106_105118_resized: The quay walls from the 19th century are due for renovation.
20181106_110324_resized: To be able to carry out the excavation of the anchor wall “dry”, heavy dewatering work must be carried out on the land side of the anchor wall.
Contour lines Scheldt quays: The G. Smeyers company worked out an extensive dewatering study on a 3D computer simulation.