Royers lock – The excavation concept

 In Geotechnics

Renovation works on the Royers lock in the Port of Antwerp is due to start shortly. SBE is responsible for the design of the new lock complex. This blog discusses the deep excavation concept and the challenges encountered during its design process. Some of the major challenges faced include the numerous structures buried on site, the planned tunnel for the Oosterweel connection which is situated close to the Royers lock and the soil structure which isn’t ideal for constructing retaining structures in steel.

The initial concept of the deep excavation pit consisted of diaphragm walls which would also later be used as the permanent lock walls with a screen of cast-in-situ concrete as finishing layer. This concept was, however, abandoned due to the numerous existing structures underground close to the existing lock including parts of an old sewage system, buried quay walls, old retaining walls from a collapsed excavation pit (a failed first attempt during the construction of the Royers lock in the 19th century) and ground anchors to stabilize the existing lock, to name a few. In order to construct the diaphragm walls while avoiding all the known underground structures, the distance between the retaining walls facing each other at the lock heads became so big that using struts wasn’t viable anymore. The use of ground anchors below groundwater level wasn’t allowed by the client, which led to a much larger retaining wall, becoming economically unviable.

Royers Lock – Section Initial Concept
Section of the lock with lined excavation - initial concept

The second and final concept of the deep excavation pit consists of a cement-bentonite screen founded in to the Boom Clay, which can be considered impermeable during the construction period. This screen is constructed approximately 40m away from the existing lock to avoid any interference with the known underground structures. On the Scheldt side an auto stable double combi wall cofferdam is constructed. On the ‘Amerikadok’ side, however, there was no room for an auto stable cofferdam due to the planned tunnel for the Oosterweel connection. The cofferdam – double wall sheet piles – is therefore supported on both ends by a rigid structure consisting of diaphragm walls and ground anchors. Both cofferdams are founded in the Boom Clay. Watertight connections are provided between the cofferdams and the cement-bentonite wall, to ensure a closed water-retaining screen around the construction site. Within this water retaining screen the groundwater level will be lowered and a dry sloped excavation pit will be constructed to a depth of 21 m below ground level.

Royers Lock – Cement-Bentonite Wall
Section of the sloped excavation with cement-bentonite wall - final concept
Section of cofferdam with double combi-wall
Section of the cofferdam with double combi-wall - final concept

The final concept still has some challenges. There is still a possibility of encountering unknown buried structures within the planned location of the cement-bentonite wall, the cofferdams and the diaphragm walls which will have to be dealt with during the construction phase. Concrete and masonry structures can be broken down and excavated from ground level, but steel structures will have to be excavated whole. The other main challenge anticipated is a glauconitic sand layer with very high levels of glauconite situated on top of the Boom Clay layer. The installation and recuperation of both temporary cofferdams will be very challenging due to high friction resistance in this layer.

To summarise, a deep excavation pit must first be constructed, in a challenging environment, to be able to start with the demolition of the existing lock and the construction of the new lock. SBE succeeded in finding a concept that is economically viable, within acceptable risk levels, while satisfying the demands of the client.

For more information about the project, click here.

Ⓒ Port of Antwerp