Royers lock – Soil investigation
For the design of a lock it is necessary to perform an extensive soil investigation, it enables the designer to calculate with the correct soil characteristics, so that no conservative assumptions need to be made. This article discusses the components of a traditional soil survey in Belgium, briefly discusses the soil survey carried out in the Royers lock project and finally explains some of the challenges that occurred during the design of this lock.
In Belgium, a traditional soil survey consists of the elements discussed below.
- Probing: a cone is pushed into the ground and the resistance is measured. At the same time, local friction is also measured; this friction number is an aid in the interpretation of the different soil layers.
- Drilling: a tube is drilled into the soil and the soil in this tube is brought to the surface. The extracted soil can be named by a geologist, possibly with the help of characterisation tests in the laboratory. A borehole can be used to calibrate the probes. During a drilling, undisturbed soil samples can also be brought to the surface. Laboratory tests can be performed on these undisturbed samples to determine the characteristics of the undisturbed soil. For example, tri-axial tests can be performed to determine the resistance characteristics, e.g. for calculations of the lock chamber walls. Or compression tests can be performed to determine the stiffness characteristics of the subsurface, e.g. for settlement calculations of the lock.
- A borehole can also be transformed into a monitoring well to determine the groundwater levels and the phreatic heads of the various soil layers. For example, overstrained groundwater can be traced.
In the case of renovation project of the Royers lock, it was initially possible to fall back on the many ground probe searches that had already been conducted in the vicinity of the project. Within the framework of this project, probing, drilling and monitoring wells were executed, to check the available data and to map the exact location of the different layers in the project area.
The soil structure consists of the following layers:
- Quaternary soil layers: heterogeneous, also many replenished and excavated soil from the time of construction of the existing lock.
- Tertiary sand layers: Formations of Kattendijk and Lillo
- Tertiary clay layer: Boom clay
The tertiary soil layers are well known, as they occur in the wide surroundings of Antwerp. A wide range of studies is available and can be used to determine the soil characteristics. The number of lab tests in the framework of this project was therefore limited. The tertiary sand layers have a strong glauconite content, which has an influence on the construction methods, e.g. the penetrability of sheet piling and tubular piles. The Boom clay will swell considerably when relieved (excavating the construction pit) and will reset when reloaded. Consequently, it is important to know the correct stiffness characteristics and permeability of this clay in order to be able to properly estimate the movements associated with it with their corresponding course over time.
Specifically, for this project there was concern about the settlement behaviour between the bridge cellar and the adjacent door chamber and the settlement course of the doorway. In addition, the future construction of the Oosterweel connection near (<30m) the new Royers lock also had to be considered. The tunnel of the Oosterweel connection will be constructed deeper than the starting level of the lock and will cause relaxation and horizontal deformation of the ground layers under the lock, which could lead to unacceptable differential settlements of the lock. To control this risk, it is planned to build a shaft next to the lock from which compensation grouting can be applied. By injecting grout under high pressure, the settlements under the lock can be counteracted. These injections will be controlled by continuous settlement monitoring of the various parts of the lock.
For more information about the project, click here.