Risk Management of Groundwater During The
Reconstruction of the Rotterdam Central area
Bert DE DOELDER and Geerhard HANNINK
City of Rotterdam, Project Management & Engineering, the Netherlands
Abstract. The rather complex subsoil and groundwater conditions have played an important role in the design and execution of projects with extensive excavations in the centre of Rotterdam. These projects comprise of the reconstruction of a subway station, two new traffic tunnels, an underground bicycle parking facility and a new railway terminal. The realisation of these projects involve a number of risks. This paper describes the main hydrological risks during the realisation of the different projects around the railway station Rotterdam Centraal. Three risks are presented in more detail, illustrating that knowledge of the local groundwater regime is a precondition for realising projects with controlled risks.
Keywords. hydrological risks, pumping test, groundwater modelling, leakage, ground freezing, thermal erosion, water tightness
1. Introduction
In the centre of Rotterdam, extensive reconstruction works have been carried out around the railway station Rotterdam Centraal between 2004 and 2014. These works comprised of the reconstruction of the subway station Rotterdam Centraal, two new traffic tunnels, an underground bicycle parking facility and a new railway terminal (Figure 1). During the design stage an extensive analysis was made of the risks of deep excavations in the centre of the town. The projects had to be carried in a limited area as public transport and related passenger flow had to go on.
This article describes three hydrological risks of the reconstruction of the subway station Rotterdam Centraal. This underground station had to be transformed from a two-track, single platform lay-out into a three-track, two platforms configuration.
The soil conditions around the subway station are heterogeneous. It was unclear during the design stage whether and to what extent dewatering of groundwater would influence the different soil layers and the surrounding buildings.
It was therefore decided to reconstruct the subway station Rotterdam Centraal within an excavation with diaphragm walls and a collar construction around the existing subway tunnel,
to be realized by ground freezing. Ground freezing is only possible with a limited flow rate of the groundwater. In this case the flow rate was influenced by the dewatering installations for nearby projects.
Due to the various excavations the precipitation will easily infiltrate into the subsoil and this may lead to an increase of the phreatic groundwater level, and to nuisance in the environment.
To control the risks various actions were taken. In the design stage a field test has been carried out to investigate the geohydrological conditions. During the construction of the subway station the water pressure has been measured with an extensive monitoring network, consisting of 130 online piezometers.
The article focusses on the approach that was followed by the City of Rotterdam to control the execution of these major projects, and describes three hydrological risks, including the mitigating measures that were taken.
2. Geohydrologic system
The subsurface of Rotterdam consists of a several meters thick anthropogenic layer on 10 to 15 m thick Holocene clay and peat layers. Below these Holocene layers is the first aquifer, made up of Pleistocene river sediments with a © 2015 The authors and IOS Press.
This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License.
thickness of 15 to 20 m. Underneath the first aquifer is a separating layer, followed by the second aquifer.
The anthropogenic layer has a reasonable permeability. Within the Holocene layers there are well drained local sandy river dune deposits, that have locally contact with the underlying first aquifer.
The subsurface conditions in the area of the subway station Rotterdam Centraal are presented in table 1.
Table 1. Subsurface conditions Elevation [m NAP] Origin- Type of soil Hydraulic head [m NAP] C[days]/ kD [m2/day] From To -0.3 -4.5 Sand fill -1,5 15 -4.5 -17.0 Holocene-clay and peat
1.500 -17.0 -35.0 Pleistocene sand -2.3 950 -35.0 -37.5 Kedichem clay -37.5 -40.0 Kedichem sand -2.4 3.000-4.500
Figure 1. Overview of projects
The following local variations in the soil stratification were identified:
- a former canal that is partly filled with sandy material goes to the east-west direction. This former canal contains the subway tunnel that has been opened in 1968;
- at the location of the high-rise office building “Delftse Poort” a sand layer of varying thickness and size interrupts the clay and peat layers at a depth of NAP -13 m to NAP -16 m. The sand layer is a river dune sediment (called "donk" in Dutch).
The remaining clay and peat layers between the bottom of the canal, the sandy layer and the Pleistocene aquifer are relatively thin and may even be locally absent (Figure 2).
Figure 2 – Geotechnical profile of the subsoil underneath the high-rise office building Delftse Poort orange = peat, green = clay, yellow = sand
3. Rotterdam approach
The City of Rotterdam realised several major projects in the City in the past. The approach for these projects is based on an extensive knowledge of the underground. In this case the following steps were taken:
1) Additional field investigations were carried out to assess the feasibility of the preferred design option(s).
2) Critical situations were modelled and investigated by FEM calculations.
3) Possible risks were identified during all stages of the project.
4) Extensive monitoring was executed during the realisation and if necessary, corrective action were taken.
4. Main Risks
During the design process different risks have been distinguished. In this chapter, the three major hydrological risks are described.
4.1. Hydrogeological situation
For several projects around the railway station Rotterdam Centraal dewatering is necessary (for
the extension of the subway line called RandstadRail, subway station Rotterdam Centraal, and the Weenatunnel). Normally, due to the Rotterdam soil conditions, the phreatic groundwater will hardly be affected by a dewatering system that decreases the hydraulic head in the Pleistocene sand layer. But with the complex soil conditions around the railway station the phreatic groundwater can be influenced by dewatering the Pleistocene layer due to a possible contact between the various hydrological aquifers.
In the 80s of last century, the high-rise office building Delftse Poort has been built above the existing subway tunnel. The pile foundation below the subway tunnel suffered from additional settlements due to the weight of the building. Therefore, an new pile foundation on both sides of the tunnel was made from which the tunnel has been suspended. For the design of the pile foundation a certain upward ground water pressure was taken into account. If the phreatic surface water level is reduced, there is an increasing risk that the connection between the tunnel and the suspension construction is overloaded. The phreatic water level beneath the subway tunnel should not become below NAP -3.3 m. Therefore the hydrological resistance between the different soil layers had to be examined thoroughly.
4.2. Influence of excavations
During the excavation works the precipitation will infiltrate directly into the subsoil. This will possibly lead to an increase of the phreatic water level around the buildings and local groundwater problems. The possible consequences, risks and mitigating measures have been analysed.
4.3. Freeze and water tightness
The excavation for the reconstruction of the subway station Rotterdam Centraal is executed with 40 m deep diaphragm walls into the layers of the Formation of Kedichem. The major reason for this choice is that the alternative (an excavation with conventional sheet piling and a large dewatering system) would lead to unacceptable settlements in the surroundings during the estimated construction period (4 years
sandcanal
or more). To limit the settlements a maximum duration of dewatering of about 20 to 25 months was possible, based on historical data. There was also a real risk of a reduction of the phreatic groundwater level below the already mentioned office building Delftse Poort, due to the detected short cut between the different hydraulic regimes.
Subway metro traffic had to proceed at all times, irrespective of the type of excavation. Therefore a collar construction had to be constructed around the existing underground tunnel at the east side of the excavation. The collar construction was realized by ground freezing. This construction method has several pre-conditions / risks:
- the freezing process is only possible in case of a limited flow of groundwater in the Pleistocene sand layer;
- the diaphragm wall and collar construction have to be waterproof at the start of the excavation;
- the water tightness should be guaranteed during the entire construction stage.
5. Pumping test
A pumping test has been carried out in 2004 with the dewatering system as close as possible to the subway tunnel. The first part of the test was performed by dewatering the groundwater from the Pleistocene sand layer, the second part of the test by dewatering the sandy fill in the former channel around the subway tunnel. The second stage of the pumping test was used for the determination of the hydraulic resistance of the aforementioned channel and of the donk.
The results of the pumping tests were analysed by a number of geohydrological models. At first the geohydrological parameters were determined by analytical methods. This approach has the disadvantage that a difference in transmittance into the horizontal direction, or a strongly varying layer thickness cannot be taken into account. Later, the geohydrological parameters were determined using a more complex numerical groundwater model.
The analyses showed that a hydraulic connection between the sand fill in the former channel and the Pleistocene sand layer exists. To ensure a certain upward pressure against the
subway tunnel the hydraulic head in the Pleistocene sand layer should not be further reduced than to NAP -6.4 m.
To mitigate the risk on a lower hydraulic head an infiltration system was installed into the subway tunnel, by which the groundwater level in the sand fill in the former channel can be controlled. This infiltration system has proved to be very effective.
The pumping test underlined that groundwater monitoring should play an important role during the execution of the projects. To make an accurate prediction of the influence of the dewatering of the Pleistocene sand layer on the phreatic groundwater the use of a numerical groundwater model was necessary.
6. Groot Handelsgebouw
It was foreseen in the design that the excavations might lead to an increase of the phreatic water level in the area. An increase in the phreatic groundwater could lead to flooding of cellars of nearby buildings so that compensatory measures were taken by installing drains.
However, during the execution, monitoring showed a decrease of the phreatic groundwater level due to the barrier that is formed by the diaphragm walls around the excavation. Moreover monitoring showed settlement of the north-east corner of the Groot Handelsgebouw. To minimize the negative impact of the groundwater decline tap water was infiltrated through the installed drain to restore the water level. It took some time before the effect of the infiltration was visible. In the piezometers around the building the groundwater level rose fairly quick but a similar effect was not noticed underneath the building. The infiltrated water could apparently not flow easily underneath the building, because of the presence of the cellar. Another problem was the leakage of infiltrated water through cracks into the cellar. In the end the groundwater level has raised sufficiently to stop the settlement of the building.
At the same time a permanent solution was created by making use of the newly built sewage system nearby. Water from the rainwater sewage system is since 2014 infiltrated into the subsoil through a so-called infiltration main. The amount
of precipitation, combined with the storage capacity of the main will be sufficient during most of the year, and will maintain a reasonable groundwater level.
The phreatic groundwater level in the area of the railway station Rotterdam Centraal will also be influenced by the large new roof of the railway station. This will particularly effect the northern side of the station. Most buildings in that neighbouring area are founded on wooden piles. As a mitigating measure rainwater from the roof of the station will be infiltrated into the subsoil.
7. Ground freezing and water tightness
7.1. Ground freezing process
The realisation of the retaining wall that is created by ground freezing strongly depends on the flow velocity of the groundwater in the Pleistocene sand layer. This flow is affected by the dewatering systems of nearby projects. Because the ground freeze wall is a vital part of the project the flow velocities had to be accurately determined with a 3D numerical groundwater model.
During the ground freezing process, the dewatering of the nearby Weenatunnel excavation was in operation, resulting in a groundwater pressure gradient of about 0.02 and a flow velocity of around 1.5 m/day as calculated by the MicroFem code. It was recognized that this would at least delay the freeze-up period. Based on additional MicroFem calculations, it was therefore decided to install so called mirror-wells nearby the ground freeze wall to reduce the water pressure gradient at the location of the collar construction, as illustrated in Figure 3.
Monitoring records showed that this measure was very effective, and that almost no pressure gradient remained at the ground freeze location.
7.2. Waterproof and thermal erosion
During the ground freezing process, the following thermal erosion factors played a role:
o a high groundwater flow velocity due to the dewatering for the Weenatunnel project;
o the configuration of the freeze pipes; o the difference in hydraulic head inside
and outside the excavation during the final stage of the freezing process. A little bit of pumping inside the excavation cancelled this difference out.
Figure 3 – Calculated water pressure contour lines in the Pleistocene sand layer inside and around the excavation, including effect of the dewatering of the Weenatunnel project
and of the mirror-wells. The window shows the hydraulic head in the east-west direction, outside the excavation
With the available temperature monitoring it appeared not to be possible to determine the thickness (and therefore the water tightness) of the collar construction with sufficient certainty.
Since relatively small leaks by thermal erosion might develop in time to difficult to control proportions (uplifting or inundation of the excavation), the determination of the water tightness was very important. Therefore, the planned pumping test to check the water tightness of the excavation was optimised. Additional groundwater calculations with the MicroFem code revealed that an increasing gap in the ground freeze wall, resulting from thermal erosion, would almost simultaneously cause significant changes of monitored hydraulic heads and water discharge volumes.
Therefore, the conclusion of a 100% water tightness of the ground freeze wall could be made immediately after the pumping test was completed, as none of the possible thermal erosion effects had shown up.
7.3. Leak
Monday morning, 17 December 2007 a major leak occurred at a joint of the diaphragm wall around the excavation for the subway station Rotterdam Centraal at the moment that the maximum excavation depth of 14 m was reached. The construction of the nearby new Weenatunnel was at that time still in execution. A dry excavation for that tunnel was made possible by using a dewatering system that decreased the hydraulic head in the Pleistocene sand layer. As a result the hydraulic head near the leak before the calamity was about NAP -5 m. A piezometer nearby the location of the leak showed a sudden drop in the hydraulic head to NAP -9 m immediately after the break through. A recovery to NAP -6 m occurred during Monday due to the automatic correction of the Weenatunnel dewatering system.
In order to reduce the hydraulic head further down at the location of the leak, two more deep wells were placed in the vicinity of the leak.
The optimum configuration and the effects of the extra dewatering in the area were visualised with the numerical groundwater model and were controlled with additional monitoring. The leakage was provisionally stopped two days later, and during the spring of 2008, the leak was definitely closed with a wall of jet grout columns.
Afterwards the question was raised whether this leakage could not be foreseen. Therefore the results of the pumping tests and the monitoring data during the execution were analysed in more detail. In addition, calculations with the numerical model were carried out, in which the leakage is modelled by a discharge that is practically equal to the amount of water that is pumped out of the excavation during the leakage.
The calculations for the situation with a leak show that both the hydraulic head inside and outside the excavation deviates significantly from the observed values during the pumping tests:
o inside the excavation the hydraulic head will, within a distance of 50 m from the leak, in case of a fixed discharge through the leak, be about 4 m higher than measured during the pumping test. This has however not been noticed;
o outside the excavation the hydraulic head will, at short distance from the leak, in case of a fixed hydraulic head inside the excavation, be about 2 m lower during the leakage. Neither this has been measured.
8. Conclusions
With a good research during the design stage and an intensive monitoring program during execution, it is possible to manage risks in an efficient way.
With the heterogeneous and complex subsoil in the area of the railway station Rotterdam Centraal a pumping test is a necessary investment to determine the risks of dewatering.
During a complicated project a solid monitoring program is essential and an investment in an automatic and on-line system can be very valuable. A conclusion about the water tightness of the ground freeze wall would not have been possible without monitoring. The situation during the leakage could only be managed effectively with the aid of groundwater measurements.
Throughout the project the use of a numerical groundwater model proved to be essential:
o for the analysis of the pumping test and for the determination of the combined effects of the dewatering for a project in the design stage (the magnitude of the discharge, the change of the hydraulic head and the critical flow rate);
o for the comparison of calculated parameters and monitoring data during the execution;
o for the analysis of damage or unforeseen circumstances.
References
Thumann, V.M., (2007). Application of ground freezing technology for a retaining wall at a large excavation in the centre of Rotterdam, The Netherlands, Proceedings 14th ECSMGE, Madrid, September 2007. Thumann, V.M., Hannink, G., Doelder, B.R. de (2009).
Ground Freezing and Groundwater Control at Underground Station CS in Rotterdam, Proceedings of the 17th Int. Conf. on Soil Mech. and Geotechn. Eng.,
2560–2567, Alexandria, 5-9 October 2009.
Thumann, V.M., Berkelaar, R., Luijten, C., Doelder, B.R. de (2013). Practical Integration of Risk Management and Monitoring During Rebuilding of Subway Station CS Rotterdam, Proceedings of IABSE Conference, Rotterdam, 6-8 May 2013.
