What is construction dewatering and why is it needed?
Dewatering is the process of removing groundwater. This is often required as a temporary measure during construction projects, hence the term construction dewatering.
A simple example is the construction of a house whose basement will be below the property's groundwater level. To excavate the site and build the basement, the groundwater must first be lowered so as not to flood the excavation pit. Once the basement has been completed and sealed off from groundwater, dewatering is no longer necessary. The house can then sit in the groundwater.
The majority of the structures being built for the second core route are underground. Due to the local geology, groundwater is regularly encountered even at greater depths. Groundwater is found in the cavities between the individual sediment grains, which are a bit like the holes in a sponge. To build the structures, especially the new stations at Munich Main Station, Marienhof and Munich East as well as the rescue shafts and the new underpass at Laim, some of the groundwater has to be extracted during the construction work.
How does construction dewatering work?
To remove groundwater, extraction wells are built. This involves drilling deep boreholes down to the groundwater. Pumps are installed in the wells to pump the groundwater to the surface.
Each extraction well removes groundwater from the area surrounding it. This first serves to reduce groundwater pressure. If more water is extracted, the surrounding soil can also be (at least partially) dewatered. By arranging multiple extraction wells in a group, dewatering can be effective in larger areas.
Construction dewatering at Marienhof
There are essentially two parts to the construction of the new S-Bahn station at Marienhof: the large excavation pit and the various tunnels leading off from it.
In the large excavation pit, the new multi-storey S-Bahn station is being built, including the entrances and exits to the platforms. The outer boundary of the excavation pit is formed by the diaphragm wall which has already been constructed. While the excavation pit is being dug beneath the top slab (upper deck) of the underground structure, horizontal slabs for each storey are quickly installed in order to hold the diaphragm walls in position (this is known as the cut-and-cover construction method).
Pressure on the outside (earth side) of the diaphragm walls is exerted not only by the soil itself but also by the groundwater. Given the depth of the excavation pit, the pressure from the groundwater poses a particular problem. For technical reasons, it is therefore necessary to reduce the water pressure.
This reduction in water pressure is achieved through construction dewatering. Groundwater is pumped from a large number of wells around the excavation pit before and during construction. The groundwater extraction process does not completely dewater or drain the soil but is designed only to reduce the water pressure on the diaphragm wall to the necessary level, i.e. to depressurise the groundwater.
In the final stage, i.e. after construction of the new station structure in the excavation pit, it will no longer be necessary to reduce groundwater pressure on the diaphragm walls. The finished structure is designed to withstand the full extent of groundwater pressure. The construction dewatering process, i.e. the pumping of groundwater to reduce pressure, can then be stopped.
Tunnelling for platforms
Tunnels forming part to the station structure, which will later house parts of the platforms, will be made in a conventional process using the sprayed concrete lining method in a pressurised environment. When this method is used, the ground is usually loosened in sections with an excavator and the tunnel wall is secured with a sprayed concrete lining. While this is happening, compressors ensure constant positive air pressure in the tunnel, which is sealed off from the excavation pit by pressurised air locks. The air pressure is adjusted so that it is greater than the pressure of the groundwater in the vicinity of the tunnelling. Combined with the use of sprayed concrete to secure the tunnel sections, this elevated air pressure prevents groundwater from flowing into the tunnel during tunnelling work.
The construction workers entering and leaving the pressurised areas need to acclimatise to the change in air pressure, a bit like divers resurfacing. This is achieved using air locks between the excavation pit and the tunnel, in which the air pressure is gradually increased or reduced.
The tunnels for the new S-Bahn station at Marienhof are located approximately 40 m below ground. The groundwater pressure there is very high, and even elevated air pressure in the tunnel is not enough to counteract the groundwater on its own. To solve this problem, the pressure from the groundwater surrounding the tunnel will therefore be lowered in advance. As the water pressure goes down, this in turn will reduce the positive air pressure needed during tunnelling.
Construction dewatering will be needed to reduce the water pressure. Groundwater will be extracted from a large number of wells in the vicinity of the tunnel construction site before and during tunnelling work. As with the excavation pit, this groundwater extraction does not completely dewater or drain the soil but is intended primarily to reduce the water pressure to the necessary level.
Once the tunnel is complete, elevated air pressure will no longer be needed. Sealing against groundwater will be achieved using a multi-layered system consisting of a sprayed concrete outer shell, a sealing membrane and an inner tunnel lining. This multi-part system will be able to permanently withstand the full extent of the original groundwater pressure. The pumping of groundwater, i.e. construction dewatering, can be stopped after the tunnels have been completed. As with the excavation pit, the groundwater will return to its original pressure around the new structures.
Where does the extracted groundwater go?
The groundwater extracted from the many wells during construction is usually returned to the ground elsewhere, away from the construction site.
However, due to the central location of the construction site at Marienhof, this is not feasible. There are no open spaces nearby where the water could seep back into the ground. For the construction of the new S-Bahn station at Marienhof, permission has been obtained to discharge the extracted groundwater into the nearby Westlicher Stadtgrabenbach, an underground stream to the west of the city centre. From there, the water will flow into the Isar river.