In the construction industry , dewatering is the term used to describe processes that have the purpose of draining the construction pit temporarily or permanently. By means of the dewatering, on the one hand the accumulating rainwater and on the other hand water that penetrates from the subsoil into the excavation pit are either prevented from entering or collected and drained from the excavation pit.
If a planned excavation is below the groundwater level, it is to be expected that groundwater-related problems may arise when the excavation is excavated . At the very least, without preventive measures, the excavation pit fills with water. For this reason, a subsoil investigation must be carried out in advance of the construction work, during which the local groundwater conditions are also examined. With the help of analyzes of the local geological and hydrological conditions, the external and groundwater levels are determined in order to derive the measures to be taken. The respective measures have to lower the water level so that the excavation floor is dry and the foundation trenches can be dug . The measures to be taken depend on the amount of water flowing in. It must also be checked where the pumped water can be discharged into the sewer system; it must be checked whether and how the pumped water can be fed back into sufficiently distant groundwater levels .
Different methods can be used with which the accumulating water is either displaced, lowered or blocked and thus kept away from the excavation pit. There are methods of open dewatering and closed dewatering, as well as methods with which the entry of water is prevented by blocking. Which method is used depends largely on two factors, the composition of the soil and the amount of water ingress.
With this form of dewatering, deeper trenches must be created in advance during the excavation work. Outside of the building plan, a deeper pump sump must be created in an expansion pit . The surface water and groundwater that arise is then collected in the open trenches or in drainage trenches within the construction pit. From there it flows into the pump sump, from which it is pumped out of the construction pit. If the excavation is deepened, the pump sump and the trench system must also be laid deeper. The pumps used must be safe to run dry and insensitive to dirt. Manhole rings closed at the bottom are used for the pump sump. As considerable amounts of water are often pumped out when the excavation is drained, the groundwater level within the excavation and, depending on the enclosure of the excavation, also the groundwater level in the aquifer surrounding the excavation is lowered. This is particularly important for larger water holdings. Open dewatering is not suitable for construction pits with large flow rates.
With this form of dewatering, the groundwater is taken from the ground via several wells distributed around the construction pit. Shallow wells, deep wells, well point systems or flushing filter systems can be used for closed dewatering. The principle of closed dewatering is the targeted lowering of the groundwater in the area surrounding the excavation. The required total pump volume can be calculated when the groundwater is lowered and the size of the wells can thus be determined in advance. The lowering of the groundwater increases the effective stress in the subsoil. This can lead to subsidence in the affected areas around the excavation, which in turn leads to damage to neighboring buildings, traffic areas and underground lines. In addition, the groundwater currents can mobilize contamination . The contaminated water must not enter the sewage system, but must be collected in tank trucks and disposed of. In addition, wood foundations come into contact with oxygen due to the lowering of the groundwater and begin to rot.
There are various options for significantly restricting or preventing the entry of water into the excavation. On the one hand, there is the possibility of enclosing the construction pit below the base of the construction pit by means of sheet pile walls and thus preventing the access of the groundwater. This method is particularly useful as a supporting measure for open dewatering, since this measure reduces the amount of water to be pumped off. However, the water that penetrates from the base of the excavation must still be pumped out by means of wells created in the excavation. Injection bodies or ground freezing can also be introduced into the construction pit. If a barrier is created from below and from the sides, this is called a trough construction. If there is a risk that water from a body of water can penetrate the excavation pit from outside , a dam must be built around the excavation pit, which is constructed in such a way that it can withstand the pressure of the water and possible waves.
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