Ground water is essential to our survival. However, while a high water table is usually welcomed as positive, it can be very disruptive to a building project. For this reason, excavations on damp ground are often equipped with piling walls or pumped out on a regular basis.
Gravel or clay?
An even more effective solution is to lower the water table in the area surrounding the building site before starting work. On permeable terrain, such as sandy or gravelly soils, gravitational force can assist this process. Wells are drilled next to the excavation so that any water present will flow there automatically, and can then simply be pumped out.
However, on more dense terrain – for example, soil with a higher clay or loam content – this method does not work. In this case, applying vacuum can increase the ease of flow. For vacuum dewatering, suction pipes measuring 15 to 30 centimeters in diameter are inserted upright into the ground. Their walls are permeable to water and protected with filter material to stop particles from penetrating them. Depending on the density of the soil, suction pipes are inserted at intervals of one to two meters.
Ground water suction
The individual pipes are joined by an above-ground suction tube and connected to a storage vessel. The vacuum pump is then connected to this storage vessel. In the case of larger building sites and long ring circuits, multiple vessels and vacuum pumps may even be used. The vacuum pump generates vacuum both within the vessel as well as in the entire system. This ensures that the ground water is suctioned out of the surrounding layers of soil and into the pipes and, from there, transported into the storage vessel. Once the storage vessel is full of water, a water pump is activated automatically. This empties the vessel and transports the water through the conduits, discharging it far enough away to stop it from seeping back in as ground water. Busch offers a range of vacuum pumps for lowering the water table.
Hydrologists have calculated that the top two kilometers of the Earth's crust contain around 22.6 million cubic kilometers of ground water. Over three quarters of this water is fossil ground water located deep underground, which was cut off from the water cycle thousands of years ago. Only a very small proportion is less than 50 years old and connected to the cycle of constant renewal through rain fall and surface water.
As it slowly seeps down into lower layers of the Earth – a process known as subsurface flow –, the water is modified in a number of processes. For instance, by absorbing carbon dioxide from the respiration of soil organisms and releasing calcium carbonate, the water accrues its specific hardness level. If the flow process takes long enough, micro-organisms like bacteria and viruses are almost completely eliminated. These processes are also referred to as the self-purification of ground water. However, harmful substances can also get into ground water during this process and limit its suitability as drinking water.