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Last edited 02 Sep 2020
In ground engineering works – such as foundations, deep basements, trenches, tunnels and other excavations – groundwater is very often present. This must be dealt with so that work on the site is not difficult, dangerous or even impossible. Modern geotechnical techniques allow groundwater to be safely managed by a process of dewatering.
Without suitable control measures, inflows of groundwater can flood excavations or tunnels, and can also lead to instability when the soils or rock around the excavation weaken and collapse – either locally or on a large scale. The objective of dewatering is to lower groundwater levels to below working level in the excavation. Examples of this group of techniques include sump pumping, well points, deep wells and ejector wells.
Sump pumping is one of the simplest dewatering techniques: groundwater seeps into the excavation and is allowed to collect in sumps – which can either be the lowest point of the excavation or may be an area specially created into which water may seep naturally e.g a well or sump. Each sump will typically have one or more robust electric-powered pumps with the capacity to handle the solids that will inevitably be present. The collected liquid is then pumped away for disposal, either with pumps running continuously or activated automatically as the water level rises.
Sump pumping can be a very effective and economic method to modestly reduce the liquid (‘drawdown’) in a sump, especially in favourable ground conditions such as well-graded course soils (e.g sandy/coarse gravels and gravelly sands).
In unfavourable ground conditions, such as in silts, fine sands and other fine-grained soils, sump pumping may lead to instability as it may draw out fine particles from the soil, potentially causing ground movement such as settlement. Furthermore, depending on the soil type, sump pumping may see high levels of sediment in the pumped water which could have an adverse environmental effect at the point of disposal.
Over the past few decades, there have been several high-profile projects around the world that have been seriously delayed by groundwater problems. However, a greater number have successfully dealt with groundwater.
In the south of England, projects such as High Speed 1 (known as the Channel Tunnel Rail Link during its construction in the early 2000s) and Crossrail have routes which successfully passed through multiple water-bearing strata, including crossing beneath the River Thames. Both projects involved successful dewatering and the use of sump pumps.
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