Dewatering is the process of removing groundwater and superficial water from a construction site. Usually, this procedure is carried out before excavations, or to lower the water table on site. Dewatering methods can use pumps or evaporation.
Removing water from construction sites is important to protect materials and to keep a safe workplace. Most construction sites require dewatering, due to the accumulation of water in trenches and excavations.
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Without adequate supervision or planning, dewatering can cause soil erosion and other problems at the construction site. Here are some tips to consider when choosing a discharge area:
- Selecting a proper discharge area is crucial.
- Watch for signs of instability or erosion in the discharge area during dewatering.
- Don’t pump water directly into slopes.
- When possible, the dewatering discharge should be directed to a forest buffer zone.
- Channels used for dewatering must be stable and protected with vegetation.
- Avoid dewatering under heavy rain.
- Water contaminated with oils, greases, or chemical products must be treated before discharge. Never discharge contaminated water.
- Investigate the locals permits and requirements for dewatering.
- Analyze the water table conditions in the project area.
The different methods available for dewatering are not interchangeable, since each method has a specific application. For this reason, selecting the proper method for a particular ground condition is critical. There are four main dewatering methods used in construction:
- Eductor Wells
- Sump Pumping
- Deep Wellpoint
In the wellpoint method, a series of wells is aligned along the excavated area. The wellpoints are connected to riser pipes, and these are connected to a common header pipe and vacuum pump. Water is discharged away from site, with the adequate treatment to remove contaminants and unwanted materials.
Since the wellpoint method uses suction, it is suitable for depths up to 5 or 6 meters. A deeper drawdown requires multiple stages of wellpoints.
The eductor well method is similar to the wellpoint method. The main difference is that eductor wells use high-pressure water instead of a vacuum to draw water from the wellpoints. This method uses the venturi principle:
- High-pressure water is circulated through eductors in the base of each well, creating a reduction in pressure.
- The pressure drop draws water through the riser pipe.
With this method, the water table can be lowered to a depth of 10m to 45m, and multiple pumps can operate in a single station. The eductor well method is suitable when the ground has low permeability materials, such as clays and silts.
Sump pumping is the most common method for dewatering, since it basically works by gravity. This method is also the most economical.
Groundwater is allowed to seep into the excavation area, where it is collected in sumps to be pumped out. This method is used in shallow excavation areas, where the soil has a high content of sand or gravel. If the excavation area is large, this method can use a ditch - a long and narrow sump along the excavation.
The deep wellpoint methods uses boreholes with submersible pumps, lowering the groundwater level below the excavation level. Wells with diameters between 15 to 20 cm are drilled around the excavated area, where groundwater falls due to gravity .This reduces the water table, depleting the groundwater in the excavated area.
Casings are used to retain wells, along with screens and filters to keep sediment out. This method is suitable for projects where a large amount of water must be removed from the well.
Dewatering is an important consideration for construction managers, especially during the excavation and foundation stage. Uncontrolled groundwater can flood excavated areas, damaging construction materials and equipment. Water can also cause trench collapses and flooding, threatening the construction workers in the area. Selecting an adequate dewatering method based on soil conditions and the water table is an important step before groundwork.