Dewatering and construction dewatering are terms used to describe the action of removing groundwater or surface water from a construction site. Normally the dewatering process is done by pumping or evaporation. It is usually done before excavation for footings and will help to lower the water table that might cause problems during excavations.
Dewatering can also refer to the process of removing water from the soil by wet classification. Wet classification is a construction process that looks at the size of particles that make up the underlying soil in a project site and the flow of fluids through those particles.
The right dewatering plan can be used to be in compliance with some of the Best Management Practices related to the SWPP plan.
Construction dewatering is used on most construction sites due to accumulated water in trenches and excavations or in places with an inadequate slope or high water table. In construction projects, this water should be removed to keep working as scheduled or to provide a safe workplace.
Normally, builders tend to use water pumps to dewater these areas, but if they are not paying attention to the place where water is discharged, erosion and other problems may occur. It is important to follow best management practices when water is being pumped to lakes, wetlands, or directly to storm sewer inlets.
Dewatering activities must be done properly to avoid eroding the soil on the construction site. It is also important to choose the best location for discharge, even when you might be far away from water bodies or catch basins. There are multiple dewatering products that can be used to remove sediment from the pumped water, such as dewatering bags. When choosing discharge areas from a dewatering process remember:
- Water should not be pumped directly into slopes.
- Dewatering activities should be directed to a wooded buffer, if available.
- It is important to pay special attention and discontinue dewatering if the area shows signs of instability or erosion.
- Channels used for dewatering must be stable and better if they have been protected with grass or vegetation.
- You should avoid dewatering under heavy rains because the infiltration rate is at a minimum and water will move slower or just the dewatering process will not function.
- Never discharge water that has been contaminated with oil, grease, or chemical products directly. In such instances, an oil/water separator may be necessary.
- Additional permits and requirements might be needed from the state, local, or federal agencies.
- It is important to understand the water table conditions in the area, perhaps the underground water is always near the surface, so your plan might not work.
- Sump pumps are the most common dewatering technique but can handle only a small volume of water.
How to Use Dewatering Bags
Dewatering bags are made of durable geotextile fabric used to filter water by removing sediments. They are used for dredging operations, construction sites, or places with a high water table near the shoreline. These bags should be sized accordingly based on the pump flow rate and type of sediment. When using dewatering bags, avoid multiple pipe discharges as it may cause the filter bag to fail. Remember to manage water runoff properly by guiding it to the nearest storm inlet.
Construction dewatering from open excavation or trenches can be done by several methods. However, the most simple of all is the gravity drain using drainage channels carrying away water from the area to be worked to the discharge point.
Other feasible ways for dewatering are water pumping, siphoning, or using large construction machinery buckets to scoop and dump water from the selected area. Earth channels used for dewatering could also be protected with ditch linings, and additional protection should be placed to reduce water velocities and minimize erosion. It is recommended to build riprap revetment protection with geotextile to prevent additional erosion at the discharge point.
Preventing Water from Affecting Your Site
When it is known that a low permeability soil can be found, a possible solution to reduce the dewatering process is isolating the permeable strata from other sources of water. Sometimes, sheet piles, slurry walls, and grout curtains can be used as a method of preventing water from entering your trench or foundation.
Although this solution will not get rid of the problem, it is a very good plan to use when the water table is high, as you will still need to pump or remove water from the foundation or trench due to rainfall and water trapped in the area. Depending on the system use, a complete engineering analysis will be required to analyze how this process affects other areas.