Dewatering is the process of removing water from a construction site and moving it to another place, like a detention pond or forest, to make the construction site safe to work on. Dewatering is a necessary step in preparing an area for construction work.
You can choose from four major dewatering methods, which include wellpoints, sump pumping, eductor wells, and deep wells. No matter which method you use, you have to follow some rules and regulations, even permits, that apply to construction dewatering, especially if the water contains pollutants.
The main factors that determine which dewatering method to use are the depth to which the drawdown has to be conducted and how much the water table has to be lowered. Whether your excavation area is superficial or delves into deep groundwater, this post helps you figure out which dewatering method is best for your construction project.
What Is Dewatering?
Dewatering refers to the process of removing surface or groundwater from a construction site and moving it to a pond, tank, or other location, depending on local regulations, so that construction work can start or continue. Although you can use different dewatering techniques, the process is usually carried out by evaporation or pumping.
Why Is Dewatering Necessary?
Dewatering may be necessary for a construction site, especially if the site is in an area that already contains a body of water or is prone to flooding or holding stormwater after heavy rains. Removing water keeps the work site safe, preventing dirt from becoming slippery mud, providing stabilization, and promoting erosion control, and dewatering activities keep the site free of delays that could result from water-related damage.
You should pump water to help:
- Promote a safe job site
- Prepare the site for construction
- Maintain project efficiency
You can utilize four main types of dewatering systems: wellpoints, sump pumping, eductor wells, and deep wells. Each dewatering method works best for certain excavations and soil types.
A wellpoint dewatering system gets its name because it involves small, individual wellpoints installed around an excavation that connect to a central, centrifugal header pipe with a vacuum function. This helps lower groundwater levels and create a stable, dry area for construction work.
This system is especially useful in shallow excavations or on job sites that have fine-grained soils with low permeability. It’s a cost-effective option with easy installation.
Sump pumping is the simplest and most cost-effective dewatering method. It uses sumps, or pits that are dug in the drainage area, to collect water, which is then removed using solids-handling pumps that pump it out to a discharge point.
It’s best for construction projects with shallow excavations, minimal surface water to remove, and low-permeability soils. Sump pumping can, however, sometimes increase the risk of erosion or collapse, and can produce water with high total suspended solids.
The eductor dewatering method uses an at-grade pumping station consisting of a series of small wells. The wells are equipped with nozzles, also called eductor bodies, to create a vacuum zone, drawing groundwater in through a foot-valve and piping system.
In addition to being low-maintenance once installed and cost-effective, an eductor dewatering system can reach significant depths, and isn’t limited by suction lift, so it works well for deep excavations. It’s especially useful in low-permeability soils and when close well-spacing or vacuuming is required. It is, however, unable to handle a high volume of water.
A deep well dewatering system uses a series of drilled wells, each equipped with a submersible pump, to lower groundwater. It’s a gravity-based system, with wells that are wider and deeper than a wellpoint system. It’s often used to remove water from previous structures that extend below the current excavation.
This dewatering method is best-suited for job sites that need to pump out large amounts of groundwater, or require significant drawdowns, but it can handle multiple excavation depths. It also works well in high-permeability soils.
What to Consider When Choosing a Dewatering Method
When looking for the best dewatering method for your construction project, some of the most important factors to consider are: soil, budget, amount of water to remove, and excavation depth.
Soil type is a major factor in what dewatering method will work best for your excavation site. The permeability of the soil, or rate of the flow of water through the soil, is especially important.
Some dewatering methods, like sump pumping and eductor wells, only work in low-permeability soil, while deep well dewatering works best in high-permeability soil. Wellpoint dewatering can work for either permeability.
The cost may play a part in what dewatering method you choose. Sump pumping is the most cost-effective, with wellpoint and eductor wells being the next most affordable.
While deep well dewatering is generally the most expensive, if it’s the most suitable for your construction project, it’s better than using a cheaper option that may not dewater your project site.
Amount of Water
The amount of water, and whether it’s surface water or groundwater, also determines what dewatering method is best to use. Sump pumping and eductor dewatering work best when you don’t have much water to pump. Wellpoint dewatering works well in shallow aquifers of 50 feet deep or less. Of the four, deep well dewatering is the only method that can handle a high volume of water.
The depth of your excavation also affects the dewatering process. Eductor wells and deep wells are great for deep excavations, while wellpoint and sump pumping work best in small and shallow excavations.
Water Removal Options
Once you’ve removed the water from your job site, where does it go? You can choose from a few methods for storing the water that has been dewatered, depending on your project site, budget, water turbidity, and applicable state and federal laws.
A few common removal options include:
- Detention ponds or basins are man-made bodies of water created to store runoff. Using these to remove water can be beneficial for the environment because it reduces the transfer of pollutants and contaminants to other bodies of water. If you use this method, be sure to follow federal and state requirements for checking the ponds and basins regularly.
- Tanks or boxes not only allow you to transport dewatered water from your construction site to another place, but they also can be used for drainage, filtering out solids, sediments, and sludge, and reducing wastewater. Be sure to check with federal and state regulations about the water you’re trying to move or drain.
- Release or redistribution as an endpoint of the dewatering process allows you to return your water to rivers, wetlands, or lakes. It’s important, however, that the wastewater undergoes filtration, or is treated, prior to release, and that you secure permission from federal and state environmental authorities.
The next time you’re planning an excavation, make sure your job site is fully prepared for construction work by dewatering it. You can’t start moving earth without safe, solid ground to dig into.