7 Questions for creating an effective dewatering system
WATER
As land becomes more expensive, leveraging the real estate below the earth’s surface for parking garages, new utilities and even meeting rooms has become a reality. And, as any experienced contractor knows, excavating in these spaces often turns up something other than soil—namely, groundwater. All construction foundations and structures below the water table will require some type of dewatering. Contractors or engineering firms often assume the dewatering system used on previous projects (usually a simple wellpoint system) will work on all projects. But looks can be deceiving. Companies may underestimate the amount of water they will encounter or fail to realize where it might come from. The risks of inadequate dewatering When dewatering is not considered or the wrong dewatering method is chosen, the results can be costly in terms of time, money, reputation, and safety. If the dewatering system proves inadequate, work may halt while additional pumps are installed, and every day work does not get done, users lose money and fall behind schedule. The costs can mount quickly. To meet the project deadline and avoid penalties, companies may need to rent additional excavators, loaders, and dump trucks. There may be extra fuel costs at a time when fuel prices are skyrocketing, and companies may need to hire additional crews, which on large projects can run $25,000 (R380 597.50) a day or more. Water from the ground and surface due to rain events can damage construction equipment and reduce productivity. In the worst-case scenario, water could end up destroying work that has been performed. Wet or muddy jobsites also pose safety issues for crews, especially when water
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Pipes, Pumps and Valves Africa - Jan/Feb 2022
compromises the stability of slopes. Planning for all possibilities Choosing the right dewatering method requires considering the current conditions as well as potential future conditions. The soil types, excavation depth and buildings and infrastructure surrounding the site can influence a project. Taking a detailed soil boring to understand the soil components and the strata (including where the permeable layers are and the depth and formation of each layer) and performing a pump test are critical to determining the best method. It is also important to anticipate weather events. For example, a contractor was putting in a sewer line and an engineer specified a single wellpoint system for dewatering, but it was not robust enough to control flooding when it rained. So, an equipment rental company created a backup dewatering system by installing wellpoints on both sides of the excavation and additional sump pumps to handle the surface water. That dried out the area enough for the contractor to work but subcontractors had to work around several additional 6-inch pumps. Had the weather contingencies been anticipated upfront, the problem might have been avoided. For waterfront projects, factoring in the tidal system and water fluctuation is essential. When a contractor in Mississippi was working on a drop-in structure, its engineers also recommended a wellpoint system. Then the
