Dewatering refers to the removal of groundwater and/or surface water from a site before construction work commences. This may be necessary on sites where there are trenches and excavations in which water can accumulate, in places where there is an inadequate slope for water run-off, or where there is a high water table. In most construction projects the water should be removed to keep the production as scheduled or to provide a safe workplace. In most cases, the contractors tend to use water pumps to dewater these areas.
Contractors have a variety of pumps from which to choose. Some of the most common types used on construction sites include trash pumps, diaphragm pumps, and submersible pumps. Another type of pump that is gaining in popularity is the well point pump, most commonly used for groundwater dewatering tasks.
Each type of pump has its strengths and weaknesses. Centrifugal trash pumps move large amounts of water quickly but can jam if large or heavy solids clog the impeller. Diaphragm pump works sort of like a piston, so there's no impeller to clog, making them handy for moving water with a lot of mud or silt, especially in shallow depths where submersible and centrifugal pumps are ineffective. That's why they're often called “mud hogs” or “mud suckers.” Submersible dewatering pump works on the water that is relatively clean—the sort found in sumps, excavations, or collection ponds.
Pump capabilities vary from specialized to highly versatile and from low pressure to high pressure. Some are more capable of pumping fluid vertically than others, and, while some work well for moving water with debris, others will clog under those same conditions. A pump that is not designed to move solids may cost less, but it will likely clog and break if any type of debris is pumped through it. Choosing the correct pump for the application can help ensure a safe and successful job.
Trash Pumps
Portable trash pumps are commonly used in construction projects where there is an open body of water that can be accessed by the piping or hose. These pumps can handle clean, muddy, mucky, or sandy water with solids up to 2 inches in diameter (depending on pump size) and between 10 to 25 percent by weight. Applications include pumping unwanted water from excavations, flooded basements, manholes, utility vaults, mining work, retention ponds, lakes, and barge holds. Trash pumps are very popular in the construction industry because of their reliability, versatility, and durability under a variety of site conditions.
Trash pumps are available in either wet priming or dry priming versions. Wet priming pumps require that the casing be filled with water before operation. Dry priming pumps utilize an external priming system, usually a compressor or a vacuum pump, to assist in purging air from the suction line of the pump system.
These types of pumps typically have high suction lifts and relatively high discharge heads, as well as a simple design with few parts. However, they are not a good choice for pumping thick mud. And when there is only a small quantity of water to pump, priming will be lost.
Diaphragm Pumps
Diaphragm pumps are effective in muddy applications and are often used where the content of solids is high and/or a condition of seepage exists. Because of this muddy application, diaphragm pumps are commonly referred to as mud hogs, mud suckers, or simply mud pumps. While diaphragm pumps do not pump high volumes or distances for their weight, they are very useful because of their versatility. Diaphragm pumps have the capability of pumping water with greater than 25 percent solids by weight. Diaphragm pumps do not require a steady flow of water and can handle large amounts of solids.
One of the best features of the diaphragm pump is that it can run dry indefinitely without damage. These pumps are ideally suited for the construction industry, as well as for municipal where dry priming pumps are required.
Diaphragm pumps handle slow seepage or full capacity with each stroke. These pumps also keep job sites dry when there is a small inflow. Another plus is that the diaphragm and valves are typically easily replaced.
Drawbacks include less capacity than a self-priming centrifugal pump for the same investment, and limited to low discharge pressure.
Submersible Pumps
Submersible pumps can be an effective, inexpensive solution for dewatering a construction site. They provide instant priming and are capable of moving large amounts of water with high discharge heads. These pumps are primarily for water containing solids up to ¼ in. in diameter and less than 10 percent by weight. They can run unattended and are ideal where quiet operation is mandatory. Pumping unwanted water from well casings, tunnels, shafts, flooded excavations, manholes, and vaults are some applications appropriate for submersible pumps. Submersible pumps are generally powered by a variety of single or three-phase electric motors, some equipped with automatic float options, and are lowered directly into the liquid to be pumped.
Today's submersibles feature improvements in both pump and impeller designs that make them more efficient than past models. For example, non-clog impellers have become standard in most makes, and variable frequency drives (VFD) automatically regulate the speed of the pump for maximum efficiency.
Well point Pumps
Well point pumps are specifically designed and engineered for wellpoint and sock dewatering with high-air handling, large water volume, and high-vacuum capability. Well point dewatering is a simple method of temporarily lowering the groundwater table in a localized area to accomplish the construction of a foundation, pipeline, or other structure. A well point system consists of a series of shallow wells—called well points—installed around the excavation and connected to a well point pump by means of a set of header pipes with couplings and fittings.
A well point pump is a combination of two pumps, one of which pumps water from the header and the other of which is a vacuum pump to remove air that enters the system. Control of the air is important, as excessive air causes cavitations which reduce pump efficiency
