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High groundwater level

Water entering the basement through the floor slab is an indication that water pressure is being exerted on the underside of the floor. When the level of the water below the house is sufficiently high (due to a seasonal high water table or improper drainage) so that it pushes on the underside of the floor slab, it seeps into the basement through cracks, open joints, or porous sections of the slab. If the pressure is great enough, it can cause the floor to crack and heave.

If the seepage is minor, it can often be controlled by sealing cracks and open joints with a hydraulic cement and coating any porous areas of the slab with a cement-base or epoxy sealant. However, a better solution would be to lower the level of the water below the floor slab. This can be done by installing a sump pump below the slab. Subsurface water then flows into the sump pit in the manner of water flowing into a hole dug at the seashore. The water is then removed by the pump and discharged either into a storm drain or at a point sufficiently far from the house so that it will not be absorbed by the ground and flow back under the basement floor slab.

Depending on how the floor slab was constructed, a single sump pump might or might not be adequate to lower the level of the subsurface water. In areas with a seasonal high water table, a concrete floor slab should be installed over a gravel base. Water that accumulates below the slab can then flow through the voids between the gravel and drain away or flow into a sump pit. However, in many houses the floor slab has been installed directly over soil with poor drainage characteristics or over an inadequate gravel bed. In this case, water in the saturated area below the slab will not readily flow into a sump pit, and to control the water buildup, it is necessary to install a series of perforated drainpipes below the floor slab that terminate in the sump pit. Caution should be observed when lowering the level of the groundwater below the basement floor. With some slow-draining soils such as silts and clays, some soil can wash out from around the foundation footing. This can result in unequal settlement, which could crack the walls. Whether a sump pump or drainpipes are needed below the floor slab is an evaluation that should be determined by a professional.

If the house is located in an area with a high incidence of power failures, you should not depend solely on an electrically driven sump pump to control groundwater seepage. It is possible for the power to be knocked out when the water level below the floor slab is rising. As a precautionary measure, there should be an auxiliary backup sump pump in the sump pit. A backup system is particularly helpful in vacation homes where the house will be vacant for extended periods. One type of backup pump is a water-actuated (nonelectrical) ejector pump. The Zoeller Pump Company, Louisville, Kentucky, manufactures this type of pump. The pump is connected to the house water supply and is activated by a float control. However, it will be of no help if the water to the house is supplied by an electrically driven well pump. If you install a water-actuated sump pump, it’s important that you include a backflow preventer on the water supply because of the potential for contamination as a result of the cross connection. Another type of backup system is a battery-operated sump pump. This system will take over automatically to protect against flood damage when the power fails. The Zoeller Pump Company also manufactures this type of system.

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