

Specialty drainage products. Today there are several products on the market, such as Form-A-Drain (CertainTeed Corp., P.O. Box 860, Valley Forge, PA 19482; 800/233-8990;, that provide both the footing form and the drain tile (Figure 8).

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Figure 8. Form-A-Drain stay-in-place footing forms ensure a level perimeter drain and have a larger capacity than pipe systems (left). To control hydrostatic pressure, dimpled drainage panels fastened against the foundation wall carry water from the backfill into the perimeter drains (right). These systems not only ensure that the drainage system is level, they often provide more flow capacity than traditional pipe systems. On sites where an exceptionally high ground water table creates intermittent hydrostatic pressure on the foundation walls, dimpled sheets can be used in conjunction with standard drain tile. These membrane systems provide a waterproof barrier while also directing excess ground water from higher up on the foundation walls into the perimeter drains.

Discharging Collected Water

Capturing ground water in a perimeter drainage system is only half the battle — once you’ve collected water in the drain tile, you have to dispose of it somewhere. Discharging water into sanitary sewer systems is generally illegal, which leaves two basic ways to get rid of the water: On sloped sites, you can extend unperforated drain tile to daylight and discharge the water on the ground; on flat sites, you can collect the water in a sump basket and pump it to a discharge area away from the basement.

Gravity discharge.

Two elements are critical to proper function of a gravity drainage system. First, although the perforated drain tile around the foundation itself may be level, solid pipe running from the foundation to daylight should slope at the rate of 1/16- to 1/8-inch per foot. Second, the open end of the discharge line should prevent entry by rodents, frogs, snakes, and reptiles. One method is to cover the exposed end of the pipe with 1/4-inch hardware cloth. Alternatively, you can bury the end of the pipe in crushed stone, which will allow the water to seep out below grade.

Pumped discharge.

While gravity discharge to daylight is cheap and easy, I recommend installing a sump basket as a backup. A submersible sump in the bottom of the sump basket connects to a hose or rigid pipe system that carries the collected water out of the basement. If you provide for the collection sump at the time the foundation and slab are placed, the pump and discharge piping can be installed later if needed. The sump basket should be located inside the foundation, where it can pick up ground water that rises under the slab. On a flat site where all ground water must be pumped away, water from perimeter drains should also be directed into the sump through drainage sleeves in the footing (Figures 9a & 9b).

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Figure 9a. An interior sump basket picks up excess water flowing through sleeves in the footing.

Figure 9b. A submersible pump connected to a hose or rigid pipe discharges the water on the ground away from the foundation. To avoid having to excavate later, be sure to place sleeves before the footings are poured. Use 4-inch-diameter pipe, and space sleeves 6 to 8 feet apart around the entire perimeter of the footing. In special cases where the slab is placed a foot or more above the top of the footings, you can locate sleeves in the foundation wall. Although water passing through the sleeves or under the footing will generally find the sump basket on its own, I recommend an interior drain pipe at the perimeter, terminating in the sump basket. Brent Anderson owns and operates Brent Anderson Associates, a concrete contracting and consulting firm in Fridley, Minn..