Groundwater, water table
Water that soaks into the ground eventually percolates downward under the influence of gravity until it reaches an impervious layer it cannot penetrate. After the water reaches the impervious layer, it begins to move in a lateral direction. This underground flow is known as groundwater, the top surface of which is commonly called the water table. The level of the water table will vary with the amount of rainfall. Consequently, the water table might be several feet higher after a prolonged rainy period than during a prolonged spell of dry weather.
A high water table can result in a flooded basement or failure of the septic system’s leaching field. In many parts of the United States, the seasonal high water table is only 2 to 5 feet below the surface. In those areas, houses should be built on a slab or over a crawl space rather than over a full basement. Unfortunately, houses with full basements have been built in areas where the water table (during the wet season) is above the level of the basement floor. This invariably results in water penetration into the basement. Depending on the soil, even when the water table is several feet below the basement floor, some water might seep into the area as a result of the capillary rise of groundwater. (See chapter 11.)
Homes located in areas where the seasonal high water table is only a few feet below the ground surface should not have septic systems for waste disposal. Ideally, they should be connected to a sewer system. For proper operation of a septic system, the water table during the wet season should be at least 4 feet below the bottom of the leaching field or seepage pit. The operation of a septic system is explained in chapter 13.
Basically, the top surface of flowing streams, rivers, lakes, and oceans is the water table. Consequently, the terrain that gradually slopes into the waterway has a high water table. Homes built in these areas are not only vulnerable to problems associated with a high water table but are also vulnerable to flooding.
The water table tends to follow the general contour of the land and in some areas might intersect the ground surface, thus forming marshy wetlands. To a homeowner, these wetlands are quite undesirable, because not only are they costly to drain, but they are also a breeding place for insects.
Excessive grading or reshaping of the ground surface (such as cutting out the side of a hill to locate a house, FIG. 6-4) can change the natural drainage patterns and cause groundwater to seep to the surface. I have inspected many such houses and have found (in the early spring or after an excessively heavy rain) water oozing out of the cut side of the hill. This water, if not redirected away from the house, can work its way into the basement or crawl space.
In areas with a seasonal high water table, if the topography is such that the land slopes toward one side of the house, in addition to surface-water runoff, subsurface water will flow toward the house. This water, if allowed to accumulate around the foundation, can seep into the basement or crawl space. This condition can usually be controlled by installing a curtain drain in the hillside parallel to the house to divert the water away from the house.
Fig. 6-4. House located on the cutout side of a hill. The building site requires special provisions to minimize erosion and drainage problems.
A curtain drain consists of a perforated drainpipe installed in a trench that is filled with gravel and covered with soil. The trench normally extends several feet beyond the house, with one end leading to a suitable disposal area. Incidentally, the perforations in the pipe should be facing downward, not upward as is popularly believed. As the subsurface water level rises, it enters the holes along the length of the pipe. Since water always takes the path of least resistance, once inside the pipe it flows to the outlet, which must be located away from the house and must be unobstructed. The outlet, however, should have an animal screen to prevent a small animal (such as a rabbit) from entering, becoming lodged, and blocking the flow.
In areas with a seasonal high water table or a potential for surface water to accumulate around the foundation, it is advisable to have foundation footing drains (perforated drainpipes) installed parallel and adjacent to the foundation footing. (See FIG. 6-5.) As with curtain drains, footing drains are installed with the holes facing downward. The purpose of the footing drain is to channel the water that accumulates around the foundation to another location. Footing and curtain drains either must have a free-flowing outlet or discharge into a sump pit where the accumulated water can be pumped to the desired location.