Posted on 2019-12-03 10:25:11
Few, if any, basements are truly waterproof. To remain dry, basements need a properly designed and constructed drainage system to intercept water moving through the soil and drain it away from the structure, preventing the water from ever reaching the basement walls or floor.
Each year, the Water Resources Center receives dozens of phone calls and emails from people experiencing problems with unwanted water. Many of the calls start with “I think my house is built on a spring”. The symptoms may include water leaking into the basement, the sump pump operating frequently, a perpetually wet area in the yard or a similar complaint. Some of these people live where springs are fairly common, but others are in parts of the state where springs are very rare. In talking with them, we find that many people do not understand what a spring actually is and how it functions.
A spring is a natural geologic feature. It is a place on the Earth’s surface where groundwater naturally discharges from the subsurface. Most springs in Missouri are associated with limestone and dolomite bedrock units that have been partly dissolved, leaving behind conduits or cave-like openings, large or small, which transport water through the subsurface from recharge sources to springs. These types of springs are also commonly associated with sinkholes and losing streams and occur in karst areas. Karst is a term used to denote areas where the dissolution of soluble rock has created features such as caves, springs, sinkholes, and losing streams. Springs in karst areas vary greatly in size, from flows of a few gallons per minute to several hundred million gallons per day. The flows generally vary with local precipitation conditions, and are highest during the spring and early summer and lowest in late summer and fall. Some of the springs, especially ones with small flows, can cease discharging during prolonged dry weather. Others, termed karst resurgences, function only briefly following heavy precipitation, and remain dry other times.
Springs can also occur in areas not characterized by karst conditions such as most of northern Missouri where glacial materials have been deposited on top of the bedrock, or in west-central Missouri where the bedrock is Pennsylvanian age strata. In these areas, springs are fewer in number and smaller in size than in the Ozarks. Many, in dry weather, are little more than a damp area with little or no flow discharging from them. In areas where shale units are present, water tends to move downward through the shallow subsurface, then moves laterally when it encounters a low-permeability unit such as shale. Seeps and small springs are common on hillsides where shale units crop out. Sand lenses in glacial drift also account for seeps and small springs in the glaciated regions.
There is usually a much simpler explanation for wet basement problems than a spring or cavern beneath the house. Realistically, the possibility of a contractor building a house where water is naturally discharging from the ground on a frequent or continuous basis is poor. The problem typically has nothing to do with natural groundwater flow, but rather is related to surface drainage and basement construction.
Few, if any, basements are truly waterproof, at least not in the sense that they can remain dry while surrounded with water-saturated soil materials. To remain dry, basements need a properly designed and constructed drainage system to intercept water moving through the soil and drain it away from the structure, preventing the water from ever reaching the basement walls or floor. During wet weather, the soil materials around a house can become saturated to a depth of several feet, especially in places where the soils have been disturbed and then replaced, such as in the excavation made for a basement. Undisturbed soils generally are much less permeable than soils that have been excavated and then replaced. During and after rainfall, some of the water flows laterally across the surface of the ground away from a house, but some of the water moves vertically downward through the soil materials. Whether water is moving across the land surface or through the subsurface, it moves due to gravity. Water moves from an area of high head (or elevation) potential toward an area of low head (or elevation) potential. If the water accumulates around a basement and saturates the soil materials around and beneath it, leakage commonly results. Water accumulating around the walls of a basement will exert a pressure of 0.43 pounds per square inch for each foot of water depth. If it rains very hard for an extended period and the soil materials are saturated from land surface to the floor of the basement, the water pressure against the basement floor would be about 3 pounds per square inch, more than enough to cause water to leak through cracks in concrete, around the perimeter of the basement floor, and any other openings.
Most basements are constructed in three separate steps. The footings that the basement walls rest upon are poured first. The forms for the walls are set on the footings and then the walls are poured. Finally, the basement floor is poured after the forms are removed from the walls. Concrete shrinks slightly as it cures. Even though the concrete floor completely filled the space between the basement walls, a small opening generally appears between the basement floor and the walls, at least in places. Since the floor is not sealed against the walls, if water under any pressure is present beneath the floor it will commonly leak upward through the joint between the walls and floor. If there are cracks in the basement walls, a common occurrence in older homes or houses built upon swelling-clay type soils, they can also be an entry point for leakage.
To remain dry, a basement needs to have footing drains completely around the house. There should also be drains beneath the basement floor. The drains are perforated pipe, usually PVC or some other type of thermoplastic. The drain pipes are laid on several inches of gravel at the base of the footings, and covered by gravel. The space above the drain pipe between the concrete basement walls and the dirt should be filled with several feet of gravel. The gravel should be covered with some type of filter fabric that allows water to drain through it but holds back soil particles, preventing the fine-grained materials from plugging the pore spaces in the gravel. Also, the outside wall of the basement should be coated with a waterproofing material and a vapor barrier to help prevent moisture from seeping through. The gravel, being much more permeable than the soils, will allow water to rapidly drain into the perforated pipes. The water is channeled away before it can ever contact the basement wall.
Good Drainage. For best results the footing drains should "daylight", meaning the pipes that carry the water away from the footing drains should slope away from the house and discharge the water to land surface some distance downhill from the basement. The basement floor should rest on a bed of gravel, and the gravel should have perforated drain pipes laid in it to prevent any water from accumulating beneath the basement floor. Like the footing drains, the floor drains should also "daylight". The figures below show cross-sections of basements with effective drain systems, and without suitable drainage.
Poor Drainage. Some contractors forgo using gravity drains beneath the basement floor,and instead installone or more sumps in the floor which can be equipped with sump pumps. The sump pumps remove the water that accumulates in the gravel bed beneath the basementbefore it can cause a seepage problem. In some cases, sumps can be added in older basements to help alleviate seepage problem. Sump pumps can be very effective in preventing wet basements. However, they are mechanical devices that operate on electricity. If they fail, or if electrical service is disrupted, the result can be a wet, or even flooded, basement. A backup sump pump is recommended, and also a high water-level alarm to alert the home owner that the sump pump is not functioning is an inexpensive precaution. A portable generator, which must be placed outdoors, may also be a worthwhile investment to keep the sumps operating during power outages.
Wet basements are more of a problem in houses that are on very flat land where walk-out basements cannot be used. It is often difficult to construct basement footing and floor drains that "daylight" to land surface in these types of homes. Typically either a dry well (a gravel-filled pit where the drainage from around the basement is channeled) or sump pumps are used to drain the water from the drainage system. Unfortunately, many home builders do not take the necessary steps to prevent basement leakage. Footing and floor drain tiles, gravel, vapor barrier, and the labor to install them, are not terribly expensive. Yet, a surprising number of homes are built with inadequate or ineffective drainage systems. Adding footing drains to an existing dwelling is considerably more expensive than doing it when the home is built.
There are several things a homeowner can do to help decrease the potential for basement leakage. Rain gutters and downspouts must be used to help prevent roof runoff from accumulating around the basement walls. Two inches of rain falling on a 1,500 square foot roof will produce about 1,870 gallons of water. If that amount of water is allowed to drain off a roof and collect around a house, it can overwhelm even a properly installed basement drainage system. Gutters and downspouts are critical to maintaining a dry basement. The downspouts should empty on the undisturbed soils beyond the backfill around the basement. The land should be sloped away from the house on all sides. A commonly recommended minimum slope is at least 1 inch of vertical drop for each foot of horizontal distance.
Homes built on steep slopes commonly have basement leakage problems on the uphill side of house. To minimize dirt work, contractors commonly leave the yard on the uphill side of the house very flat. If there is not adequate drainage around the basement, the water will tend to saturate the soil materials and pond against the basement wall on the uphill side of the house. Landscaping to channel water away before it reaches the house will help alleviate the problem. Installation of soil drains such as a French drain may be necessary to reduce the buildup of moisture in flat areas.
Homes constructed on footings and foundations can also experience water problems similar to those associated with poorly drained basements. Typically, the footings are poured a few inches below the frost line, and the foundations are short walls that rest atop the footings, much like basement walls rest on their footings. Footing drains should be placed around the outside of the structure to intercept water in the soils, and direct it away from the building. Otherwise, water may move beneath the footings and surface in the crawl space beneath the house.
There are, of course, places where the water table is only a few feet below land surface. The water table is the boundary between unsaturated materials above it, and water-saturated materials below it. The water table is typically closest to land surface in low-lying areas, and a greater distance below land surface on higher ground. Basements are generally not recommended where homes are constructed on floodplains unless the basement is essentially built on top of the ground and an artificial “hill” of fill is placed around it. During dryer times of the year when stream flow is low, the water table beneath a floodplain may be well below the typical depth of a basement, but other times of the year when the stream is at a higher stage, the water table may rise to only a few feet below land surface. If this occurs, the basement may have much more serious problems than leakage, the hydrostatic pressure may cause the floor to buckle or walls to bulge or collapse. A 40-ft long basement wall with 5 feet of water on the outside of it would have to withstand an external pressure of about 31,000 pounds. A basement floor beneath a 1,200 square-foot house under the same conditions would have to withstand an upward hydraulic pressure of nearly 374,000 pounds.
Basement water problems can occur in rural as well as urban areas. But other types of excessive water problems are predominantly found in developed areas, especially those where extensive landscape modifications have been made that alter surface drainage patterns, and where dwellings are closely spaced. Surface-water runoff is a natural result of rainfall, but surface-water runoff rates and amounts change when the landscape is modified. The addition of streets, driveways, buildings, patios, swimming pools, etc., creates impervious areas where surface water can no longer infiltrate. The greater the impervious area, the greater the runoff. In carefully planned developments, these drainage changes are mitigated by the addition of storm sewers, drainage ditches, storm-water detention basins, or other structures that carry much of the excess runoff away from the development or store part of the runoff for later release. Drainage in many developments receives less attention, and the result is greater than normal runoff occurring in the higher parts of the development, and the excess water flowing to the lower elevations in the development. The problems are almost always transferred downhill. If the lots in the lower areas are notched into the hill sides, then the nearly flat yards below the over-steepened slopes may receive so much water that they stay wet for long periods of time.
Water from precipitation is not always the only source of moisture. Urban development usually includes a centralized public water supply for the development. If the supply were through private domestic wells, then waterline leaks would be easy to identify. Simply turn off all the faucets and water-consuming appliances in the home, and watch the pressure gauge on the pressure tank. A leak will be indicated by dropping pressure (assuming the check valves at the pump are working and the pump pipe has no leaks).
In a public water supply system, leaks may be much more difficult to detect. If the leak is between the water meter and the dwelling, then it can be detected by turning off all faucets and water-consuming devices, and seeing if the meter shows water use. Detecting a leak on the supply side of the meter is more problematic, particularly if the leak is relatively small. When a water main ruptures, there is usually no problem identifying the site of the leak. There are usually copious quantities of water flowing from the ground above the leak where formerly there was none. Small leaks, though, are much less spectacular and more difficult to find. The ground above the leak may show no sign of it. Water lines are commonly laid on beds of sand or gravel, which can allow water to travel a significant distance laterally from where the actual leak is occurring. When a water problem suddenly appears where there has not been one, a leaking waterline is a logical suspect. Unfortunately, not all water purveyors diligently try to locate and repair leaks. The first test is usually to check for chlorine in the water (assuming the water supply is chlorinated). Such tests are commonly negative, even if the source of the water is a chlorinated supply. Chlorine is a volatile gas that is dissolved in the water. When the water is released from the waterline, the chlorine gasses off fairly quickly. When it is not found in the water, the supplier may quickly lose interest.
Some other clues that can help determine if the water is from a leaking waterline or another source is to check the temperature of the water. Spring-water temperature is generally very close to the average annual temperature of the area, roughly 56° F in the northern part of the state, and 59° F in the south. If the temperature of the water is substantially different than this, it can indicate a water line leak. An easy test is to let a cold-water faucet run for a few minutes and measure the temperature. Water from by a leaking waterline should be a similar temperature. Other times it may be necessary to collect samples of the unwanted water and from the water supply, and have them analyzed. Water chemistries that are similar indicate a common source. Those with major differences indicate the water is not due to a waterline leak. The specific constituents dissolved in the water can also be useful. For example, most municipal water supplies in southwestern Missouri produce water from the Ozark aquifer, which is mostly from bedrock formations composed of dolomite, calcium-magnesium carbonate. Water produced from dolomitic aquifers contains considerable dissolved magnesium. The shallow bedrock units in southwestern Missouri are mostly limestone. Limestone is calcium carbonate, and water produced from it contains very little dissolved magnesium. Water from a leaking municipal water line in that area would contain enough magnesium to distinguish it from shallow, natural spring water.