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:. Hydrostatic Pressure*" Hydrostatic pressure affects underwater portions of a house in the same way that water pressure affects a diver. Greater depths of water exert,greater pressures on the structure. See Figure 2.2. At a depth of 5 feet (an average below grade depth of basement walls) water exerts over 300 pounds of pressure per square foot of surface. Fortunately, there is frequently a tortuous path through relatively impervious soil from water on the surface to basement walls and to the under side of the basement floor. If it were not for relatively impervious soil, water pressure from a flood or a heavy rain, could easily break the average basement slab floor. The alternative to relying upon this uncertain protection is to build a reinforced or braced basement that is designed to withstand full pressure.*'" Builders have learned from many years of experience that certain conventional structural specifications will usually withstand the water pressures to which they are subjected. Occasionally, however, the variable factors combine adversely resulting in structural failure. ' Pressure on structures is caused by differential hydrostatic pressures; that is, the difference between outside pressure and inside pressure. If this cannot be lowered by reducing the outside hydrostatic pressure, the alternative is to increase the inside pressure. This can often be accomplished by providing an inside water level (inside the basement or house) that will offset the outside hydrostatic pressure. For this reason, some home owners deliberately fill their basements with water or leave the doors open to allow water to enter. These procedures can cause some damage. but they are often preferred to the chance of structural failures. Buoyancy An object in water is buoyed by an upward force equal to the weight of the water displaced. Each cubic foot of water that is displaced by the structure exerts enough buoyant force to float about 62 pounds. This upward movement is opposed by two forces - first, the weight of the house and its furnishings, and second, by the frictional shear forces of the soil against the underground foundation walls and footings. See Figure 2-3. An average 1.story house with basement could reach a buoyant condition and begin to float out of the ground when outside water has reached about 3 feet above the level of the basement floor. Fortunately, the entire wall and underfloor areas are not fully saturated in most cases. The buoyant condition is caused by only those areas that are saturated. As a consequence, buoyant basements are less frequent than a simple application of the basic laws of physics would predict. Most basements also have an effective, but not very efficient, built-in protective device. Under- slab upward hydrostatic loads will usually rupture the floor slab before a buoyant condition is reached. This allows water to flow into the basement and reduces the amount of water displaced. The basement less house will float if trapped air inside the house displaces enough water. An average 1-story house will float when water reaches about to the eaves. An important factor here is the strength of the sill anchor system. Effective anchor bolts or straps tie the house to its foundation and greatly improve its resistance to the buoyant force. Length of arrows indicates relative amounts of pressure Figure 2-2 Hydrostatic Presssure Figure 2-3 Buoyancy .Hydrostatic Pressure: the force resulting when a head (depth) of water, or of wet soil, is opposed by a surface such as a wall or basement slab. Hydrostatic pressure is on all surfaces facing the water. The deeper the water, the greater the force exerted. "Anderson, Charles M., Manual for the Construction of Residential Basements in Non.Coastal Flood Environs, Federal Insurance Administration, Department of Housing and Urban Development, Washington, D.C. 20410 2-3