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24 ment of the wall caused by the backfill and foundation deformation, if any. Where the top of the zone of saturation (water table) is at an elevation above the base of the foun- dation, the pressure on the wall and floor slab is due to the bauyant weight of the soil plus the full hydrostatic pressure of the water. When a workable subdrainage system is provided to lower the elevation of the water table, the pressure on the wall will be reduced. The degree to which the water table can be lowered will depend upon the per- meability of the soil and the efficiency of the subdrainage system. Flood loading without subdrainage is the force of the full hydrostatic pressure of the water above as well as below the ground line plus the buoyant weight of the soil. As schematically illustrated in Figure 18, the magnitude of this force can be considerably larger than the force developed under nonflood. conditions (shown in Figure 18 for com- paris on). When subdrainage is provided, this flood loading force is reduced. However, in the case of an existing building with an existing unmodified, subdrainage system, prudence would dictate that no load reduction be assumed. Subdrains, where already installed, are gen~rally provided only to intercept seepage and control uplift on basement floors due to ground water. If such a subdrainage system were to be modified to attain a known degree of effectiveness, a load reduction could be determined. Obviously, for new construction the subdrainage system can be designed and constructed to afford a predetermined degree of reduction of flood loads. The magnitude of the flood induced forces that will be encountered is indicated by the fact that a one story brick building (3 5/8 inches of brick over wood frame) can be expected to withstand no more than two feet of water above the ground line providing the wall is in good condition. For brick with concrete block backup this height would be somewhat greater. Subsurface Drainage Ground-water conditions may adversely affect the stability of a building or structure either through uplift which tends to "float" the building or by erosion which can under- mine the support. Investigation and analysis of the factors involved at any specific build- ing and the design of control or corrective measures are endeavors requiring the atten- tion of professionally trained personnel. Ground-water problems can be controlled by the installation of subdrainage systems (see Figures 17 and 19) to reduce the lateral forces on the foundation walls and floor slabs. Experience has shown that the composition of soils in a particular area can vary widely, with extreme ranges of permeability existing in areas of the same general geo- logical origin. Such ranges in permeability argue further for careful investigation and analysis. The design of a subsurface drainage system must be based on the results of soil investigations of permeability and analyses of structural strength. A sump and pump system can be employed to help protect the subsurface part of a building. The pump could be designed to accept storm and seepage flows and pump them to a point above the flood waters. The sump should be open to the soil at the bottom and to atmospheric pressure at the top within the basement. This would provide a fail-safe feature, in that power or pump failure would allow water to flood the basement and there- by tend to balance the outside flood induced pressures upon the basement walls and floor slab. As an alternative, a prearranged program of deliberate flooding with clean water could be employed to minimize the cost of clean up after a flood. Seepage Control. - Foundation walls can be made watertight to minimize water infil- tration through cracks and crevices in the walls. In buildings under construction, this can be accomplished through the use of waterproof membranes and seals. Construction joints can be protected by the use of a neoprene or other similar waterstop. Existing masonry or stone foundations are more difficult to waterproof, particularly if the mor- tar joints have deteriorated with age. Sealing of walls to prevent seepage can be