【正文】 arge bearing areas are thus obtained。 however , these tables give only general classifications and descriptions of the soil or rock must be used with caution. More specific information about the soil or rock is normally obtained by drilling test borings, extracting soil or rock samples, performing laboratory tests on the samples, and making engineering must be given to the amount of settlement which may occur and the capability of the structure to withstand such settement. If settlement is problem it may be necessary to use an alternate foundation type rather than footings or to enlarge the footings and decrease the bearing pressure.Grade beams may be used between exterior column footings to support walls, with the beams transferring the weight of the walls to the column footings. Beams are also used between interior column footings to act as braces or to support interior walls. Retaining walls are those walls subject to horizontal earth pressures due to the retention of earth behind them. The foundation for these will not slide when subjected to the horizontal earth pressure. In addition,retaining walls must be designed so they will not overturn. In frostsusceptible areas, footings must be placed below the frost line.Mat foundations Mat or raft foundations are large, thick, and usually heavily reinforced concrete mats which transfer loads from a number or colums and walls to the underlying soil or rock. Mats are also bined footings.,but are much larger than a footing uniform pressure to the underlying soil or rock. Mats are rigid and will act as a bridge over discontinuities in the soil or rock on which they are founded. Mats founded several meters below the ground surface, when bined with external walls, are termed floating foundations. The weight of the soil excavated from the ground surface to the bottom of the mat may be equal to approach the total weight of the structure. In this case, little or no new load is applied to the underlying supporting soil, and settlements of a structure may be minimal after construction.Slab foundations Slab foundations are used for light structures wherein the columns and walls are supportes directly on the floor slab. The floor slab is thickened and more heavily reinforced at the places where the column and wall loads are imposed.Special problems Groundwater is a major problem in connection with the design and installation of foundations where a substructure is to be placed below the groundwater level. Well points, pumping from deep wells, or pumping from sumps are methods used to dewater construction sites during foundation installation. Other methods which are less often used are freezing of the water in the soul, removal of water by electroosmosis, and the installation of cutoff walls made of pilling or grout around the periphery from within the excavation. If dewatering operations are perfromed in an area surrounded by existing structures, precautions must be taken to project them, as the lowering of the geoundwater may cause the soil on which they are supported to subside.If a basement is partially or totally below the groundwater level, its walls must be designed to withstand the hydrostatic pressure of the water on the outside in addition to the pressure from the soil backfill. An alternate procedure is to install a permanent system to remove watet outside the walls. Some substructure below groundwater level may at times be subjected to hydrostaic uplift forces which are greater than the downward forces imposed by the structures. In these cases, provisions must be to anchor the structures to prevent them from floating upward.Groundwater also causes problems by infiltrating though basement walls., slabs, and joints into the basement itself. This can be prevented or reduced by providing an external permanent drainage system that carries water away from the basenment, by encasing the walls and slabs in an permeability. Combinations of the foregoing are also used. Retaining walls and abutments ofen abutments can escape. The water pressure behind the walls is relieved as the water flows though the walls into an open external drainage system.Foundations placed on expansive soil are often subje