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  Mat foundation.  Based on economic consideration, mat foundations are constructed for the following reasons:

  Large individual footings. A mat foundation is often constructedwhen the sum of individual footing area exceed one half of the total foundation area.

  Cavities or compressible lenses . A mat foundation can be used when the subsurface exploration indicates that there will be unequal settlement caused by small cavities or compressible lenses below the foundation. A mat foundation would tend to span over the small cavities or weak lenses and create a more uniform settlement condition. An example of this could also be, if a foundation is built over a sanitary fill and as decomposition occurs in the soil it creates void spaces, these voids are then compressed under vertical load of the structure causing settlement.

  Shallow settlement. A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.

  Unequal distribution of loads. For some structures, there could be a large difference in building loads acting on different areas of the foundation. Conventional Spread footings could be subjected to excessive differential settlement but a mat foundation would tend to distribute the unequal building loads and reduce the unequal settlement.

  Hydrostatic uplift.  When the foundation will be subject to hydrostatic uplift due to a high groundwater table, a mat foundation could be used to resist the uplift forces.

  Post-Tensioned Slab-On-Grade.Post tensioned slab-on-grade are common in southern California and other parts of the United States. They are an economical foundation type when there is no ground freezing or the depth of frost penetration is low. The most common use of the post-tensioned slab-on grade are to resist expansive soils forces or when the projected differential settlementexceeds the tolerable value for a conventional ( lightly reinforced ) slab-on-grade. For example, Post-tensioned slab-on grade are frequently recommended if the projected differential settlement is expected to exceed (2 cm or 0.75 in.). Instillation and field inspection procedures for post-tensioned slab-on grade have been prepared by the Post Tensioning Institute. 

Foundation selection criterion

  The selection of the foundation is without saying crucial to everything else that follows. The selection of a particular type of foundation is often based on a number of factors, such as:

  Adequate Depth- It must have an adequate depth to prevent frost damage. For such foundations as the use of piers, the depth of the foundation must be sufficient to prevent undermining by scour. This is a problem that your geotechnician will solve, because of the area and soil involved, they'll have historical data etc, that they use to evaluate evidences.

  Or one might ask the question; Is there a standard depth which to bore for a foundation? I suppose a rule of thumb would be to bore 10 feet below the footing elevation. This could change from historical data of the site or the discovery of problem soils. However; an often sited guideline, Some Geo-techs have said; the depth should be at least two times the foundation width below the bottom elevation of the planned footings with a minimum of at least 5 to 10 feet depending on loads etc. And spread footing loads should be four times the width and is considered the most likely range to clear the seat of settlement. Consequently; neither case takes into account how earthwork may affect the seat of settlement.

  Bearing Capacity Failure - The foundation must be safe against a bearing capacity failure. Bearing capacity of the soil is to support applied loads to the ground. The bearing capacity of the soil is the maximum average contact pressure exerted between the foundation and the soil which should not exceed capacity or produce shear failure.

  There are three modes of failure that limit bearing capacity: general shear failure, local shear failure, and punching shear failure.

  General shear failure is movement caused by shearing stresses in a soil mass and is of sufficient magnitude to destroy or seriously impair a structure. When a load is applied to the foundation, settlement occurs, and if certain load limits are exceeded, ultimate load shear failure results, a plastic condition under the footings, cause stress forces to push the soil outward and upward to the ground surface, and rotation of the foundation occurs.

  Local shear failureoccurs in moderately compressible soils, and soils of medium relative density. Significant vertical settlement may take place due to local shear failure, i.e. Soil yielding close to the edge of the footing, and the yield surfaces often do not reach ground level. Similar to general shear failure but local shear failure is vertical stress that pushes outward and downward with no visible upward or turning shear stress. and like the general shear failure, may not be as catastrophic as to rotate the footer.

  Punching shear failure happens in weak compressible soils, and soilsof low relative density. Considerable vertical settlement may take place with the yield surfaces restricted to vertical planes immediately adjacent to the sides of the foundation; the ground surface may be drawn down. After the first yield has occurred the load-settlement curve will steepen slightly, but remain flat.

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