Mat foundations - Design.

a. Stability.   The bearing pressure on mat foundations should be selected to provide a factor of safety of at least 2.0 for dead load plus normal live load and 1.5 for dead load plus maximum live load.  By lowering the base elevation of the mat, the pressure that can be exerted safely by the building is correspondingly increased, and the net increase in loading is reduced.  The bearing pressure should be selected so that the settlement of the mat foundation will be within limits that the structure can safely tolerate as a flexible structure.  If settlements beneath the mat foundation are more than the rigidity of the structure will permit, a
redistribution of loads takes place that will change the pressure distribution beneath the structure, as subsequently described.  The bearing capacity of loose sands, saturated silts, and low-density loess can be
altered significantly as a result of saturation, vibrations, or shock.  Therefore, the allowable bearing pressure and settlement of these soils cannot be determined in the usual manner for the foundation soils may be subject to such effects.  Replace or stabilize such foundation soils, as discussed later, if these effects are anticipated.

b. Conventional analysis.   Where the differential settlement between columns will be small, design the mat as reinforced concrete flat slab assuming planar soil pressure distribution.  The method is generally applicable where columns are more or less equally spaced.  For analysis,  the mat is divided into mutually perpendicular strips.

c. Approximate plate analysis.   When the column loads differ appreciably or the columns are irregularly spaced, the conventional method of analysis becomes seriously in error.  For these cases,  use an analysis based on the theory for beams or plates on elastic foundations.  Determine the subgrade modulus by the use of plate load tests.  The method is suitable, particularly for mats on coarse-grained soils where rigidity increases with depth.

d. Analysis of mats on compressible soils.   If the mat is founded on compressible soils, determination of the distribution of the foundation pressures beneath the mat is complex.  The distribution of foundation
pressures varies with time and depends on the construction sequence and procedure,  elastic and plastic deformation properties of the foundation concrete, and time-settlement characteristics of foundation soils.  As a conservative approach, mats founded on compressible soils should be designed for two limiting conditions: assuming a uniform distribution of soil pressure, and assuming a pressure that varies linearly from a minimum of zero at the middle to twice the uniform pressure at the edge.  The mat should be designed structurally for whichever distribution leads to the more severe conditions.

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