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C CORNERSTONE EARTH GROUP 7.2 MAT FOUNDATIONS 7.2.1 Post-Tensioned Mats In our opinion, the proposed residential structures may be supported on post-tensioned mat foundations provided the recommendations in the "Earthwork" section and the sections below are followed. Post-tensioned (PT) concrete mat foundations may be designed in accordance with the parameters in the table below based on procedures developed by the Post-Tensioning Institute (latest edition) and as referenced in the 2007 California Building Code. Table 6: Post-Tensioned Mat Design Criteria Differential Movement Condition Center Lift Edge Lift Edge Moisture Variation (feet) 5.5 3.0 Differential Soil Movement (inches) 0.5 0.8 To reduce potential differential movement, all mats should be designed for a maximum average allowable bearing pressure of 750 psf for dead plus live loads; at column or wall loading, the maximum localized bearing pressure should be limited to 2,500 psf. When evaluating wind and seismic conditions, allowable bearing pressures may be increased byone-third. These pressures are net values; the weight of the mat may be neglected for the portion of the mat extending below grade. Top and bottom mats of reinforcing steel should be included as required to help span irregularities and differential settlement. 7.2.2 Mat Foundation Settlement We estimate that total foundation movement under static loads will be less than'/: inch, with post-construction differential movement of less than %2 inch across the building footprint. 7.2.3 Lateral Loading Lateral loads may be resisted by friction between the bottom of mat foundation and the supporting subgrade, and also by passive pressures generated against deepened mat edges. An ultimate frictional resistance of 0.4 applied to the mat dead load, and an ultimate passive pressure based on an equivalent fluid pressure of 450 pcf may be used in design. The structural engineer should apply an appropriate factor of safety (such as 1.5) to the ultimate values above. The upper 12 inches of soil should be neglected when determining passive pressure capacity. 7.2.4 Moisture Protection Considerations for Mat Foundations The following general guidelines for concrete mat construction where floor coverings are planned are presented for the consideration by the developer, design team, and contractor. Ponderosa Active Adult Community Page 18 132-2-1