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4.2.5 Mat Foundation <br />A mat foundation can be used for support of proposed residential buildings. An allowable <br />soil bearing pressure of 1,250 psf may be used for the design of the mat at the surface under <br />the slab area. <br />The allowable bearing value is for total dead loads and frequently applied live loads and may <br />be increased by one-third for short durations of loading which will include the effect of wind or <br />seismic forces. A coefficient of vertical subgrade reaction, k, of 85 pounds per cubic inch <br />(pci) may be used to evaluate the pressure distribution beneath the mat foundation. <br />The magnitude of total and differential settlements of the mat foundation will be a function of <br />the structural design and stiffness of the mat. Based on assumed structural loads, we estimate <br />that total static settlement will be on the order of an inch at the center of the mat foundation. <br />Post construction differential settlement can be taken as one-half of the maximum estimated <br />settlement <br />Resistance to lateral loads can be provided by friction acting at the base of foundations and <br />by passive earth pressure. Foundations may be designed for a coefficient of friction of 0.35. <br />Coordination with the structural engineer will be required in order to ensure structural loads <br />are adequately distributed throughout the mat foundation to avoid localized stress <br />concentrations resulting in potential settlement. The foundation plan should be reviewed by <br />LGC to confirm preliminary estimated total and differential static settlements. <br />4.2.6 Foundation Settlement <br />Based on our current understanding of the project, the results of our site investigation and the <br />recommended remedial grading with shallow foundations embedded into compacted fills or <br />competent native soils, we estimate the post -construction static settlement of the site to be less <br />than 1 inch with a differential settlement of approximately of 0.5-inches in 30 feet. Post - <br />construction settlements should also include the estimated differential seismically induced <br />settlements of 1.5-inches in 30 feet. For a total static and seismically induced differential <br />settlement of up 2-inches in 30 feet. <br />4.3 Lateral Earth Pressures for Retaininiz Walls <br />The following lateral earth pressures may be used for the design of any future site retaining walls, if <br />any. Due to the variable nature of onsite soils, we recommend site retaining walls be backfilled with <br />select soils or clean sand having a sand equivalence of greater than 30. Select soils should consist of <br />clean, granular soils (less than 15 percent passing the No. 200 sieve) of very low expansion potential <br />(expansion index 20 or less based on U.B.C. 18-2). The recommended lateral pressures for clean sand <br />or approved select soils for level or sloping backfill are presented in Table 4. <br />Project i y ouncil 18 — 836 1 3 , 2021 <br />