<br />23 <br /> <br />certerra.com 4660 Main Street, Suite 100, Springfield, Oregon 97478 T: 541.393.6340 <br />Please note that our subsurface investigation was limited to the areas explored. At a minimum, we recommend that <br />during construction we observe all footing and floor slab excavations to observe that the material is similar to what <br />we observed during our subsurface investigation and, subsequently, is similar to the material our recommendations <br />are based on. Unsuitable soil zones encountered at the bottom of the foundation excavations should be removed to <br />the level of suitable soils or properly compacted structural fill as directed by the Geotechnical Engineer. Cavities <br />formed as a result of excavation of unsuitable soil zones should be backfilled and compacted with structural fill in <br />accordance with Section 3.3 above. In addition, our observations would include verifying that the footings have been <br />placed on at least 18 inches of properly compacted structural fill if overlying the stiff native clay stratum and <br />embedded as recommended in Figure 6 above. <br /> <br />Exterior footings and foundations in unheated areas should be located at a depth of at least 12 inches below the final <br />exterior grade to provide adequate frost protection. If the building is to be constructed during the winter months or <br />if the foundation soils will likely be subjected to freezing temperatures after foundation construction, then the <br />foundation soils should be adequately protected from freezing. Surface run-off water should be permanently drained <br />away from the foundation excavations and not allowed to pond. <br /> <br />Lateral frictional resistance between the base of footings and the subgrade can be expressed as the applied vertical <br />load multiplied by a coefficient of friction of 0.35 for concrete foundations bearing directly on rock gravel structural <br />fill. In addition, lateral loads may be resisted by passive earth pressures based on an equivalent fluid density of 300 <br />pounds per cubic foot (pcf) for footings poured "neat" against in-situ soils, or properly backfilled with structural fill. <br />These are ultimate values - we recommend a factor of safety of 1.5 be applied to the equivalent fluid pressure, which <br />is appropriate due to the amount of movement required to develop full passive resistance. <br /> <br />Provided our recommendations above are followed, we do not anticipate that total and differential settlement will <br />exceed the typical values of 1 inch and ½-inch of total and differential settlement, respectively, between foundation <br />elements that are, at most, 20 feet apart (Note: complies with National Home Builders Association <br />recommendations). <br /> <br /> <br />3.5 Slab on Grade Recommendations <br /> <br />For the purposes of this report, we have assumed that maximum floor slab loads will not exceed 150 psf. <br /> <br />As mentioned above, ideally, no slabs for this project would be supported on the potentially highly expansive clay <br />encountered in our explorations. However, given the thickness of the clay encountered (up to 7 feet thick), it may <br />not be economical to remove the clay in its entirety. It may be more economical to partially remove the clay with the <br />understanding that there is still some risk of excessive cracking due to settlement or heaving of the potentially <br />expansive clay soils with our recommended layer of structural fill under proposed slabs. Note that the thicker the lift <br />of properly compacted structural fill underlying proposed slabs, the lower the risk. If the risk of excessive cracking <br />due to the presence of the shrink/swell clays is not acceptable, the clay should be removed and slabs should be