<br />21 <br /> <br />certerra.com 4660 Main Street, Suite 100, Springfield, Oregon 97478 T: 541.393.6340 <br />We do not recommend use of the clay as structural fill due to the high expansive potential of the clay. <br /> <br />Structural fill should be compacted to a minimum of 92 percent of the maximum dry density as determined by <br />the Modified Proctor. Structural fill should be placed within +/- 2 percent of the optimum moisture for that material. <br />The Modified Proctor can be either the ASTM D1557 or AASHTO T180 test methods. When placed, lift thickness <br />should generally not exceed 12 inches prior to compacting. The type of compaction equipment used will <br />ultimately determine the maximum lift thickness. <br /> <br />A representative of the Geotechnical Engineer should approve any selected granular fill material before importing <br />it to the site. Each lift of compacted engineered fill should be evaluated by a representative of the Geotechnical <br />Engineer prior to placement of subsequent lifts. Fill should extend horizontally outward beyond the exterior <br />perimeter of pavements and slabs at least three and five feet, respectively, prior to sloping. <br /> <br /> <br />3.4 Foundation Recommendations <br /> <br />We have not been provided with any anticipated foundation loading information from the project structural <br />engineer, and have assumed typical maximum column, maximum wall and floor loads for the proposed <br />construction will not exceed 30 kips, 4 kips per linear foot and 150 psf, respectively. Based on the soils <br />encountered in our test pits, assumed maximum column load of 30 kips, the results of our laboratory testing, and <br />our current understanding of the project, we recommend the proposed structures be supported on conventional <br />shallow foundations. <br /> <br />Ideally, no foundations for this project would be supported on the potentially highly expansive clay encountered in <br />our explorations. However, given the thickness of the clay encountered (up to 7 feet thick), it may not be economical <br />to remove the clay in its entirety. It may be more economical to partially remove the clay with the understanding that <br />there is still some risk of excessive foundation cracking due to settlement or heaving of the potentially expansive <br />clay soils with our recommended 18-inch-thick minimum layer of structural fill under proposed footings. Note that <br />the thicker the lift of properly compacted structural fill underlying footings, the lower the risk. If the risk of excessive <br />foundation cracking due to the presence of the shrink/swell clays is not acceptable, the clay should be removed and <br />foundations should be supported on properly compacted structural fill overlying the hard undisturbed Eugene <br />Formation. <br /> <br />Proposed footings should not be supported directly supported on the clay stratum, due to the high shrink/sell <br />potential of the clay. We recommend footings be designed for a maximum net allowable soil bearing pressure of <br />2,500 psf, when supported on at least an 18-inch-thick layer of properly compacted structural fill (as outlined in <br />Section 3.3 of this report) when overlying the stiff undisturbed native clay stratum. In addition, the depth of footing <br />excavations (which includes the thickness of the recommended structural fill), should be extended to a depth of at <br />least 36 inches below adjacent finished grade (see Figure 6 below). This minimum footing excavation depth is <br />recommended to help reduce the effects of seasonal moisture changes, which could cause the potentially expansive