<br />24 <br /> <br />certerra.com 4660 Main Street, Suite 100, Springfield, Oregon 97478 T: 541.393.6340 <br />supported on properly compacted structural fill overlying the hard undisturbed Eugene Formation. In addition, floor <br />slabs could be constructed with a thicker concrete section and additional rebar to make them more rigid. The option <br />to thicken floor slabs and/or add additional rebar should be evaluated by the project Structural Engineer. <br /> <br />In order to provide uniform subgrade reaction beneath floor slabs, we recommend supporting floor slabs on a <br />minimum 12-inch-thick layer of properly compacted granular structural fill if overlying firm clay subgrade. <br />Alternatively, proposed slabs may be supported on a 4-inch-thick leveling course of properly compacted structural <br />fill (as outlined in Section 3.3 of this report) overlying the hard undisturbed Eugene Formation stratum. <br /> <br />Based on the native clay encountered at this site, the design of slabs-on-grade overlying the clay can be based on <br />a subgrade modulus (k) of 150 pci. This subgrade modulus value represents an anticipated value which would be <br />obtained in a standard in-situ plate test with a 1-foot square plate. Use of this subgrade modulus for design or other <br />on-grade structural elements should include appropriate modification based on dimensions as necessary. <br /> <br />Imported granular structural fill should be placed and compacted as required in Section 3.3 of this report. Prior to <br />placing the structural fill, the subgrade surface should be prepared and then proofroll tested as discussed in Section <br />3.2 of this report in order to identify any soft or unstable areas that should be removed prior to structural fill <br />placement. The proofroll should be observed by a representative of the Geotechnical Engineer. Note: as mentioned <br />above, during the dry season the exposed clay subgrade should be covered with structural fill the same day it is <br />exposed to help prevent it from drying out. <br /> <br />The recommended structural fill should provide a capillary break to limit migration of moisture through the concrete <br />slab, however, if additional protection against moisture vapor is desired (i.e. areas where there will be floor coverings) <br />a moisture vapor retarding membrane should be incorporated into the design. Factors such as cost, special <br />considerations for construction, and the type of floor coverings suggest that decisions on the use of vapor retarding <br />membranes be made by the architect and the owner. <br /> <br /> <br />3.6 Pavement Recommendations <br /> <br />Our scope of services included evaluating the surface soils for the specific purpose of a detailed pavement analysis. <br />As discussed above, the primary factor in the development of pavement areas on this site is the presence of <br />potentially highly expansive clay soils. The only way to completely mitigate the potential for the potentially expansive <br />fat clay to reduce the life of proposed pavements would be wholesale removal of the clay soils from beneath <br />pavement areas. 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 cracking due to settlement or heaving of the potentially expansive clay soils with <br />our recommended layer of structural fill. Note that the thicker the lift of properly compacted structural fill underlying <br />proposed pavements, the lower the risk. If the risk of excessive cracking due to the presence of the shrink/swell