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The final subgrade surface should be proof rolled with a heavily loaded truck prior to placing the subbase <br />and base course gravels. Areas that deform or pump excessively under the wheel loads should be stabilized <br />prior to placement of subbase gravels. A layer of stabilization fabric may be required on the finished <br />subgrade surface, below the gravel materials to help stabilize and separate the gravels from the subgrade <br />soils. The soils engineer should make this determination after the subgrade surface has been properly <br />prepared. <br />The base course and subbase gravels should be compaMed to at ]east 95% of the maximum modified <br />Proctor density, neaz optimum moisture content determined by ASTM Dt557. We recommend that the <br />subbase gravels meet the requirements for Class I base course aggregates in accordance with the Colorado <br />Department of Transportation (CDOT) Specifications, 1999. The base course gravels should meet the <br />requirements for Class V or VI base course aggregates in accordance with CDOT Specifications. We <br />recommend that the subbase gravels and base course gravels be tested for compaction and the thickness of <br />the completed lifts be measured to verify that the required gravel sections have been constructed. <br />Drainase: For adequate performance of any roadway, positive surface drainage must be provided and <br />maintained. This includes periodic cleaning and regazding of the drainage ditches along the edges of the <br />roadway. In addition, the roadway surface should be graded on a periodic basis to ensure that sufficient <br />crown or super elevations are maintained so that the roadway surface adequately drains into the ditches. <br />We recommend that the roadway be constructed with a minimum crown or super elevation of 2 percent. <br />• The collection and diversion of surface drainage away from the roadway azea is extremely important to <br />satisfactory performance of the roadway. The design of surface drainage features should be carefully <br />considered to remove all water from the roadway surface and embankment azeas. In addition, the <br />maintenance of al! drainage structures and roadway surface should be conducted pcriodically to ensure <br />proper performance. Subsurface drainage at the site may contribute to roadway distress. Groundwater at <br />the site is capable of entering the subgrade soils and base course and causes them to lose strength. <br />Foundation Recommendations: Based on the soils encountered in the test holes, the results of the field <br />and laboratory investigations and the proposed construction, we believe a safe type of foundation system is <br />spread footings founded on a minimum of 2 feet of recompacted natural soils or on the sandstone bedrock. <br />The design and construction details presented below should be observed if a shallow foundation system is <br />opted for. The precautions and recommendations itemized below will not prevent the movement of the <br />foundations if the underlying soils materials consolidate or swell; however, they should reduce the amount <br />of differential movement beneath the foundation system. After the clay materials aze properly moisture <br />conditioned and recompacted, the materials may still exhibit a low to moderate swell potential if the clay <br />materials aze wetted. <br />1) The footings placed on a minimum of 2 feet of recompacted naturzl soils should be designed using <br />an allowable soil bearing pressure of 2,500 psf. The footings placed on the undisturbed sandstone <br />bedrock should be designed using an allowable soil bearing pressure of 5,000 psf The footings <br />should also be designed with a minimum dead load pressure of at least 600 psf. <br />Job Numtrer: 03-5838 NWCC, Inc. Page4 <br />