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• The subsurface conditions encountered in the test holes were variable and generally consisted of a layer <br />of existing road base gravel materials overlying natural clays. <br />The road base gravel layer ranged from 5 to 6 inches in thickness. The near surface subgrade soils <br />encountered beneath [he gravel layer generally consisted of clays to the maximum depth investigated. <br />The clays generally classified as CL soils in accordance with the Unified Soil Classification System. <br />The clays will generally provide poor to fair subgrade support for the proposed gravel sections. Free <br />groundwater was not encountered in the test holes at the time of drilling. <br />Pavement Recommendations: The subgrade soils encountered along [he proposed roadway were <br />somewhat variable and generally consisted of a layer of etas[ing road base gravel materials overlying <br />natural clays. The existing road base gravels were in good condition; therefore, a strength coefficient of <br />0.14 was used in the design of [he new gravel section. The underlying clays classified as A-7-6 [o A-6 <br />soils in accordance with the American Association of State Highway Transportation Officials <br />(AASHTO) classiFcation system. Using the group index values obtained on the subgrade soil samples, <br />a correlated soils resilient modulus of 5,000 psi was used in the design process. <br />It is our understanding that the subject roadway will be used as an access road which will generally be <br />subjected to light traffic volumes related to pickup trucks and occasional delivery and construction <br />trucks; therefore, we have assumed an 18 kip equivalent single axle loading (ESAL) of 6,500 for [his <br />roadway. <br />• The gravel sections presented below are based on the field investigation, laboratory test results, the <br />assumed traffic loadings and the AASHTO Guide for Design of Pavement Structures (1986). The <br />design calculations are shown in Appendix A. <br />We recommend that an additional 4 inches of road base gravels be placed over the existing gravels.. <br />Suberade/Gravel Preparation: Prior [o placing the new gravel section on the roadway, we recommend <br />that all of the loose and soft materials be removed and the existing gravels be reshaped and <br />recompacted to a[ leas[ 95% of [he standard Proctor density determined in accordance with ASTM <br />D698. <br />The new base course materials placed on the roadway should be compacted to at leas[ 95% of the <br />maximum modified Proctor density a[ or near the optimum moisture content, determined in <br />accordance with ASTM D1557. The base course gravels should be approved by the soils engineer, <br />prior to placement. We recommend that the road base gravels placed on the roadways meet the <br />requirements for Class V or VI base course in accordance with the Colorado Department of <br />Transportation specifications. <br />Drainaee: The collection and diversion of surface drainage away from the roadway area is extremely <br />important for satisfactory performance of the roadway. The design of surface drainage features should <br />be carefully considered to remove all water from all roadway areas. In addition, the maintenance of all <br />u <br />l (S a`S° <br />