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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 silty sands. <br />The road base gravel layer ranged from 4 to 6 inches in thickness. The near surface subgrade soils <br />encountered beneath the gravel layer generally consisted of very silty sands [o [he maximum depth <br />investigated. The sands generally classified as SM soils in accordance with [he Unified Soil <br />Classification System. The sands will generally provide fair subgrade support for the gravel sections. <br />Free groundwater was not encountered in the test holes at the time of this investigation. <br />Pavement Recommendations: The subgrade soils encountered along the subject roadway were <br />somewhat variable and generally consisted of a layer of existing road base gravel materials overlying <br />natural sands. The existing road base gravels were in fair to good condition; therefore, a strength <br />coefficient of 0.11 was used in [he design of the new gravel section. The sands classified as A-4 soils in <br />accordance with the American Association of State Highway Transportation Officials (AASHTO) <br />classification system. Using the group index values obtained on the subgrade soil samples, a correlated <br />soils resilient modulus of 6,00 psi was used in the design process. <br />It is our understanding that the subject roadway is used as an access road and is typically subjected [o <br />light traffic volumes related to pickup trucks and occasional delivery and construction trucks; therefore, <br />we have assumed an 18 kip equivalent single axle loading (ESAL) of 6,500 for this roadway. <br />• The gravel section presented below is based on the field investigation, laboratory test results, [he <br />assumed traffic loadings and [he AASHTO Guide for Design of Pavement Structures (1986). The <br />design calculations are shown in Appendix A. <br />We recommend [hat a minimum of ~ inches of new road base gravels be added to the existing gravel <br />section. <br />Subgrade/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 at least 95% of the 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 at 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 />• <br />'CRac~ <br />