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6/24/75 AC 150/5300-4B <br />e. The distribution and physical properties of the various soils <br />encountered on the site should be investigated, and this information <br />combined with data on topography and climatic records will provide <br />planning material essential to airport development. An investiga- <br />tion of soil conditions at an airport site should include: <br />(1) Soil borings to determine the arrangement of different layers <br />of the soil profile with relation to the proposed subgrade <br />elevation. <br />(2) Sampling of the layers of soil. <br />(3) Testing of samples to determine the physical properties of the <br />various soil materials with respect to stability and subgrade <br />support. <br />(4) A survey to determine the availability of materials for use in <br />construction of the subgrade and pavement. <br />65. PAVEMENT THICKNESS. The determination of pavement thicknessis based <br />on a theoretical analysis of load distribution through pavements and <br />soils, the analysis of experimental pavement data, and a study of the <br />performance of pavements under actual service conditions. Pavement <br />thickness curves presented in Figure 12-1 have been developed from a <br />correlation of the data obtained from these analyses. Pavements con- <br />structed in accordance with these standards have proved satisfactory. <br />a. Under certain conditions, it may be necessary to utilize a <br />bituminous surface treatment on a prepared base course in lieu of <br />a more durable surface. If such is the case, a pavement so con- <br />structed is a temporary one with no inherent strength other than <br />that furnished by the underlying base, and the application of a <br />higher -type surface course is recommended at the earliest possible <br />date. <br />b. The base course thicknesses in Figure 12-1 range from 4 inches to 7 <br />inches, while the subbase thicknesses vary from 0 inch to 14 inches. <br />The subgrade classes shown are obtained from the corresponding soil <br />group and frost and drainage conditions stated in <br />AO 150/5320-6. <br />c. Use the pavement thickness determined from the curves in Figure 12-1 <br />for all areas of the airport pavement. Make no reduction in thick- <br />ness for "noncritical" areas of runways for utility aircraft. <br />Note, however, that in some cases, the loads imposed by maintenance <br />equipment are more severe than the aircraft loadings. In such <br />cases, the pavement must be designed to support the equipment <br />rather than the aircraft. <br />• Chap 12 <br />Par 64 <br />Page 75 <br />