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AC 150/5300-4B 6/24/75 structed and drained pavement base may serve the airport's immediate operational requirements and be adequately protected by a limited surfacing such as a surface treatment or a single bituminous and stone chip seal application. b. It should also be recognized, when the thinner surfaces are utilized in stage construction, that constant surveillance and timely main- tenance are required. Even so, this may be the most satisfactory approach to construction with limited funds. 64. SOIL INVESTIGATION AND EVALUATION. Accurate identification and evalua- tion of the pavement foundation soils are a basic consideration in the design of pavements. The classification of these soils is discussed in detail in AC 150/5320-6 However, aspects which are particularly important to the airport designer are highlighted in the text that follows. a. Soil conditions and the availability of suitable construction materials are the most important items affecting construction costs of landing areas and pavements. Grading costs are directly related to the difficulty with which excavation can be accomplished and compaction obtained. b. The subgrade soil carries the load imposed by aircraft utilizing the facility. The pavement serves to distribute this load to the subgrade over a greater area than that of the tire contact. Thicker pavements distribute the load over a greater subgrade area. Therefore, the more unstable the subgrade soil, the greater is the required area of load distribution and consequently the greater is the required thickness of pavement. The soils having the best engineering characteristics encountered in the grading and excavating operations should be used in the upper layers of the subgrade. c. In this manner, utilization of satisfactory local material can reduce the cost of pavements. Quite often, however, the soils encountered at the site are unsuitable and will not carry the loads intended. In such cases, the physical properties of unsuit- able soils may be changed by admixtures, such as cement, lime, etc., to produce the desired qualities or may be overcome by capping or topping with suitable, locally available, borrow material. d. In addition to the relationship which soil conditions bear to grading and paving operations, they determine the necessity for underdrains and materially influence the amount of surface runoff. Thus, they have a consequent effect on the size and extent of other drainage, structures, and facilities. (See FAA publication AC 150/5320-5, Airport Drainage.) Page 74 Chap 12 Par 63