Page 298 - Planning and Design of Airports

Structural Design of Airport Pavements 259

The subbase course is also composed of treated or untreated
material, typically unprocessed pit-run material or material selected
form a suitable excavation on the site. The function of the subbase is
the same as that of the base. Whether or not a subbase is required, or
how many subbase layers are required, is a function of the type of
loads on the pavement, as well as the type and quality of soil, or sub-
grade, on which the pavement will be resting. For most rigid pave-
ments, the surface course rests directly on the subbase.
The design of the thickness of each of the above layers is of primary
concern to airport pavement engineers. The two primary factors that
contribute to the design thickness of airfield pavement layers are the
soil base and the volume and weight of the traffic using the pavement.
As such, the first steps in pavement analysis are an investigation of the
soil on which the pavement will be placed, and an estimation of the
annual traffic volume on the pavement.

Soil Investigation and Evaluation
Accurate identification and evaluation of pavement foundations are
essential to the proper design of the pavement structure. The sub-
grade supports the pavement and the loads placed upon the pave-
ment surface. The function of the pavement is to distribute the loads
to the subgrade, and the greater the capability of the subgrade to sup-
port the loads, the less the required thickness of the pavement.
Soil investigation consists of a soil survey to determine the
arrangement of the different layers of soil in relation to the subgrade
elevation, a sampling and testing of the various layers of soil to deter-
mine the physical properties of the soil, and a survey to determine the
availability and suitability of local materials for use in the construc-
tion of the subgrade and pavement. Surveys and sampling are usu-
ally accomplished through soil borings to determine the soil of rock
profile and its lateral extent. The sampled materials are then tested to
determine soil types, gradation or particle sizes, liquid and plastic
limits, moisture-density relationships, shrinkage factors, permeabil-
ity, organic content, and strength properties. In the United States, soil
surveys are often conducted using a variety of methods, including
referring to U.S. Geological Survey (USGS) geodetic maps, aerial
photography, and soil borings. The FAA recommends borings of
given spacing and depths for soil surveys as illustrated in Table 7-1.
In the United States, evaluation of sampled soils for the purpose
of airfield pavement design is performed according to the U.S. Army
Corps of Engineers Unified Soil Classification (USC or “unified”)
System, as illustrated in Table 7-2. Under the unified system, soils are
initially classified as either coarse-grained, fine-grained, or highly
organic soils. Coarse-grained soils are those that do not filter through
a No. 200 grade sieve. Coarse-grained soils are further divided into
gravels and sands, as a function of the percentage of soil that filters