The FAA methods (8) of pavement design calls for accurate identification and
evaluation of pavement foundation (subgrade) conditions. The recommended method
requires thorough investigations to determine the distribution and physical properties
of pavement foundation soils. A soil survey should be made to describe the soils that
comprise the soil profile and to indicate subsurface water conditions. It is recommended
that representative samples of soil be taken by means of a soil auger. Generally, borings
should be taken along runway and taxiway centerlines at 200-ft intervals. One boring
for every 10,000 ft2 should be made under other pavement areas. Such borings normally
are made to a depth of 10 ft below the finished grade in cut areas and 10 ft below the
existing ground surface in fill areas. Borrow areas should be adequately sampled to
establish the physical characteristics of the borrow material.