Lesson 3/Learning Event 2
Select Material and Subbase
Areas within the airfield site and within a reasonable haul from the site should be explored for possible
sources of select material and subbase. Exploration procedures are similar to those described for
subgrades since the select material and subbase generally are natural materials. Test pits or large auger
borings put down with power augers are needed in gravelly materials.
Base and Pavement Aggregates.
Since these materials are generally crushed and processed, you should make a survey of existing
producers plus other possible sources in the general area. Significant savings have been made by
developing possible alternate quarry sites near the airfield location. This is particularly important in
remote areas where no commercial producers are operating and in areas where commercial production is
limited in quantity.
The engineer in charge of the soil survey is responsible for properly surveying, numbering, and
recording each auger boring, test pit, or other exploration investigation. A log is kept of each test hole
which shows the elevation (or depth below the surface) of the top and bottom of each soil layer, the field
identification of each soil encountered, and the number and type of each sample taken. Other
information which should be included in the log is that relating to density of the soil, changes in
moisture content, depth to ground water, and depth to rock. A typical boring log from a deliberate
survey is shown in Figure 25. The next page shows what to include on that log.