UFC 3-260-02
30 June 2001
CHAPTER 5
SITE INVESTIGATIONS
1. GENERAL. The design of pavements must be based on a complete and thorough investigation of
climatic conditions, topographic conditions, subgrade conditions, borrow areas, and sources of base
course, subbase course paving, and other materials. These preliminary investigations will necessitate
use of standard tests and all other available information such as aerial photographs, pavement
evaluations, condition surveys, construction records, soil maps, geologic maps, topographic maps, and
meteorological data. Table 5-1 lists sampling and testing standards used in soil investigations. Although
previous investigations should be used to establish preliminary soil characteristics, additional
investigations must be performed for final design.
2.
SUBGRADE INVESTIGATIONS.
a. Field Reconnaissance. Conduct field reconnaissance with the available topographical,
geographical, and soil maps; aerial photographs; meteorological data; previous investigations; and
condition surveys and pavement evaluation reports. This step should precede an exploratory boring
program.
b. Spacing of Preliminary Borings. The subgrade conditions in the area to be used for airfield
pavement construction should be determined by exploratory borings. The recommended maximum
spacing of borings should be as shown in the following tabulation, and should be supplemented with
additional borings whenever variations in soil conditions or unusual features are encountered.
Item
Spacing of Borings
Runway and taxiways
One boring every 61 to 152 meters (200 to 500 feet)
# 61 meters (200 ft) wide
longitudinally on alternating side of pavement centerline
Runways >61 meters
Two borings every 61 to 152 meters (200 to 500 feet
(200 feet) wide
longitudinally (one boring on each side of centerline)
Parking aprons and pads
One boring per 2,325-square-meter (25,000-square-foot) area
c. Depth of Borings. In cut sections, borings should extend to a minimum depth of 3 meters
(10 feet) below the finished grade or to rock. In shallow fill sections, borings should extend to a
minimum depth of 3 meters (10 feet) below the surface of the natural subgrade or to rock. Shallow fills
are those where the effect of the weight of the fill on the natural subgrade is small compared to the
weight of the design aircraft (generally 1.8 meters (6 feet) or less). In high-fill sections, borings should
extend to a minimum depth of 15.2 meters (50 feet) below the surface of the natural subgrade or to rock.
Results of borings will be used to develop boring logs as illustrated in Figure 5-1.
d. Samples. Soil samples should be obtained from the borings for classification purposes. After
these samples are classified, soil profiles should be developed and representative soils selected for
testing. A typical soil profile is shown in Figure 5-2. Test pits or large-diameter borings may be required
to obtain the samples needed for CBR testing, or to permit in-place tests of the various soil layers. The
types and number of samples required will depend on the characteristics of the subgrade soils. Subsoil
5-1
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