Page 303 - Planning and Design of Airports

Structural Design of Airport Pavements 263

program. Consequently, it was decided to review all available meth-
ods for the design of highway pavements and to select one which
could be adopted for airfield use. The criteria for selecting a method
were many. Among the more important were (1) simplicity in proce-
dures for testing the subgrade and the pavement components, (2) a
record of satisfactory experience, and (3) adaptation to the airport
problem in a reasonable time. After several months of investigating
of suggested methods, the CBR method was tentatively adopted.
Application of the CBR method enables the designer to determine
the required thickness of the subbase, base, and surface course by
entering a set of design curves using the results of a relatively simple
soil test.

The CBR Test
The CBR test expresses an index of the shearing strength of soil.
Essentially the test consists of compacting about 10 lb of soil into a
6-in-diameter mold, placing a weight, known as a surcharge, on the
surface of the sample, immersing the sample in water for 4 days, and
penetrating the soaked sample with the steel piston approximately
2 in in diameter at a specified rate of loading. The resistance of the soil
to penetration, expressed as a percentage of the resistance for a stand-
ard crushed limestone, is the CBR value for the soil. Thus, a CBR of
50 means that the stress necessary for the surcharge to penetrate the
soil sample a specified distance is one-half that required for the sur-
charge to penetrate the same distance in the standard crushed lime-
stone. The relationship is usually based on a penetration of the piston
2
of 0.1 in with 1000 lb/in used as the stress required to penetrate the
crushed limestone at 0.1 in penetration. As illustrated in Table 7-3,
soils range in CBR values from relatively weak fine-grained and
highly organic soils with CBR values as low as 3, to wide-grained
coarse soils with CBR values as high as 80 (although CBR testing has
been found to be somewhat inaccurate for very gravelly soils, and for
application a CBR value of no higher of 50 should be applied).
The Plate Bearing Test
The modulus subgrade of reaction, or k value of the subgrade is deter-
mined by what is known as a field plate bearing test. This test con-
sists of applying loads by means of a hydraulic jack through a jacking
frame on to a steel plate 30 in in diameter on the soil. By loading the
plate, a load-versus-deformation curve is obtained. The k value is
determined to be the pressure required to produce a unit deflection of
the pavement foundation, measured in pounds per cubic inch. k val-
ues range from less than 150 (considered “very poor”) to more than
300 (considered “very good”). In general, the greater the coarseness
of the soil, the higher k value and the less deflection for a given load-
ing can be expected.