Page 300 - Planning and Design of Airports

Structural Design of Airport Pavements 261

through a No. 4 sieve. Fine grained soils, known also as silts and clays,
are subdivided into two groups on the basis of their liquid limits.
These soils are finally grouped into one of 15 different groupings.
These groupings are

GW: Well-graded gravels and gravel-sand mixtures, little or no
fines
GP: Poorly graded gravels and gravel-sand mixtures, little or no
fi nes
GM: Silty gravels, gravel-sand-silt mixtures
GC: Clayey gravels, gravel-sand-clay mixtures
SW: Well-graded sands and gravelly sands, little or no fi nes
SP: Poorly graded sands and gravelly sands, little or no fi nes
SM: Silty sands, sand-silt mixtures
SC: Clayey sands, sand-clay mixtures

ML: Inorganic silts, very fine sands, rock flour, silty or clayey fi ne

sands
CL: Inorganic clays of low to medium plasticity, gravelly clays,
silty clays, lean clays
OL: Organic silts and organic silty clays of low plasticity
MH: Inorganic silts, micaceous or diatomaceous fine sands or silts,

plastic silts
CH: Inorganic clays or high plasticity, fat clays
OH: Organic clays of medium to high plasticity
PT: Peat, muck, and other highly organic soils

A flowchart illustrating the procedure for the classification of
soils by the unified system is given in Fig. 7-2. The uses of the various
soil materials for pavement foundations are described in Table 7-3.
It should be noted that column 11 in Table 7-3 refers to the soil’s
“field CBR” value, or “California Bearing Ratio,” a value of the
strength of material used in flexible pavement bases, and column 12
in Table 7-3 refers to the soil’s “subgrade modulus” or “k value,” a
value of the bearing capacity of the soil, estimated using a plate bear-
ing test.
The soil’s field CBR value is determined by the CBR method of
pavement design, which is applied primarily to flexible pavements.
The CBR method of design was developed by the California Division
in 1928. The method subsequently was adopted for military airport
use by the Corps of Engineers, U.S. Army, shortly after the outbreak of
World War II. The outbreak of the war required that a decision be
made with little delay concerning a design method. At the time, there
were no methods available specifically developed for airport pave-
ments. It was apparent that the time required to develop a completed
new method of design would preclude its use in a war emergency