California Bearing Ratio Test - Soil Properties: Testing, Measurement, and Evaluation - page 393

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CHAPTER TWENTY-FOUR

California Bearing
Ratio Test

(Referenced Document: ASTM D 1883)

The California Bearing Ratio (CBR) test is a relatively simple test that
is commonly used to obtain an indication of the strength of a subgrade
soil, sub-base, and base course material for use in road and airfield pave-
ments. The test is used primarily to determine empirically required
thicknesses of flexible pavements for highways and airfield pavements.
CBR tests are normally performed on remolded (compacted) speci-
mens, although they may be conducted on undisturbed soils or on soil
in situ . Remolded specimens may be compacted to their maximum unit
weights at their optimum moisture contents (see Chapter 12 and ASTM
Methods D 698 and D 1557) if the CBR is desired at 100% maximum dry
unit weight and optimum moisture content. CBR tests may be per-
formed, however, over the ranges of unit weights and moisture contents
that are expected during construction. Soil specimens may be tested
unsoaked or soaked—the latter by immersing them in water for a
certain period of time in order to simulate very poor soil conditions.
The CBR for a soil is the ratio (expressed as a percentage) obtained
by dividing the penetration stress required to cause a 3-in. 2 area (hence,
a 1.95-in. diameter) piston to penetrate 0.10 in. into the soil by a stan-
dard penetration stress of 1,000 psi. This standard penetration stress is
roughly what is required to cause the same piston to penetrate 0.10 in.
into a mass of crushed rock. The CBR may be thought of, therefore, as an
indication of the strength of the soil relative to that of crushed rock.

INTRODUCTION

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