Geology Reference
In-Depth Information
Table 24-1
Soil Ratings for Roads and Runways [2, 3]
Classification System
CBR
General
No.
Rating
Uses
Unified
AASHTO
0-3
Very poor
Subgrade
OH, CH, MH, OL
A5, A6, A7
3-7
Poor to fair
Subgrade
OH, CH, MH, OL
A4, A5, A6, A7
7-20
Fair
Sub-base
OL, CL, ML, SC, SM, SP
A2, A4, A6, A7
20-50
Good
Base, sub-base
GM, GC, SW, SM, SP, GP
A1 b, A2-5, A3, A2-6
50
Excellent
Base
GW, GM
A1 a, A2-4, A3
The major result of a CBR test is, of course, the CBR value. Reports of this
test should also include the values of other parameters determined dur-
ing the test, including moisture contents of the soil sample (before and
after compaction, of the top 1-in. layer after soaking, and the average
after soaking), dry unit weights of the soil specimen before and after
soaking, and the swell. The condition of the sample (soaked or unsoaked),
the method used for preparation and compaction of the specimen [either
ASTM Method D 698 (Chapter 12) or D 1557], and the curve of penetra-
tion stress versus penetration should also be reported.
ASTM D 1883-05 permits the omission of load readings at penetra-
tions of 0.40 in. and 0.50 in. if the capacity of the testing machine has
been reached. If, however, bearing ratio values are desired for penetra-
tions of 0.30 in., 0.40 in., and 0.50 in., corrected penetration stress values
for these penetrations should be divided by the standard penetration
stresses of 1,900, 2,300 and 2,600 psi, respectively (and then multiplied
by 100 to express the answer as a percentage).
As mentioned in the introduction, the CBR of a soil gives an indi-
cation of its strength as subgrade, sub-base, and base course material.
The CBR is used primarily to determine empirically the required thick-
ness of flexible pavements for highways and airfield pavements. Some
general ratings of soils to be used as subgrade, sub-base, and base
courses of roads and airfield runways corresponding to various ranges
of CBR values are given in Table 24-1.
SUMMARY
[1]
ASTM, 2007 Annual Book of ASTM Standards , West Conshohocken,
PA, 2007. Copyright, American Society for Testing and Materials, 100
Barr Harbor Drive, West Conshohocken, PA 19428-2959. Reprinted with
permission.
REFERENCES
[2]
Joseph E. Bowles, Engineering Properties of Soils and Their Measurement ,
2d ed., McGraw-Hill Book Company, New York, 1978.
[3]
The Asphalt Institute, The Asphalt Handbook , College Park, MD, 1962.
 
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