Guideline for the Design and Use of Asphalt Pavements for ...

General
Classification
Group Classification
Sieve Analysis:
Percent Passing:
Table 2-9 Classification of Soil-Aggregate Mixtures with Suggested Subgroups
No. 10 50 Max
Granular Materials
(35% or less passing No. 200)
Silt-Clay Materials
(More than 35% passing No. 200)
A-1 A-3 A-2 A–4 A-5 A-6 A-7
A-1-a A-1-b A-2-4 A-2-5 A-2-6 A-2-7
No. 40 30 Max 50 Max 51 Min
No. 200 15 Max 25 Max 10 Max 35 Max 35 Max 35 Max 35 Max 36 Min 36 Min 36 Min 36 Min
Characteristics of
fraction passing
No. 40:
Liquid Limit 40 Max 41 Min 40 Max 41 Min 40 Max 41 Min 40 Max 41 Min
Plasticity Index 6 Max N.P. 10 Max 10 Max 11 Min 11 Min 10 Max 10 Max 11 Min 11 Min
Usual types of Significant
Constituent Materials
General Rating
as Subgrade
SUBGRADE STRENGTH
Stone Fragments,
Gravel & Sand
Fine
Sand
Silty or Clayey Gravel and Sand Silty Soils Clayey Soils
Excellent to Good Fair to Poor Poor
Within a given soil group the strength properties of the soil are a function of the density, void ratio, and
moisture content. Fine grained soils are much more influenced by changes in these properties than are
coarse grained soils. Because thickness calculations depend on the strength of the finished subgrade, the
soil should be tested for this information. Tests are based on bearing capacity related to the moisture and
density of the soil. The California Bearing Ratio (CBR) is one of the most widely used methods of measuring
the strength of the subgrade. The CBR value can be measured directly on the in-situ soils in the field or on
remolded samples in the laboratory. Determining an R-Value (Hveem method) is another method of
measuring the subgrade strength, but can only be performed on a laboratory prepared sample.
The lower the CBR or R-Value of a particular soil, the less strength it has to support the pavement. This
means that a thicker pavement structure is needed on a soil with a low CBR or R-Value than on a soil with a
high CBR or R-Value. Generally, clays have a CBR classification of six or less. Silty loam and sandy loam
soils are next with CBR values of six to eight. The best soils for road building purposes are the sands and
gravels whose CBR ratings normally exceed ten. Corresponding R-Values are ten or less for clays, ten to
thirty for silty loam and sandy loam soils, and thirty or higher for sands and gravels.
The change in pavement thickness needed to carry a given traffic load is not directly proportional to the
change in CBR or R-value of the subgrade soil. For example, a one-unit change in CBR from five to four
requires a greater increase in pavement thickness than does a one-unit CBR change from ten to nine.
For the design method presented in this Guide, CBR and R-Values are converted to a Resilient Modulus
(MR) so the CDOT design procedure, which parallels the AASHTO design procedure, can be used. MR is a
measure of the elastic property of soil recognizing certain nonlinear characteristics. MR can be used directly
for the design of flexible pavements. Direct measurement of subgrade reaction can be made if such
procedures are considered preferable to the designer.
The resilient modulus was selected to replace a soil support value for the following reasons:
A-7-5;
A-7-6