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The "subgrade" is the in situ material upon which the pavement structure is placed. Although there is a tendency to look at pavement performance in terms of pavement structure and mix design alone, the subgrade can often be the overriding factor in pavement performance.

Figures 1 and 2: Subgrade preparation on Kaua'i.


Subgrade Performance

A subgrade’s performance generally depends on two interrelated characteristics:

  1. Load bearing capacity. The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good.
  2. Volume changes. Most soils undergo some amount of volume change when exposed to excessive moisture or freezing conditions. Some clay soils shrink and swell depending upon their moisture content, while soils with excessive fines may be susceptible to frost heave in freezing areas (not really a concern in Hawai'i).  Ash, especially on the Big Island, can present volume change problems.

Poor subgrade should be avoided if possible, but when it is necessary to build over weak soils there are several methods used to improved subgrade performance:

Subgrade Physical Properties

HDOT characterizes subgrade by its R-Value while Counties most often characterize subgrade using CBR.

Subgrade materials are typically characterized by (1) their resistance to deformation under load, in other words, their stiffness or (2) their bearing capacity, in other words, their strength. In general, the more resistant to deformation a subgrade is, the more load it can support before reaching a critical deformation value. Although there are other factors involved when evaluating subgrade materials (such as shrink/swell in the case of certain clays and ash), stiffness is the most common characterization. There are three basic subgrade stiffness/strength characterizations commonly used in the U.S.:

Table 1: Typical CBR and Modulus of Elasticity Values for Various Materials

(USC given where appropriate)
Elastic or Resilient Modulus (psi)
1 - 2,000,000
Crushed Stone
(GW, GP, GM)
20 - 100
30 - 50
20,000 - 40,000
Sandy Soils
(SW, SP, SM, SC)
5 - 40
7 - 40
7,000 - 30,000
Silty Soils
(ML, MH)
3 - 15
5 - 25
5,000 - 20,000
Clay Soils
(CL, CH)
3 - 10
5 - 20
5,000 - 15,000
Organic Soils
(OH, OL, PT)
1 - 5
< 7

< 5,000


There are many different correlation equations between CBR, R-value and resilient modulus.  Each one has its limitations, which should be headed.  Table 2 presents some of the more popular correlation equations.

Table 2: Selected Subgrade Strength/Stiffness Correlation Equations

MR = (1500)(CBR)
Heukelom & Klomp (1962)
Only for fine-grained non-expansive soils with a soaked CBR of 10 or less.
MR = 1,000 + (555)(R-value)
1993 AASHTO Guide
Only for fine-grained non-expansive soils with R-values of 20 or less.
R-value = [1500(CBR) - 1155]/555
Only for fine-grained non-expansive soils with a soaked CBR of 8 or less.

MR = 2555 x CBR0.64

AASHTO 2002 Design Guide (not yet released)
A fair conversion over a wide range of values.
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