Typical Values for Compacted Soils

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Subgrade | Loads | Environment | Drainage | References

Subgrade

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 ofpavement structure and mix design alone, the subgrade

can often be the overriding factor in pavement performance.

Su b g r a d e Pe r f o r m a n c e

A subgrades 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 (normally

a concern east of the Cascade mountains).

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:

z Removal and replacement (over-

excavation). Poor subgrade soil can

simply be removed and replaced

with higher quality fill. Although

this is simple in concept, it can be

expensive.

z Stabilization with a cementitious or

asphaltic binder. The addition of an

appropriate binder (such as lime, portland cement or emulsified asphalt) can increase subgrade

stiffness and/or reduce swelling tendencies.

Figure 1: Subgrade Preparation on SR 528

in Marysville

Figure 2: Subgrade Preparation on

University Avenue in Seattle

WAPA P a v em e n t N o t e on Expansive Soils

Expansive soils are ones that swell in volume when

subjected to moisture. These swelling soils typically

contain clay minerals that attract and absorb water. When

water is introduced to expansive clays, the water molecules

are pulled into gaps between the clay plates. As more

water is absorbed, the plates are forced further apart,

leading to an increase in soil pore pressure (Handy, 1995).

If this increased pressure exceeds surcharge pressure

(including the weight of the pavement) the soil will expand

in volume to a point where these pressures are once again

in balance.

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z Additional base layers. Marginally poor subgrade soils may be made acceptable by using additional

base layers. These layers spread pavement loads over a larger subgrade area. This option is rather

perilous; when designing pavements for poor subgrades the temptation may be to just design a

thicker section with more base material because the thicker section will satisfy most design

equations. However, these equations are at least in part empirical and were usually not intended to

be used in extreme cases. In short, a thick pavement structure over a poor subgrade may not make

a good pavement.

S u b g r a d e P h y s i c a l P r o p e r t i e s

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 swell in the

case of certain clays), stiffness is the most common characterization. There are three basic subgrade

stiffness/strength characterizations commonly used in the U.S.:

z California bearing ratio (CBR). A simple test that compares the bearing capacity of a material with

that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR

100%). CBR is basically a measure of strength. It is primarily intended for, but not limited to,

evaluating the strength of cohesive materials having maximum particle sizes less than 0.75 inches

(AASHTO, 2000). It was developed by the California Division of Highways around 1930 and was

subsequently adopted by numerous states, counties, U.S. federal agencies and internationally. Most

agency and commercial geotechnical laboratories in the U.S. are equipped to perform CBR tests.

z Resistance value (R-Value). A test that expresses a material's resistance to deformation as a

function of the ratio of transmitted lateral pressure to applied vertical pressure. It is essentially a

modified triaxial compression test. Materials tested are assigned an R-value. The testing apparatus

used in the R-value test is called a stabilometer and is identical to the one used in Hveem HMA mix

design. The R-Value is basically a measure of stiffness.

z Resilient modulus (MR). A test used to estimate elastic modulus (a material's stress-strain

relationship). The resilient modulus test applies a repeated axial cyclic stress of fixed magnitude,

load duration and cyclic duration to a cylindrical test specimen. While the specimen is subjected to

this dynamic cyclic stress, it is also subjected to a static confining stress provided by a triaxial

pressure chamber. It is essentially a cyclic version of a triaxial compression test; the cyclic load

application is thought to more accurately simulate actual traffic loading. Resilient modulus is

basically a measure of stiffness.

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

Material

(USC given where

appropriate)

CBRElastic Modulus

(psi)

Diamond - 170,000,000

Steel - 30,000,000

Aluminum - 10,000,000

Wood - 1 - 2,000,000

Crushed Stone

(GW, GP, GM)20 - 100 20,000 - 40,000

Sandy Soils

(SW, SP, SM, SC)5 - 40 7,000 - 30,000

Silty Soils(ML, MH)

3 - 15 5,000-20,000

Clay Soils


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A widely used empirical relationship developed by Heukelom and Klomp (1962) and used in the 1993 AASHTO

Guide is:

MR

= (1500) (CBR)

This equation is restricted to fine grained materials with soaked CBR values of 10 or less. Like all such

correlations, it should be used with caution.

Similarly, the 1993 AASHTO Guide offers the following correlation equation between R-value and elastic

modulus for fine-grained soils with R-values less than or equal to 20.

MR

= 1,000 + (555)(R-value)

A WSDOT developed relationship between the R-value and resilient modulus is shown below.

MR

= 720.5 (e0.0521(R-value)-1)

(CL, CH) 3 - 10 5,000 - 15,000

Organic Soils

(OH, OL, PT)1 - 5 < 5,000

WAPA P a v em e n t N o t e on Subgrade Stiffness/Strength Tests

All three types of subgrade stiffness/strength tests are used in Washington State to some degree. WSDOT

uses the resilient modulus when possible, while many geotechnical firms typically use CBR. Although not

common any more, WSDOT still has an R-Value procedure. It is possible to convert one value to another

but these conversions are based on empirically derived equations and may not be appropriate for the

specific conditions being considered. Use the below conversion equations with extreme caution. For

instance, in WSDOT Test Method 611 design R-Values are determined at an exudation pressure of 400 psi,

while AASHTO T 190 allows the use of a 300 psi exudation pressure. Thus, WSDOT R-Values may differ

from AASHTO R-Values.

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Typical Values for Compacted Soils