Subgrades and pavements - Geotechnical Investigation and Design Tables - page 158

- Soil classification.
- Compaction level.
- Confinement.

Table 13.9 Typical values of subgrade CBR.

Soil type USC Description Drainage CBR %

symbol (standard)

Competent broken rock, GW, GP eg Sandstone, granite, All 20

Gravel sizes greywacke

Well graded, poorly graded

Competent broken rock - some GM, GC eg Phyllites, siltstones Silty, All 15

fines formed during construction SW, SP Clayey, well graded,

Gravel sizes, sands Poorly graded

Weathered Rock likely to weather ALL eg Shales, mudstones All Treat as

or degrade during construction soil below

Sands SM, SC Silty, clayey Good 10

Sands SM, SC Silty, clayey Poor 7

Inorganic silts ML Low plasticity Good

Inorganic silts ML Low plasticity Poor 5

Inorganic clays CL Low plasticity Good

Inorganic clays CH High plasticity Good

Inorganic silts MH High plasticity Good 3

Inorganic clays CL Low plasticity Poor

Inorganic silts MH High plasticity Poor 3 <

Inorganic clays CH High plasticity Poor

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The issues with converting CBR to modulus values are discussed in later sections.

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Underlying support is also required to obtain the above CBR values (Chapter 11).

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At the edge of an embankment (lack of edge support), CBR value is not applicable.

Figure 13.2 Laboratory CBR model versus field condition.

13.10 Properties of mechanically stable gradings
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The gradation is the key aspect to obtaining a mechanically stable pavement.

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This is the first step in development of a suitable specifications.