Subgrades and pavements - Geotechnical Investigation and Design Tables - page 161

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For strength improvements requirements, the UCS or CBR test is used in the
literature.

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Test results may show CBR values above 100%. Irrespective of test results a
subgrade design CBR of 20% maximum should be used.

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For strength, a target CBR value (at 7 days) of 60% used.

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For strength, a target UCS value (at 28 days) of 1MPa used. 7Day UCS ∼ ½ 28Day

UCS.

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Add 1% additional lime above the laboratory test requirements to account for
unevenness in mixing in the field.

13.15 Soil stabilisation with bitumen
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Bitumen is a good waterproofing agent, and preserves the natural dry strength.

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Asphalt, Bitumen and Tar should be distinguished (Ingles, 1987). These material
properties are temperature dependent:
- Asphalt - most water repellent, but most expensive.
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Bitumen - most widely available.

Table 13.15 Typical bitumen content for various soil types (Ingles, 1987).

Soil type Bitumen requirement

Fine crushed rock - open graded 3.5%-6.5%

Fine crushed rock - dense graded 4.5-7.5%

Well graded and poorly graded gravels GW, GP

Silty and clayey gravels GM, GC,

Well graded and poorly graded sands SW, SP 2%-6%

Silty sands SM

Clayey sands SC

Sandy clay, silty clays, low plasticity inorganic clays and silts ML, CL,

Highly plastic inorganic silts MH

Highly Plastic inorganic clays CH 4%-7%

Highly organic OL, OH, Pt Not recommended

13.16 Pavement strength for gravels
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The pavement strength requirement is based on the type of road.

Table 13.16 Typical pavement strength requirements.

Conditions CBR strength Comments

“Standard'' requirements 80% Soaked On major roads at least 100 mm of pavement

layer

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80% CBR

Low traffic roads 60% unsoaked Top 100 mm of base layer

30% Sub base

Rural traffic roads/arid 30% unsoaked Upper sub base

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to semi - arid regions 15% Lower sub base

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