Material and testing variability - Geotechnical Investigation and Design Tables - page 114

10.5 Guidelines for inherent soil variability
•

Variability is therefore the sum of natural variability and the testing variability.

10.6 Compaction testing
•

In a compaction specification, the density ratio has less variation than the moisture
ratio.

•

The density ratio controls can be based on a standard deviation of 3% or less
(Hilf, 1991).

Table 10.6 Precision values (MTRD, 1994).

Conditions Maximum dry density Optimum moisture content

Granular materials Clay

Repeatability 1% of mean 10% of mean 13% of mean

Reproducibility 2.5% of mean 12% of mean 19% of mean

•

The placement moisture is therefore only a guide to achieving the target density,
and one should not place undue emphasis on such a variable parameter.

10.7 Guidelines for compaction control testing
•

Clays tend to be more variable than granular materials.

•

At higher moisture contents, the variation in densities is reduced.

Table 10.7 Guidelines for compaction control testing.

Test control Coefficient of variation

Homogeneous conditions Typical Highly variable

Maximum dry density 1.5% 3% 5%

Optimum moisture content 15% 20% 30%

10.8 Subgrade and road material variability
•

Testing for road materials is the more common type of test.

Table 10.8 Coefficient of variations for road materials (extracted from Lee et al., 1983).

Test type Test Coefficient of variation

Strength Cohesion (undrained) 20-50%

Angle of friction (clays) 12-50%

Angle of friction (sands) 5-15%

CBR 17-58%

( Continued )