Material and testing variability - Geotechnical Investigation and Design Tables - page 115

Table 10.8 ( Continued )

Test type Test Coefficient of variation

Compaction Maximum dry density 1-7%

Optimum moisture content 200-300%

Durability Absorption 25%

Crushing value 8-14%

Flakiness 13-40%

Los angeles abrasion 31%

Sulphate soundness 92%

Deformation Compressibility 18-73%

Consolidation coefficient 25-100%

Elastic modulus 2-42%

Flow Permeability 200-300%

10.9 Distribution functions
•

Variability can be assessed by distribution functions.

•

The Normal distribution is the taught fundamental distribution, in maths and
engineering courses. It is the simplest distribution to understand, but is not directly
relevant to soils and rocks.

•

When applied to soil or rock strength properties, negative values can result
at say lower 5 percentile if a normal distribution used (Look and Griffiths,
2004).

•

The assumed distribution can affect the results considerably. For example the
probability of failure of a slope can vary by a factor of 10 if a normally distributed
or gamma distribution used.

Table 10.9 Appropriate distribution functions in Rock property assessment (Look and Griffiths,
2004).

Distribution Overall Typical application outside of geotechnical engineering

type rank

Pearson VI 1 Time to perform a task.

Lognormal 2 Measurement errors. Quantities that are the product of a large

number of other quantities. Distribution of physical quantities
such as the size of an oil field.

Gamma 3 Time to complete some task, such as building a facility,

servicing a request.

Weibull 4 Lifetime of a service for reliability index.

Beta 5 Approximate activity time in a PERT network. Used as a rough

model in the absence of data.

Normal 11 Distribution characteristics of a population (height, weight);

size of quantities that are the sum of other quantities (because
of central limit theorem).

•

Above rank is based on various goodness of fit tests for 25 distribution types.

•

Due to non normality of distribution, the median is recommended instead of mean
in characterisation of a site.