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CV Soil allows the determination of characteristic geotechnical parameters according to a probabilistic approach.
The geotechnical parameters are treated as random variables, and the set of values assumed by the parameters such as statistic population.
It is assumed that the investigated variables (f, cu, etc.) vary randomly within the soil.
The derivation of the characteristic value must be such that the calculated probability of a worst value (worst case) that governs the occurrence of the limit state into consideration is not greater than 5%.
It is therefore a conservative margin of 5% (which may coincide with a 5° percentile or the 95° percentile of a statistical distribution into account), which probabilistically guarantees us to have a 95% certainty in the computation of the characteristic value.
There are indications in the literature that the angle of shearing resistance ϕ does not follow a normal distribution, but its tangent yes, so the variable to be included in the formulas is not ϕ but tan(ϕ). In addition, the undrained cohesion cu seems to follow a lognormal distribution, so the variable to use for the estimates should not be directly cu, but his natural logarithm ln(cu).
The ϕ e cu characteristic are obtained by calculating the arctangent, respectively the exponential of the variable xk obtained as a result.
The criteria under which to choose the calculation procedure of the characteristic parameters within a homogeneous layer of soil are two:
1) Based on the number of measurements taken. With the increase of the size of the sample improves the estimate of the mean and standard deviation of the population that are used to construct the curve of the probability density, and then estimate the value corresponding to the probability of not exceeding 5%.
2) Based on the presence or absence of resistance compensation of the soil
See also Elements of statistics and probability.

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