Structural Design of Pavements PART VI Structural ... - TU Delft

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4. Measurement plan:
The question always is how many measurements should be taken and where should the
measurements be taken on a specific stretch of pavement in order to get a reliable picture of
the flexural stiffness of the pavement. Some guidelines for this will be given in this chapter.
4.1 Estimation of the number to test points per section:
In this section the method presented in [2] is described which allow the number of tests to be
determined that are needed on a particular road section to obtain a proper insight in the
bearing capacity of the pavement.
One can calculate a statistical quantity R, called the limit of accuracy, which represents the
probable range the true mean differs from the average obtained by “n” tests at a given degree
of confidence. The larger n is, the smaller value will be obtained for R which means that the
mean value calculated from the data obtained from the tests will differ less from the true mean
value. The mathematical expression is:
R = K α . ( σ / √ n )
Where: K α
σ
= standardised normal deviate which is a function of the desired confidence
level 100 . (1 - α),
= true standard deviation of the random variable (parameter) considered.
If the confidence level is chosen and if a proper estimate for σ is obtained, R is inversely
proportional to the square root of the number of tests.
Figure 7shows the basic shape of the relation between n and R.
Figure 7: Typical limit of accuracy curve for all pavement variables showing general zones.