Determination of the vertical extent of transition zones based on the dispersion measures of CPTu tests and the impact of excluding these zones in estimating characteristic values of parameters

The natural spatial variability of soil properties is particularly evident near the boundaries between distinct soil layers. In such transition zones, there is usually a noticeable differentiation and increased dynamics of changes in the values of geotechnical parameters. The identified soil layers were assumed to have similar properties based on lithological and geotechnical criteria. Zones surrounding the interpreted contact surface of the layers with significant variability in geotechnical and physical parameters were considered as transition zones. This variability is reflected in the variability of the basic Cone Penetration Test with pore pressure measurement behind the tip called u ₂ (CPTu) sounding parameters. The application of basic statistical functions such as the local expected value μ _L (X) calculated in a given neighborhood of sounding depth H, the dispersion measures σ _L (X) and V _L (X) ( local population standard deviation and local coefficient of variation defined at each depth H for a given neighborhood of that depth H) to the analysis of CPTu soundings enabled the development of a method for determining the depth range of transition zones near the boundaries between distinct layers and an algorithm enabling the implementation of this method in computer programs. Using the determined vertical ranges of transition zones, it was proposed to estimate characteristic parameter values based on the central parts of the layers, excluding the measurements in the transition zones from the calculations. In the next step, using the same CPTu tests and focusing on the same layers, the characteristic values of selected geotechnical parameters were compared, calculated both with and without inclusion of the measurements from the estimated transition zones in the calculations. Finally, the calculation results were summarized and discussed.