Importance of the detection of the low strength zones for the engineering geological and geotechnical modelling is stressed as well as illustrated with an example, and an appropriate method is described. The highest values of liquid limit and plasticity index in a geotechnical column regularly correspond to the lowest values of angle of internal friction. RNK method singles out one layer, often one of the weakest layers, as a reference, and uses relatively simple but numerous and very carefully performed tests to organize and compare data obtained in the area—allowing very useful extrapolations, and detection of all weak layers. Correlation of plasticity index and angle of internal friction is presented with several new series of very carefully obtained data. An instability in very fractured limestone is analysed in detail, which occurred during excavation at the shipyard near Rijeka in Croatia, because the low strength zone was undetected and its possible importance underestimated.

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