Previous construction works in regions of permo-carboniferous slates have led to difficulties with slopes. Three cases are presented where shear characteristics after investigations were estimated to be much higher than was established later when problems arose during construction works. Detailed investigations of standard in-situ and laboratory tests and sets of cylindrical triaxial tests were carried out to find out reasons for the stability problems. Drained and undrained incremental or continuous loading and unloading tests that considered conditions due to construction works were performed. Parameters determined with detailed investigations have much lower values than those found with previous investigations. The analyses performed, which are not presented in this paper, have shown suitability of the presented constitutive model.


An extensive programme of motorway network construction is being carried out in Slovenia where very different geological regions are being encountered. The most interesting are regions formed of permocarboniferous slates. During the existing practice they have proved to be as very exacting and unpredictable. Investigations often show much better stiffness characteristics of permo-carboniferous slates than they display after construction works. The most influential condition change is large excavations of cuts and the consequent changes of stress states and ground water regimes. It is obvious that mechanical characteristics are very difficult to precisely establish by in-situ tests, because the influences of construction works can not be considered. The results of laboratory tests are unreliable as well, because it is difficult to obtain representative samples from critical layers. Therefore the models for analyses are inaccurate. Agitated stratification of land can make precise model determination more difficult. Three cases of large construction works in permo-carboniferous slopes are presented. Further investigations and analyses to perform sanitation works have shown a distinct decrease of shear strength in particular layers of permo-carboniferous slates in all three cases.

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