Mio-Pliocene clays will increasingly constitute the subgrade for building structures, not only in the Warsaw region, but within the whole of central Poland, where the depths of their occurrences range from 2 to 100 m below groundlevel and their average thickness is 50 m. In their geological history these clays became overconsolidated. They include numerous weakness surfaces, are sensitive to the influence of exogenic processes, and can be classified as soils of specific properties. It is necessary to take into account the state and history of loads when testing these soils. This paper deals with the testing of overconsolidated soils. It contains full characteristics of the engineering-geological properties of the Mio-Pliocene clays of the Warsaw area.On the basis of the determined properties, the load-bearing capabilities of unweathered and weathered clays as the construction subgrade have been analysed. Special attention was paid to the quantitative expression of the influence of wetting (swelling), structure disturbance, and occurrences of shear strength weakness surfaces (cohesion and angle of internal friction). In the case of clays, it is possible that horizontal stresses are higher than vertical stresses (K0 = 0.74-1.44). The behaviour of clays occurring in slopes has been assessed. Laboratory and field (static penetration and dilatometer) test results are presented. Obtained results can be used directly for the evaluation of Mio-Pliocene clays as a construction subgrade, and indirectly for other overconsolidated soils, especially in the context of testing methodology.
