The largest part of the hilly Belgrade area is built of Neogene complex of clayey deposits. Therefore, every engineering intervention in clays calls for full attention and responsibility on the part of the investigators, especially when it's a sufficiently known that physical and mechanical properties of soil vary and weaken in time and in contact with water. Using the experiences of the expert team from a Department of Geotechnics of the Faculty of Mining and Geology and the results obtained from a few very significant underground construction projects in Belgrade clayey complex, this paper confirms a well known fact that interaction between mineral fragments and fluids represents the most important factor which controls decrease of soil shear strength. In extreme cases, the shear strength can be reduced to approximately a half or even a third and that may result in disastrous consequences, unless this particular problem is realized and solved in time.
A knowledge of shear strength of soil is an indispensable condition for successful designing and realization of each construction, whether it is a building, a mining structure or a structure of any kind, which can be leaned on and/or buried in a soil. Determination of these parameters is very important and a complicated task. The major reason for these problems is the most complex nature of soil construction, which represents, at the same time, the reflection of geological past and contemporary geological and anthropological processes and phenomena. Shear strength of soil and even its residue, so-called residual strength, became an object of interest for a great number of expert teams worldwide. Skempton (1959, 1967, 1985) gave a significant contribution in that particular field with his remarkable work, which considered construction problems in London clay deposits of different age and strength (consolidation ratio).
