The increasing application of joint elenents in construction of the underground structures affects the statical analysis by a number of new calculation procedures. Theories are mostly based on the assumption of the homogeneity of rock masses and the elastic behaviour of tunnel structures. A great number of issued papers have considered this problem based on the fact that the rock mass around the underground structure acts strictly by means of active forces which may be considered as external forces, i.e. as a loading of a tunnel structure. If it is supposed that a pressure of rock masses, as a system of external forces, both in a qualitative and a quantitative sense, is approximately specified to a sufficient extent, dimensioning of the structure shall be executed by known procedures of the statics of Engineering Structures. Although such approach to a stability problem of the underground structures is possible, there is no doubt that, in many cases, it can be neither technically nor economically justified, especially, if a co-action between the rock mass and tunnel structure is achieved by technological procedures. Such problem certainly becomes more complex when it is analyzed from the aspect that the idea of a structure includes both a tunnel structure and a rock mass in a zone of action of the secondary stresses. It is also known that a system of tunnel linings and a rock mass have mechanical properties of a rheological character and in a course of time, the state of stresses and deformations is being changed. Rheological properties of certain components of a tunnel structure - rock mass system have been investigated and cleared. Neverthless, it is not sufficiently theoretically cleared the integral behaviour of complex tunnel structure - rock mass systems. The temporal process of changes of the relationship between the stress and deformations and their final values essentially depend on rheological characteristics of a mutual co-action, i.e. mutual behaviour within the system as a whole. When underground structures are being constructed in soft rock masses, it is necessary, immediately after excavation, to provide temporary secuity for the excavated contours and after that, to place a permanent, structure of a monolithic concrete or of joint elements, taking care of a minimum distance between the face of excavation and the concrete. Due to this, it is emphasized the mutuality of a technological process of construction and an appropriate choice of structure, with purpose to find out an optimal solution.
In order to foresee a co-action between the tunnel structure and rock mass, it is used a physical-mathematical model. By means of II in situ II measurings and laboratory testings, the essential physical and mechanical properties of rock masses are being specified and on their basis, the mathematical model is being formed, on the basis of a rheological model, primarily, of the environment (rock mass), an adequate project conception for the technology of construction maybe perceived in order to enable to a certain degree the co-action between the environment and tunnel structure.
