Origin of clay in granite intrusive contact zone and its influence on tunnel deformation and instability.

The special properties of the clay derived alteration developed in the granite intrusive contact zone under the condition of humidity variation have extremely complex effects on engineering. In view of the deformation and destabilization of Yuxi Tunnel in China, detailed geological investigation has found that the deformation and destabilization section is located in the intrusive contact zone of granite and sandstone, in which the clay and marble are developed. The lithofacies and mineral composition test results of the clay show that it is derived from the alteration of hydrothermal fluids and belongs to the altered rock, rather than product of weathering or faulting. Results of laboratory tests show that the clay derived alteration has the properties of strong water absorption and expansion. In addition, its shear strength decreases significantly with the increase of water content. The characteristics of special topography, occurrence conditions, mineral composition and structure of the clay derived alteration are the inherent causes of the deformation and instability of the tunnel. Furthermore, numerical simulation of the response characteristics of the support structure under humidification conditions is realized. The calculation results show that when considering negative effects such as expansion and strength degradation of the surrounding rock, the disturbed range of surrounding rock and the deformation of tunnel vault are greatly increased, and significant stress concentration occurs at tunnel spandrel and arch foot. At the same time, the deformation and stress evolution of the supporting structure show certain accumulation and delay characteristics during the humidification process. The research in this paper not only clarifies the evolution of the displacement and stress of the tunnel support structure with the water content, but also reveals the origin of instability phenomena such as ground fissures, surface collapse, large deformation and collapse in the tunnel. This research methods and results can be used as reference for similar projects. • The deformation and instability characteristics of tunnel crossing altered clay are introduced. • The origin and formation mechanism of Clay in Granite Intrusive Contact Zone is revealed. • The deformation mechanism of tunnel crossing altered clay is revealed. [ABSTRACT FROM AUTHOR]