Study On Temperature Effect And Deterioration Mechanism Of Sandstone Surface In The Yungang Grottoes

The Yungang Grottoes is based on natural cliff of the mountain to construction. It is both the construction and the geological body. Caves are occured in the geological environment.The grotes produce a variety of environmental and geological diseases by the geological formations and a variety of geological forces in the long historical period. It caused a serious threat to the preservation of the grotto even destruction the grotto rock. The site survey found that the surface sheet cracking of the grotto legislation wall rock is one of the main diseases of the Yungang Grottoes. The Grottoes rock surface is the carrier of the engraving art, cracking spalling of the surface of both the destruction of the grotto carving art, but also weakening the physical and mechanical properties of rock, leading to the cave instability, such as constantly peeling off in the cave before the column will eventually lead to the caves loss the steady destruction.The Yungang Grottoes sandstone shallow temperature effect degradation study is an important part of the national scientific and technological support projects’grotto rock stability analysis project of key technology of stone conservation’(No.2009BAK53B03). The main findings are as follows:(1) Temperature effect can be divided into thermal expansion&contraction and ice split in the physical mechanisms of rock weathering in the Yungang Grottoes. The site survey found that in the intact rock overall stability of the caves, a shallow form of instability and failure mainly to the surface of the sheet of cliff rock cracking. Such cracking body is usually under the thermal stress along the native micro-structural surface crack extension to. Therefore, it is necessary to study the temperature change on the rock ofthe Yungang Grottoes depth and shallow rock temperature field changes with the season’s law. Through on-site monitoring of shallow rock temperature field, to explore the variation of the Yungang Grottoes sandstone shallow temperature field distribution. By testing the rock thermal physical parameters, the Yungang sandstone main thermal physical parameters:λ=3.516[w/(m·k)]0-35℃, Yungang Grottoes sandstone specific heat for0.63,35and60℃, the specific heat of0.85, coefficient of linear expansion of11.7(10-6). Test by the analysis of monitoring data and the Yungang Grottoes sandstone thermal physical properties obtained sandstone shallow temperature field calculation formulas, and provide basic data and the theoretical basis for the post-rock, thermal-mechanical coupling analysis.(2) The Yungang Grottoes is a cold region, we can see from the on-site monitoring results, the Grottoes rock temperature long-term lower than0℃in winter. Rock fracture water at low temperature phase transition of water ice on the rock formation of frost heaving force. Rock freeze-thaw cycle effects on the volume, mass, the longitudinal wave velocity and uniaxial compressive strength of the attenuation effect. Freeze-thaw cycle effects on the macroscopic shape of the rock mass, the microstructure will be injurious effect. Indoor freeze-thaw cycle test simulates the actual situation, come to the attenuation law of the Yungang Grottoes area rock. Finally, summarize sandstone freeze-thaw damage and failure of two degradation mechanism in the Yungang Grottoes region.(3) On the basis of rock temperature field monitoring tests and indoor freeze-thaw cycle test, using ANSYS finite element analysis to analysis thermal mechanical coupling. The simulation results show that using ANSYS software is feasible for the rock temperature field calculation. Considering the thermal coupling of the rock mass stress, strain showed a larger increase. The simulation results show that, in high temperature conditions, the rock showing exfoliated and thermal stress, rock frost heaving damage and moisture content of fissures phase transitions at low temperatures.The innovation of this paper:(1) Independently developed by the temperature monitoring equipment, access to the complete data of a weather the Yungang Grottoes sandstone shallow temperature, lay a data base for the future of the region of temperature field research. The Yungang Grottoes atmosphere and shallow rock temperature variation for the first time. Test the thermal physical parameters of the sandstone, the temperature of the shallow rock distribution formula.(2) Increase in conventional freeze-thaw tests on the basis of the displacement monitoring of the freezing process, reveal the Yungang Grottoes sandstone freeze the deterioration process. Freezing and thawing rock samples SEM Scanning analysis, to reveal the microscopic mechanism of rock samples freeze-thaw damage.(3) Using ANSYS software the grotto column under high temperature thermal coupling analysis reveals the thermal nature of the column weathering. Contained in the rock under the conditions of water temperature phase transitions, thermal coupling analysis of the slope of the Caves, the local rock mass stability was reduced in the freezing conditions.