Freeze-thaw problems need to be solved urgently for the construction of projects in the alpine mountain areas. Freeze-thaw cycle tests with different cycles were carried out after water filled in the crack. Combined with the theory of frost heaving mechanics and fracture mechanics, the failure modes and mechanical characteristics of crack growth are analyzed after the freeze-thaw cycle test. The research results showed the crack growth increase with the number of freezing-thawing cycles. Cracked rock failure types are divided into four types. The Poisson's ratio, elastic modulus of rock and water-ice medium, and the equivalent volume expansion coefficient of water-ice medium determine the size of frost heaving force generated in fractured rock. The upper-saturated crack of limestone and red sandstone produces pure mode-I fracture under the action of the freeze-thaw cycle. The cracks in the fractured rock mass will extend along the direction of this structural plane when there is a visible weak structural surface in the rock mass. These research results can provide a reference for the freezing and thawing splitting effect caused by rainwater infiltration in mountainous alpine areas.
Satellite thermal infrared remote sensing has received worldwide attention in the exploration for earthquake precursors; however, this method faces great controversy. Obtaining repeatable phenomena related to earthquakes is helpful to reduce this controversy. In this paper, a total of 15 or 17 years of Moderate-resolution Imaging Spectroradiometer (MODIS)/Aqua and MODIS/Terra satellite remote sensing land surface temperature (LST) products is selected to analyze the temperature changes before and after the Mw 7.9 earthquake in Nepal on 25 April 2015 and to explore possible thermal information associated with this earthquake. Major findings are given as follows: (1) from the time course, the temperature slowly cooled before the earthquake, reached a minimum at the time of the earthquake, and returned to normal after the earthquake. Since these changes were initiated before the earthquake, they may even have been precursors to the Nepal earthquake. (2) From the space distribution, the cooling areas correspond to the seismogenic structure during the earthquake. These cooling areas are distributed along the Himalayas and are approximately 1300 km long. The widths of the East and West sides are slightly different, with an average temperature decrease of 5.6 °C. For these cooling areas, the Western section is approximately 90 km wide and 500 km long; the East side is approximately 190 km wide and 800 km long. The Western side of the cooling strips appeared before the earthquake. In short, these kinds of spatial and temporal changes are tectonically related to the earthquake and may have been caused by the tectonic activity associated with the Nepal earthquake. This process began before the earthquake and therefore might even be potentially premonitory information associated with the Nepal earthquake.
The initial in situ stress field is the fundamental factor causing the deformation and failure of underground engineering and is an important basis for the feasibility analysis, design, and construction of underground engineering. However, it is difficult to obtain the whole in situ stress field of large-scale underground engineering in difficult and dangerous areas by field measurement. In view of the fact that the measured in situ stress components (σxx, σyy, σzz, τxy, τxz, τyz) of Sichuan-Tibet Railway in China are linear with the buried depth, a method is proposed to solve the in situ stress by applying corresponding loads to all unit bodies in the calculation area based on BP neural network and FLAC3D. Through this method, the in situ stress of the tunnel is inverted. The results show that both the maximum principal stress and minimum principal stress increase with the increase of buried depth, and when the tunnel passes through faults or anticlines, the main stress will suddenly drop. Furthermore, compared with the results of the multiple linear regression method, it is found that the proposed method has higher accuracy; especially for the simulation of the maximum horizontal principal stress and vertical stress, the average relative error is reduced by 26.44% and 77.27%, respectively. The research in this paper can provide a new idea for the initial in situ stress inversion of engineering.
Water content significantly affects the physical and mechanical properties of rock and can cause rock mass to become unstable. This, in turn, can cause geologic disasters such as water inrush and surrounding rock deformation. Uniaxial compression experiments on slate samples drilled from the surrounding rock of a tunnel in the Chinese Sichuan-Tibet Plateau area were performed coupled with acoustic emission (AE) monitoring. The changes in mechanical properties and failure processes of unsaturated and saturated slates were investigated comparatively through test results. Phenomena of a slight drop of σ1 in the compaction and elasticity stage were observed, and it is due to the sliding of slate along the layered surface. According to the results, the average compressive strength of saturated slates was reduced by 24.3% compared to the unsaturated slates. The spatiotemporal evolution characteristics of AE indicated that the deformation and fracturing of unsaturated slates happened concentratedly and violently in the accelerated crack growth stage, while that of saturated slates occurred with low energy and uniform distribution spatially and temporally in the whole process. Moreover, water content reduced the brittleness index of unsaturated slate by 17.1%. For both conditions, the abundance of AE activity before failure shifted from a high level to a low level and lasted until failure. This can be used as a preliminary failure prediction of slates.
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