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The mechanical properties of fractured rock mass are largely dependent on the fracture structure under the coupling of freeze-thaw cycles and large temperature difference. Based on the traditional macroscopic continuum theory, the thermal and mechanical model and the corresponding theories ignore the material internal structure characteristics, which add difficulty in describing the mesoscopic thermal and mechanical behavior of the fractured rock mass among different phases. In order to uncover the inherent relationship and laws among the internal crack development, structural change and the physical and mechanical properties of rock under strong cold and frost weathering in cold area, typical granite and sandstone in cold region were analyzed in laboratory tests. The SEM scanning technology was introduced to record the microstructural change of rock samples subject to freeze-thaw cycles and large temperature difference. Association rules between the microstructure and the physical mechanical properties of rock mass were analyzed. The results indicated that, with the increase of the cyclic number, the macroscopic physical and mechanical indexes and the microscopic fracture index of granite and sandstone continuously and gradually deteriorate. The width of original micro crack continues to expand and extend and new local micro cracks are generated and continue to expand. The fracture area and width of the rock increase and the strength of the rock is continuously damaged. In particular, the strength and elastic modulus of granite decrease by 20.2% and 33.36%, respectively; the strength and elastic modulus of sandstone decrease by 33.4% and 36.43%, respectively.

The proper design of underground powerhouse support is the key engineering technique to guarantee the safe construction and operation of underground works. By regression analysis of the surrounding rock support parameters of 29 underground powerhouses with a span of 18.0–34.0 m, the empirical formula of the relationship between the support strength of anchor bar, strength-stress ratio, and plant span and the relationship among the support strength of the anchor cable, strength-stress ratio, and plant span are proposed. Furthermore, an intelligent design model for the anchor support of the underground powerhouse was trained by a BP (back propagation) neural network. Research shows that the support strength index of the anchor bolt and the anchor cable of these 29 plants are all distributed around 1.0. Therefore, a support strength index of 0.8–1.2 can be used as a reference for practical engineering support design. Finally, the reliability of the intelligent design model for the anchor support of the underground powerhouse was verified by comparison with actual engineering and support strength index. This shows that the intelligent design model can provide a reference for engineering design and construction.