In the process of underground construction, it is common to spray concrete on the working surface, which will form a concrete-rock composite structure, to achieve the expected reinforcement effect. Compared with pure concrete or rock structures, composite structures have different mechanical properties, energy evolution and damage characteristics. Understanding the dynamic mechanics and damage characteristics of composite structures under impact loads is important to the safety of underground engineering construction. First, dynamic compression tests on different grades of mortar-sandstone composite specimens were carried out by using a Hopkinson pressure bar system to discuss the effect of impact velocities on the mechanical properties and failure modes of composite specimens. The results show that the dynamic peak stress, strain energy density and fragmentation block of the mortar-sandstone composite have an obvious strain rate effect. With an increase in impact velocity, the internal cracks of the sample do not have sufficient time to expand, and the energy absorbed by the sample leads to the continuous initiation of new cracks in the sample, resulting in an increasing damage degree of composite specimens. The increase in mortar strength can significantly increase the peak stress, improve the load-bearing capacity of the sample, and reduce the degree of sample damage. At the same impact velocity, the fragmentation degree of the mortar sample is more severe than that of sandstone, and the mortar sample is broken into powder debris, while the sandstone is broken into fragments. In addition, according to the failure properties of the composite sample, the mortar and sandstone samples are described by different damage models, and the dynamic damage constitutive model of the mortar-sandstone composite is established based on the overlay principle. The dynamic test results were fitted. The research shows that the fitting curves of the model are well fitted with the test curves and that the model accurately reflects the changes in the mechanical parameters of the composite specimens.

To ensure the safe and efficient excavation of ultradeep foundation pits in a complex urban environment, the ultradeep foundation pit excavation project of Liuguangmen Urban Complex in Guiyang City was taken as the study point. A high-efficiency blasting method was proposed for the excavation of vibration-isolating slot and electronic detonators, and a three-dimensional spatial calculation model of the foundation pit structure was established. In addition, the field test and numerical simulation of the blasting vibration were developed. The feasibility of the high-efficiency blasting method was demonstrated, and the propagation law of the dynamic response characteristic parameters of the structure was explored using an electronic detonator and vibration-isolating slot. The results show that the electronic detonator carried out peak shaving within the group, unloading waves between the groups, and the blocking effect of the vibration-isolating slot, and all these effectively reduced the peak stress and peak particle vibration velocity, evenly distributed the vibration velocity, and guaranteed the safety of the surrounding existing buildings, proving the feasibility of the efficient blasting method. There is a wall effect at the supporting pile, whose propagation velocity is lower than that inside the rock mass, and the PPV is radial X>tangential Y>vertical Z. The peak values of stress, strain, and PPV all decrease with increasing supporting pile height, and there is an inflection point at 20m. The internal nodes of the pile reciprocate without sharp change and failure in displacement. The peak displacement of different nodes is in the order X>Y>Z with increasing pile height, and the maximum peak value is 0.08mm.

Based on the complex variable function theory, the test principle of the strain gauge method (SGM) at specific orientation angles were proposed, and the dynamic caustic method (DCM) was supplemented to perform a mode I crack propagation test of single-hole prefabricated crack defects under the effect of explosive stress waves. The propagation effect, propagation characteristics, and mechanical behaviour characteristics of mode I branch cracks under slitting blasting loading were analysed. The results show that propagation of the mode I branch crack along a straight line is initiated after the main cracks of the defect model penetrate the defect under the slitting blasting load. The strain—time curves obtained using several strain gauges with specific orientation angles can characterize the dynamic mechanical behaviour parameters of mode I branch cracks in a defective medium, and the results are consistent with those of the DCM. The SGM is not limited by defects and can be applied to examine the propagation process of open-mode branch cracks with defects. The research process provides an experimental basis for the analysis of the explosive crack propagation behaviour of nontransparent materials.