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To recommend the excavation procedures and design parameters for underground powerhouses, excavation procedures of fifty-one underground powerhouses in China were summarized and analyzed based on in situ stress conditions. Firstly, the complex stress environment in China was introduced and fifty-one underground powerhouses with their engineering scale, size, lithology, rock classification and in situ stress level were listed in detail. Subsequently, to evaluate the influence of in situ stress levels on excavation procedure design, the correlation between excavation procedures and in situ stress level in three main excavation zones were analyzed accordingly. Moreover, to provide the excavation design recommendations, the strength–stress ratio (SSR) was promoted to analyze and recommend the design parameters, and the blasting excavation design based on the stress transient unloading control was also supplemented. The results show that excavation procedures have different priorities under different in situ stress levels, and the design parameters show an obvious relationship with in situ stress levels. Moreover, the excavation procedure parameters are suggested to adjust accordingly under different SSR. The discussion of influencing factors and specification ensures its rationality and accuracy. It is believed that the summary and recommendations can provide a good reference for excavation procedure optimization of underground powerhouse under high in situ stress. [ABSTRACT FROM AUTHOR]

In this paper, a new safety control method of the jointed slope under blasting excavation is presented. The spatial distribution of blasting damage and time-history characteristic of dynamic stability around the joint were investigated carefully based on the secondary development of the explicit FE code, LS-DYNA. Results clearly indicated that the evolution law of damage and stability are opposite with the change of joint angle, and the dual-controlled mechanism of damage extension and stability fluctuation exists in the safety control of the high rock slope with joints. Then a new safety control method is proposed by implementing the equivalent Blasting Safety Control Standard (BSCS) which includes coupled effects of damage and stability. Results reveal that a critical joint angle exists, and if the inclination is larger than the threshold value, the BSCS depends on damage extension, otherwise it was determined by stability fluctuation. Finally, site experiments are implemented carefully based on the blasting excavation of the famous Baihetan high rock slope with columnar joint in China to verify the rationality of the new control approach. Sonic wave test, displacement measurement and blasting vibration test are implemented for nearly two years to track the evolution process of damage and stability of the jointed slope. Experimental results demonstrate that the new approach could improve the accuracy rate of control by 40%. [ABSTRACT FROM AUTHOR]

• Using new numerical simulation method (FDEM) and theoretical analysis on smooth blasting research. • Investigating the influence of in-situ stress, hole spacing, and radial decoupling coefficient on the superposition of blasting stress and the cracking characteristic between holes. • Proposing the design of smooth blasting parameters considering the influence of in-situ stress and verify it based on specific project. Smooth blasting is an important excavation method in tunnel construction, creating a smooth and even excavation profile during the excavation process. In this study, the influence of in-situ stress, hole spacing, and radial decoupling coefficient on the superposition of blasting stress and the cracking characteristic between holes were investigated by using integrated analytical and numerical analyses (FDEM). Based on those results, the design of smooth blasting parameters considering the influence of in-situ stress were suggested, and verified against the data collected during the blasting excavation of diversion tunnel at Jinping II hydropower station. The results indicated that the propagation of blast-induced fractures are significantly influenced by the insitu stress conditions, tending to extend towards the direction of the maximum principal stress thus the holes should be arranged along the direction of the maximum principal stress to ensure the formation of cracks between holes and smooth excavation of the wall. [ABSTRACT FROM AUTHOR]

• Stability problems caused by lower bench excavation were analysed. • A new support system controlling squeezing deformation was proposed during the lower bench excavation of the headrace tunnel. • Polar coordinate description methods for the maximum displacement and associated position were presented. Jinping II hydropower station in Sichuan Province, China, is one of the largest tunnel groups in the world. The tunnel was excavated using bench excavation with a top heading and a bench. Severe squeezing problems were reported immediately after the excavation of the top heading when crossing the chlorite schist stratum at Ch.1+537 m. Squeezing deformation could last for a long period during tunnelling, similar to the problem of rockburst reported in the marble stratum distributed in the middle and east of the headrace tunnels. One year after the top heading excavation, the profile of the top heading still violated the profile tolerance and had to be reshaped due to the large convergence. A new support scheme was proposed to ensure the stability of the surrounding rock mass and control the excessive squeezing deformation during the bench excavation. For this, laboratory triaxial tests were first conducted under saturated and dry conditions to acquire the mechanical properties of chlorite schist. Then, the mechanical parameters of intact and damaged surrounding rock mass were determined using Hoek empirical estimation and a displacement inversion method, respectively. Next, initial support design was re-reviewed and evaluated based on the field displacement measurements, and the potential for squeezing in the overstressed rock mass was estimated. Thereafter, 10 supplementary support schemes were proposed using the numerical method in combination with laboratory data and field observations. Finally, the optimised support scheme was selected based on the results of comparative analysis. Field application of the suggested scheme showed that the convergence was controlled to within an acceptable level throughout the entire period of excavation of the lower bench. The experiences of controlling excessive squeezing deformation and optimisation of the support scheme for rock strata in a high geo-stress environment provide a reference for similar projects in the presence of soft rocks. [ABSTRACT FROM AUTHOR]