Search

Tensile failure (spalling or slabbing) often occurs on the sidewall of deep tunnel, which is closely related to the coupled stress state of deep rock mass under high pre-static load and dynamic disturbance. To reveal the mechanism of rock tensile failure caused by this coupled stress mode, the Brazilian disc tests were carried on red sandstone under high pre-static load induced by dynamic disturbance. Based on the pure static tensile fracture load of red sandstone specimen, two static load levels (80% and 90% of the pure static tensile fracture load) were selected as the initial high pre-static loading state, and then the dynamic disturbance load was applied until the rock specimen was destroyed. The dynamic disturbance loading mode adopted a sinusoidal wave (sine-wave) load, and the loading wave amplitude was 20% and 10% of the pure static tensile fracture load, respectively. The dynamic disturbance frequencies were set to 1, 10, 20, 30, 40, and 50 Hz. The results show that the tensile failure strength and peak displacement of red sandstone specimens under coupled load actions are lower than those under pure static tensile load, and both parameters decrease significantly with the increase of dynamic disturbance frequency. With the increase of dynamic disturbance frequency, the decrease range of tensile strength of red sandstone increased from 3.3% to 9.4% when the pre-static load level is 80%. While when the pre-static load level is 90%, the decrease range will increase from 7.4% to 11.6%. This weakening effect of tensile strength shows that the deep surrounding rock is more likely to fail under the coupled load actions of pre-static load and dynamic disturbance. In this tensile failure mechanism of the deep surrounding rock, the stress environment of deep sidewall rock determines that the failure mode of rock is a tensile failure, the pre-static load level dominates the tensile failure strength of surrounding rock, and dynamic disturbance promotes the strength-weakening effect and affects the weakening range.

Spalling (Slabbing) is a typical failure phenomenon, which often occurs after excavation of rectangular tunnel. It has a serious impact on the construction, support and stability of the tunnel. To investigate the characteristics and mechanism of spalling-failure in rectangular tunnel, a series of tests were carried out on granite rectangular tunnel with different height-to-width ratios (H/Ws) (1/2, 3/4, 1/1, 4/3 and 2/1) by using true triaxial system. The whole process of spalling was monitored and recorded by wireless micro camera. According to the test results, the failure mode and failure characteristics of the sidewall spalling were summarized, and the influence of the H/Ws of the rectangular hole on the failure characteristics and stability of the tunnel were discussed in detail. The test results indicate that when the vertical principal stress is the maximum principal stress, the sidewalls of the specimens experience spalling failure. When the spalling occurs, the crack occurs at the shoulder corner of the sidewall and propagates along the axis, resulting in the spalling damage gradually developing from the local area to the whole sidewall along the axis, and forming the thin rock slab parallel to the sidewall. In addition, rectangular tunnels with different H/Ws clearly influence spalling failure. When the height was the same, the degree of spalling damage decreases with the decrease of its width. On the contrary, when the width was the same, the degree of spalling damage does not decrease with the increase in the height. When the height was equal to the width, the damage degree of the tunnel spalling is the largest, which seriously affects the safety of the tunnel. In the case of the same cross-section area of the tunnel, the stability of the tunnel can be improved by selecting the layout mode with the height greater than the width. It is necessary to strengthen the monitoring of the cracks at the shoulder corner of the tunnel, which can reduce the safety risk of the tunnel. These conclusions are helpful to the design of the tunnel, the prevention of spalling and the improvement of the stability of the tunnel.

Spalling (or slabbing) in the sidewalls of deep rectangular tunnels is a type of rock failure phenomenon under the three-dimensional stress environment, which often occurs on the surrounding rocks after the tunnel face excavation is completed and has a significant impact on the safety of construction and support. In order to investigate the spalling process in deep rectangular excavated tunnels under three-dimensional stress, a granite material was firstly processed into cubic specimens (100 mm × 100 mm × 100 mm) with prefabricated rectangular holes (40 mm × 40 mm). Simulation experiments were performed using a rock true-triaxial electro-hydraulic servo mutagenesis testing system under four different initial stress states. Meanwhile, a wireless micro-camera was used to record and monitor the damage process on the sidewalls of the rectangular tunnel in real time. The results showed that under the four stress conditions, an evident spalling failure occurred on the entire sidewalls of the rectangular hole. It was observed that the spalling-damaged zone gradually developed toward the deep part of the hole in a horizontal direction. Finally, it formed a penetrating symmetric arc-shaped groove along the axial direction , whereas the roof and bottom slab remained stable. The spalling failure process of the sidewalls had four stages: the calm period, particle ejection at the upper and lower shoulders, sidewall crack propagation , and crack penetration spalling failure. Compared with the dynamic failure (rockburst) of a circular tunnel under the same three-dimensional high stress conditions, the spalling failure of the rectangular tunnel sidewalls was of the static failure mode. Under the three-dimensional stress condition, the radial stress had a greater effect on the spalling failure than the axial stress , by increasing the radial stress can be significantly reduced the degree of spalling failure and effectively improved the stability of the surroundings.

This paper investigated the respective applications of zinc and copper ions as inhibitors in the recycling water. The results show when their concentrations are not less than 0.5 mg L−1, the treated water is clear as usual. However, their scaling on the tube wall still lasts. In the solution of zinc ion, the weight of precipitation on the tube wall always increases stably; but in the solution of copper ion, that increases less stably; besides, when the concentration of copper ion is beyond 2 mg L−1, the increase of precipitation weight on the tube wall has become small. All of which showed different scaling mechanisms. For the solution of zinc ion, the scaling on the tube wall increases stably because of the homogeneous germs. However, in the solution of copper ion, the different types of copper ion germs will be adsorbed selectively by the tube wall because of heterogeneous germs, or the part of having been adsorbed germs onto the tube wall will be washed out.

Finding some economical and environmentally friendly inhibitors was always one of our research goals. The present paper researched the method of rapid controlled precipitation (RCP) and the results showed it was reliable and suitable as the method of evaluating the scaling. Then through the method of RCP, the paper would investigate four kinds of metal ions as the inhibitors, the results showed the zinc ion and copper ion were suitable as the inhibitors. In the 300-mL solution (calcium ion concentration of 126.5 mg L−1), the total efficiency of 100% was reached for the copper ion concentration of 0.8 mg L−1 and for the zinc ion concentration of 0.3 mg L−1. After choosing the zinc ion and copper ion as the inhibitors, this paper also investigated the cooperative effect of the mixed metal ions, the results showed the mixed ions would not change the inhibition effectiveness of any single metal ion, but could affect the calcium carbonate germination and change the crystal morphology.