Study on observation system of seismic forward prospecting in tunnel: A case on tailrace tunnel of Wudongde hydropower station

Seismic method is a major approach for detecting the seismic geological features ahead of the tunnel, understanding the distribution of unfavorable geology, and ensuring the safety of tunnel construction. Observation system is the key for seismic detection, many studies have been conducted to optimize the observation system; however, most of them focused on the surface seismic investigation and numerical simulation rather than in tunnel field environment (limited aperture and full space environment). How to obtain better wavefield information with limited observation aperture is a great challenge. In this study numerical simulation and instrumental techniques (GPR, DC, etc.) were implemented to further check the result of seismic detection at the 1# tailrace tunnel at the Wudongde hydropower station. In the field case, observation detectors were arranged spatially in the tunnel and source points were placed in four ways: linearly along a single side, on the tunnel face, in front of the detectors, and behind the detectors. Then, after data acquisition, the data processing is conducted to carry out the migration results. The imaging results indicate that the observation system with sources and detectors in liner arrangement (with equal interval) helps to suppress artifacts, further supporting the advantages of spatial observation system with liner observation line (detectors). Moreover, the study provides suggestions for geological prospecting in similar tunnel projects.

Vibration Response Characteristics of Adjacent Tunnels under Different Blasting Schemes

The vibration caused by the tunnel blasting and excavation will harm the surrounding rock and lining structure of the adjacent existing tunnels. This paper takes a two-lane large-span highway tunnel as the research object, conducts on-site monitoring tests on the impact of vibration caused by the blasting and excavation of new tunnels on the existing tunnels under different blasting schemes, and analyses in detail the three-dimension vibration velocity by different excavation footages. From the vibration speed, it is concluded that the influence of the existing tunnel of the newly built tunnel blasting team is affected by various factors, such as distance, free surface, charge, and blasthole distribution. With different blasting schemes, the greater the amount of charge, the greater the vibration caused by blasting. Existing tunnels correspond to the front of the tunnel, and the axial and radial vibration peaks are greater than the vertical. Although the cut segment uses a less amount of explosive and has a less blasthole layout, there is only one free surface. Because of the clamping of the rock, it is compared with the other two segments. The vibration caused is the largest. Although the peripheral holes are filled with a large amount of explosive, the arrangement of the blast holes is relatively scattered and there are many free surfaces. Hence, the vibration caused is the smallest. Corresponding to the back of the tunnel face, since there is no rock clamp, the vibration caused by the cut segment is the smallest, and the vibration caused by the peripheral segment and the floor segment is relatively large. The vibration caused by the front explosion side is significantly greater than the vibration caused by the back explosion side. The vibration velocity caused by the unit charge of 1.5 m footage is greater than that of the 3.0 m footage. The vibration velocity caused by the unit charge of the cut segment is the largest, and the vibration velocity caused by the peripheral segment and the floor segment is smaller. The research results provide a reference for the blasting control of similar engineering construction.

Prediction of Conductive Anomalies Ahead of the Tunnel by the 3D-Resitivity Forward Modeling in the Whole Space

Water inrush in tunneling poses serious harm to safe construction, causing economic losses and casualties. The prediction of water hazards before tunnel excavations becomes an urgent task for governments or enterprises to ensure security. The three-dimensional (3D) direct current (DC) resistivity method is widely used in the forward-probing of tunnels because of its low cost and highly sensitive response to water-bearing structures. However, the different sizes of the tunnel will distort the distribution of the potential field, which causes an inaccurate prediction of water-bearing structures in front of the tunnels. Some studies have pointed out that the tunnel effect must be considered in the quantitative interpretation of the data. However, there is rarely a predicted model considering the tunnel effect to be reported in geophysical literature. We developed a predicted model algorithm by considering the tunnel effect for forward-probing in tunnels. The algorithm is proven to be feasible using a slab analytic model. By simulating a large number of models with different tunnel sizes, we propose an equation, which considers the tunnel effect and can predict the water-bearing structures ahead of the tunnel face. The Monte Carlo method is used to evaluate the quality of the predicted model by simulating and comparing 10,000 random models. The results show that the proposed method is accurate to forecast the water-rich structures with small errors.