Recent Advances in the GPR Detection of Grouting Defects behind Shield Tunnel Segments

Injecting grout into the gaps between tunnel shield segments and surrounding rocks can reduce ground subsidence and prevent ground water penetration. However, insufficient grouting and grouting defects may cause serious geological disasters. Ground penetrating radar (GPR) is widely used as a nondestructive testing (NDT) method to evaluate grouting quality and determine the existence of defects. This paper provides an overview of GPR applications for grouting defect detection behind tunnel shield segments. State-of-the-art methodologies, field cases, experimental tests and signal processing methods are discussed. The reported field cases and model test results show that GPR can detect grouting defects behind shield tunnel segments by identifying reflected waves. However, some subsequent problems still exist, including the interference of steel bars and small differences in the dielectric constants among media. Recent studies have focused on enhancing the signal-to-noise ratio and imaging methods. Advanced GPR signal processing methods, including full waveform inversion and machine learning methods, are promising for detecting imaging defects. Additionally, we conduct a preliminary experiment to investigate environmental noise, antenna configuration and coupling condition influences. Some promising topics, including multichannel configuration, rapid evaluation methods, elastic wave method scanning equipment for evaluating grout quality and comprehensive NDT methods, are recommended for future studies.

Study on Deformation Law of Subway Construction under Passing Existing Line in Short Distance

With the large-scale development and utilization of underground space projects, the subway-led Rail Transit Project is developing vigorously, and the safety and stability of subway under passing existing projects have become a key issue for the future. In this paper, the deformation law of subway tunnel under the existing track is studied by using the method of ROCSCIENCE software and field measurement, which is based on the construction of a new double-tunnel shield-driven subway tunnel under the existing city of pile rapid track project. In the crossing area, the distance between the top of the tunnel and the pile foundation is 6.2 m and the distance between the tunnel and the existing track surface is 19.6 m. It is required that the settlement of track caused by the new tunnel construction should not exceed 2 mm. The results show that the settlement of the existing track is greatly influenced by the new tunnel without reinforcement and grouting, and the maximum settlement is more than 9.92 mm. The settlement of the existing track can be controlled to 1.93 mm by adjusting the construction parameters of the shield machine and grouting reinforcement at different stages during construction.

Modeling the Process of Controlling the Microtunnelling Boring Machine's Motion

The problem of determining the control action, the optimal number and combination of hydraulic cylinders required to guide the tunnel shield along the project axis of the tunnel with the required accuracy is consid-ered. An algorithm for the formation of a combination of hydraulic cylinders for moving the shield is applied, creating an effort in accordance with the magnitude of the control action, determined by the deflection of the shield in the vertical and horizontal planes. Analytical expressions for determining the control parameters are given. Modeling the process of controlling the movement of the tunneling shield showed that the proposed method provides the required accuracy of guiding the tunneling shield in accordance with the design direction.

SURFACE SETTLEMENT INDUCED BY TUNNELING IN GREENFIELD CONDITION THROUGH PHYSICAL MODELLING

Geotechnical conditions such as tunnel dimensions, tunneling method and soil type are few factors influencing the ground movement or disturbance. This paper presents the effect of tunnel cover to diameter ratio and relative density of sand on surface settlement induced by tunneling using physical modelling. The aluminum casing with outer diameter of 50 mm was used to model the tunnel shield. The size of the casing was 2 mm diameter larger than the tunnel lining. The tunnel excavation was done by pulling out the tunnel shield at constant speed with a mechanical pulley. The tested variables are cover to diameter ratio (1, 2 and 3) and relative density of sand (30%, 50% and 75%). The results demonstrated that the surface settlement decreased as the relative density increased. Also, as the relative density of sand increased, the overload factor at collapse increased. The surface settlement was at the highest when the cover to diameter ratio was 2. It can be concluded that in greenfield condition, the relative density and cover to diameter ratio affect the surface settlement.