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Attributed to the opacity of soil and rock mass, seepage visualization is a thorny problem in the model test around diaphragm wall leakage, which has played an increasingly significant role in deep excavation engineering recently. Enlightened by the geoelectrical field response method, the electrical potential method was proposed in this study. By means of a model experiment, the applicability of the proposed method was verified and approaches for evaluating the seepage characteristics were discussed. Based on the analyses, several conclusions could be drawn: (i) based on the electrical potential method, a circuit is established by connecting the power supplier, the ground and several monitoring electrodes and voltmeters. When the seepage passes through a certain electrode, the increase of voltage can be observed so that the seepage flow can be visualized. (ii) The effectiveness of the electrical potential method is validated by the excellent agreement between photos of practical seepage flow and the nephograms depicted on the basis of the voltages measured on the electrodes. (iii) The time history curve of the measured voltage is S-shaped and composed of a small ladder rising stage, a sharp increasing stage and a convergence stage. The seepage flow velocity, inhomogeneity and the leakage point location can be evaluated by the time cost between the two major rise beginning points, the time cost for the major rise process and the maximum measured voltage, respectively. The proposed method can provide valuable support to the diaphragm wall leakage model test and other seepage experiments.

Materials 14(9), 2108 (2021). doi:10.3390/ma14092108, Published by MDPI, Basel

As we all know, water inrush and mud gushing is very common in broken rock tunnels under high water pressure. Water inrush and mud gushing often occurs under the action of seepage force because of high water pressure which can lead to the ground subsidence, cracking and the damage of tunnel structures. In this paper, the author takes the engineering practice for example to analyze the mechanism of water inrush and mud gushing under high water pressure and put forward appropriate treatment methods. According to numerous researches on water disasters, two main types of water inrush and mud gushing in broken rock tunnels are summarized: the partial cracking in the cross section and the longitudinal water inrush and landslide. This paper not only describe where and when these two types of water inrush and mud gushing will happen but also the different formation and mechanism of water inrush and mud gushing. In addition, prevention and treatment measures of water inrush and mud gushing are put forward. The multiple advanced geological detections combined by long-term and short-term forecasts are the fundamental measures that can effectively prevent the water inrush and mud gushing during the tunnel construction. Besides, the effective treatment measures are also summarized, including the advanced drainage and advanced grouting method. Advanced drainage is an effective method to relief energy of high water pressure, and advanced grouting can form a shelter that keep water outside by filling the fissures and cracks with grouting materials in the fracture zones.

The consolidation and pressure dissipation of the generally adopted cement based grout injected into the tail void during simultaneous backfill grouting in shield tunneling are influenced by many factors, such as grout characteristics, ground permeability, pore water pressure (PWP), dimension of the initial grout pressure, and various loads acted on the lining. Based on the typical simultaneous backfill grouting parameters in soft soils in Shanghai, and excluding the influence of grout hardening, using the theory of fluid mechanics and elastic mechanics, the principle of grout consolidation in tail void is firstly analyzed in this paper, and the changes of grout cake's thickness (GCT) and grout pressure (GP) with time are clarified. Then, as for grout pressure dissipation (GPD), the influences from ground osmotic resistance, PWP and dimension of the initial GP are discussed. The following conclusions are drawn: the ground osmotic resistance can obviously delay grout consolidation, but not the final consolidation state; the increment of soil shear modulus can accelerate the grout pressure dissipation speed (GPDS); consolidation process can be obviously restrained by the increment of PWP; and there is a positive relationship between the initial GPDS and the dimension of GP.

The shield tail void formed in unit time in tunneling is simplified as a three-dimensional annular space with shied advancing at certain speed. As the filling process cannot be observed in-situ, it is very difficult to simulate it in detail. Taking the cementitious grouts used in grouting injection as Newton fluid and Bingham fluid respectively, based on the typical four grouting holes design, the distribution of grouts pressure in cross section is obtained with the diffusion model of grouts pressure and the principle of superposition. With further consideration of grouts infiltration into the surrounding strata and its hardening process, the distribution of grouts pressure in longitudinal section is obtained. Therefore, the diffusion model of grouts pressure for simultaneous backfilling grouting in shield tunneling is founded. Finally, the comparison between theoretical result and the site observation is made.