Prediction of Preceding Crown Settlement Using Longitudinal Displacement Measured on Tunnel Face in Fault Zone

Parametric Study of Water Inrush in a Tunnel Crossing a Fault Based on the “Three Zones” Fault Structure

A measurement of tunnel displacement plays an important role for stability analysis and prediction of possible fault zone ahead of tunnel face. In this study, we evaluated characteristics of tunnel behaviour due to the existence and orientation of fault zone based on 3-dimensional finite element numerical analysis. The crown settlement representing tunnel behaviour is acquired at 5 m away from tunnel face in combination with x-Rs control chart analysis based on statistics for trend line and L/C (longitudinal/crown displacement) ratio in order to propose risk management method for fault zone. As a result, x-Rs control chart analysis can enable to predict fault zone in terms of existence and orientation in tunnelling.

Water inflow is one of the main geohazards that threaten the safety of tunnels and other underground engineering projects. Faulted zone is one of the important geological triggers for such events. Numerical investigations on the evolution of flow behavior in tunnels across fault zones are of significance to the predication and prevention of this type of geohazards. In this work, a numerical investigation model with two overlapped parallel faults is established at a steady stage according to the "Three Zones" fault structure theory. The rapid turbulent flow in the fault zone is simulated by using the improved Darcy-Brinkman seepage model, while the slow laminar flow in ordinary rock zone is described by Darcy equation. The effect of relative position and distance between the tunnel excavation face and overlapped parallel faults to the groundwater pore pressure and flow velocity is studied through several scenarios, and the water inflow rate into the tunnel is calculated. The numerical investigation results reveal that while the tunnel face is excavated into the fault center core, the fractured zone, the ordinary rock zone, and the center of the overlapped faults, the pore pressure value ahead of the excavation face increases while the flow velocity decreases sequentially. The inflow rate is the largest while the tunnel face is excavated to center of the fault center core, which is closely related to the range of the overlapped area. The investigation results offer a practical reference for predicting early warning of water inflow geohazard when a tunnel cross two overlapped parallel faults.

Summary The Tunnel Seismic While Drilling (TSWD) method has been developed and applied at several tunnel sites to predict the geological situation ahead of the tunnel face during mechanical tunnel driving. When tunneling with a Tunnel Boring Machine, the vibrations of the drilling head, resulting from the cutting process, offer to be employed as a seismic source signal, ensuring a continuous seismic monitoring without hindering the drilling and driving operations. With the appropriate signal processing the continuous monitoring data can be converted to conventional seismic traces from which relevant fault zones within a geophysical forecast window of up to 100 m ahead of the current tunnel face can be predicted. Since the implemented instrumentation, data transfer and logistics guarantee processing on a daily basis, significant geological structures can be observed over long distances. The TSWD-method gives excellent continuous seismic data, from which deeply incised valleys, karst cavities, fault zones and other unexpected degradations of rock quality can be predicted. Wider fault zones over a thickness of 10 m can be successfully resolved, smaller fault zones are largely detected, depending on seismic impedance contrast and the position relating to the tunnel axis.

Risky Ground Prediction ahead of Mechanized Tunnel Face using Electrical Methods: Laboratory Tests

With Xiamen Jiaheyuan underground access as the project background, tunnel face stability of soft shallow tunnel was analyzed under the condition of no pre-reinforcement by means of three-dimensional finite element method. The results indicated that the ground was relaxed because the tensile stress appeared in front and top of tunnel face after excavation, at the same time, the ground into the plastic state around the tunnel face. From the point of view of deformation, the displacement of tunnel face were such as the longitudinal horizontal displacement reached the maximum, the vertical deposition following by, and the lateral horizontal displacement being the least. Further analysis showed that the longitudinal horizontal displacement in front of tunnel face mostly produced at 1.0D (one excavation width) distance before tunnel face, the maximum displacement was located at the center of tunnel face. The conclusions remind that engineers also pay attention to the tunnel face reinforcement in front and top of tunnel face to minimize the impact of surface environment during tunnel construction in soft shallow tunnels.

During the construction of the Semmering Base Tunnel, Lot SBT1.1, the drives have already encountered several fault zones in the Greywacke Zone. Because of the high overburden, the exact position of these fault zones is unknown at tunnel level; a common problem for all tunnelling projects in mountainous regions. Simple exploration drilling techniques such as percussion drillings, where only cuttings and not cores are won, do not always provide enough information to precisely specify the position of the fault zones or their nature ahead of the face. This is reason enough to examine other possibilities for the short‐term prediction of fault zones with differing characteristics ahead of the face. Usually displacement data evaluation provides the basis for a short‐term prediction of the system behaviour. However, experiences from Lot SBT1.1 show that applying this approach solely does not always yield satisfying results. A further systematic analysis of selected geological data can improve the short‐term prediction. In particular, changes of discontinuity and rock mass characteristics mapped at the tunnel face are analysed to spot significant trends indicating fault zones ahead of the face. These trends are then related to and verified by the results of displacement data evaluation. This combination of rock mass characteristics mapped at the face and state‐of‐the‐art evaluation of displacement data has helped to improve the reliability of short‐term predictions during the tunnel excavation.

The problem of groundwater is very prominent in super-long tunnel construction, which brings serious potential safety hazards and economic losses to the project. The knowledge of dynamic change characteristics of groundwater and prediction of water inflow is the key to ensure rational design and safe construction in super-long tunnel. In this paper, numerical simulation and in situ observation are conducted to investigate dynamic change characteristics of groundwater and the prediction of water inflow based on the Daxiangling tunnel in Sichuan Province of China. The results show that the numerical model established with detailed geological data and validated with field monitoring data can effectively analyze dynamic change characteristics of groundwater, as well as predict water inflow. The initial state of groundwater is steady when the tunnel is unexcavated. Tunnel excavation has a significant influence on the distribution of groundwater. The flow direction of groundwater will change, and the contour lines of groundwater will be intensive at the tunnel face. These changes will be more obvious and dramatic when the tunnel is excavated into the fault zone, which is a signal that the water inrush is more likely to occur in the fault zone because of a lot of joints and fractures. A connected linear cavity is formed with tunnel holing-through and groundwater begins to flow vertically downwards to the tunnel. As far as the prediction of water inflow is concerned, the numerical method can more precisely calculate the value of water inflow with less than 15 percent relative error compared with the groundwater dynamics method.

Nowadays, with the increasing population of large cities, the need to expand public transportation, especially metro systems, is greatly increasing in urban areas. Therefore, excavation of new tunnels near the existing ones or other excavations located nearby has become inevitable. Excavation of such tunnels in urban areas should be done by considering the effects of these tunnels on buildings and other urban structures. Significant factors affecting interaction between tunnels, as well as the characteristics of surface settlement, are the existence of mixed ground (soil-rock) at tunnel faces or fault zones in the direction of tunnel excavation which have not been clearly investigated by researchers. These parameters have a great effect on the amount of maximum surface settlement and shape of surface settlement curve. Although several studies aim to analyse the interaction between newly excavated and existing tunnels and its effects on surface settlements, this subject certainly needs further investigation. This study mainly focuses on the effects of the interaction between twin tunnels mainly opened in fault zones and mixed ground on the basis of surface settlement measurements by using Earth Pressure Balance Machine (EPBM). Both numerical and empirical methods are used in this study. Observed data are used to test the validity of the results obtained from three-dimensional numerical modelling. The results from numerical methods were in good agreement with the real data. The results of this study reveal that the amount of maximum surface settlement and shape of surface settlement curve are strongly related to spacing between tunnels, fault zone thickness and type of tunnel face material. The interaction factor is almost zero when spacing is larger than 4D (D is tunnel diameter). Independent of fault zone thickness, the effects of the fault on longitudinal surface settlement continue 25 m from both sides of the fault centre.

Study of longitudinal deformation profiles in high-ground-stress mega-section tunnels based on the Hoek–Brown criterion

During tunnel construction, crossing water-rich fault fracture zones leads to water influx hazards at the tunnel face. Excavation exposes the fault, causing disturbances and changes in the hydraulic gradient. Thus, the tunnel face becomes a low hydraulic head surface. The naturally high permeability of the fracture zone provides a pathway for groundwater flow, resulting in water seepage along the fracture zone and subsequent water influx at the tunnel face. To investigate this interaction, a two-phase double-point material point method was employed aimed at developing a coupled fluid-solid numerical model for excavating submarine fault tunnels. The model incorporates solid-liquid phase interaction, including rock mass permeability, water viscosity, and tunnel geometry. It analyzes the changes in the liquid-phase flow velocity, pore water pressure, and particle trajectories in the fractured zone ahead of the working face after the fracture zone is exposed, revealing the dynamic evolution of water influx following excavation. Additionally, this study discusses the impact of the solid-liquid phase permeability coefficients on the range of water influx hazards. The research findings demonstrate that the two-phase double-point material point method effectively captures the seepage and water influx processes, offering valuable insights into the mechanisms of sudden water influx in tunnel engineering.

At present, the tunnel construction engineering is increasingly transferring to southwest mountainous areas with complex terrain and geological conditions in China and presents a trend of “large buried depth, long tunnel line, high stress, strong karst, high water pressure, complex structure and frequent disasters.” Taking water inrush disaster of karst tunnel fault zone as the research object, an evolutionary mechanical model of rock damage under the coupling action of stress-seepage is proposed in this paper. Besides, based on Comsol Multiphysics numerical software, the tunnel excavation is simulated, and the stress field, seepage field, and rock damage during the excavation are analyzed; thus, the mechanical mechanism of water inrush disaster from tunnel fault in karst area is obtained. The research results indicate that the tunnel excavation is a dynamic construction process, and the construction disturbance redistributes the original rock stress field and changes the state of seepage field. With the increase of excavation steps, the contour distribution of vertical stress ratio near the tunnel face is a circle shape, indicating that the rock mass is obviously disturbed by excavation, and the ratio of principal stress difference of rock mass at arch crown and bottom plate is large. Besides, the fault fissures expand and penetrate under the influence of tunnel excavation disturbance, increasing the permeability of fault zone in karst tunnel. In addition, the water seepage erosion takes away the granular rock mass, and the lithology becomes more weaker, which makes it possible for the occurrence of water inrush disaster in karst tunnel. Therefore, the advanced geological prediction is important in tunnel construction in karst area. The research results can be treated as an important theoretical basis for the prevention and treatment for water inrush disaster of fault zone in karst tunnel.

The pre-grouting method or grouting ahead of the excavation face in underground construction in rock can offer significant advantages in many situations. This is particularly the case in difficult rock conditions such as water ingress or mechanically difficult rock, in which pre-grouting can contribute to avoiding problems and serious delays. Modern cost-effective methods and material technology for pre-grouting in underground construction aims to achieve the desired result as quickly as possible, hence reducing the down time during excavation as much as possible. Collapses at the tunnel face or unexpected high water inrushes are not uncommon experiences when tunnelling in geologically difficult rock conditions such as fault zones in alpine terrain or tunnels with shallow location influenced by weathering or low rock stresses. Tunnelling in urban areas often involve shallow location of tunnels, proximity to existing underground structures, as well as establishing connections between underground structures. The consequences of a groundwater drawdown or deformations in the ground caused by collapses are unacceptable due to the possible impact on buildings with sensitive foundations. This paper addresses the issue of how modern pre-grouting can strongly reduce the risk of problems and offer a cost-effective reduction of water ingress into tunnels. The state-of-the art technology of rapid hardening micro-cements and liquid colloidal silica is particularly emphasized. This technology can improve the cost-effectiveness and technical feasibility of tunnelling in sensitive environment in difficult rock conditions significantly.

Laboratory simulations on hybrid non-destructive survey of electrical resistivity and induced polarization to predict geological risks ahead of a TBM tunnel

As complexity can be understood in different ways depending on the context, some conceptual clarifications are provided as a starting point (Section 2). Subsequently, we address selected aspects of complex formations: the variability of squeezing, the adverse effects of water, tunnelling in fault zones and the applicability of closed shields in weak rocks (Sections 3 to 6). Finally, we present two case histories addressing two key problems of TBM tunnelling in complex rock formations: the loads that develop on the shield under variable squeezing conditions (Section 7) and the stability of the tunnel face in faults or weak water-bearing rocks (Section 8).