Lessons learned from field measurements in tunnelling

doi:10.1016/S0886-7798(98)00004-2

Tunnelling and Underground Space Technology

Volume 12, Issue 4, October–December 1997, Pages 453-460

https://doi.org/10.1016/S0886-7798(98)00004-2 Get rights and content

Abstract

Over the past two decades, various numerical methods of analysis have become popular in the field of geotechnical engineering. However, the accuracy of numerical analyses varies considerably, primarily because of the uncertainties involved in modelling ocmplex geological formations with the complex geomechanical characteristics of soils and rocks. Field measurements carried out during construction can be used to overcome this difficulty. The author reviews ways of using measurement results to improve numerical analyses, including the determination of a “hazard warning level” for each measurement item prior to the start of construction, the use of back analysis. The importance of choosing a proper model is also discussed.

References (7)

S. Sakurai
Direct strain evaluation technique in construction of underground openings
S. Sakurai
Displacement measurements associated with the design of underground openings
S. Sakurai
The assessment of tunnel stability on the basis of field measurements

There are more references available in the full text version of this article.

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Numerical back analysis based on incomplete in situ stress measurements is widely used to estimate the three-dimensional (3D) in situ stress fields of large deep underground caverns. However, the variable collinearity caused by complex geological environments has rarely been considered. This paper proposes a numerical back analysis method for 3D in situ stress fields based on stepwise regression (BSSR). In BSSR, the collinearity between independent variables is associated with the surface topography, and insignificant variables caused by variable collinearity are eliminated through stepwise regression. BSSR was applied to the underground powerhouse of a hydropower station for validation, and the results showed that it can reliably estimate the 3D in situ stress field despite the complex geological environments. Multidimensional mathematical models of in situ stress fields were established with the improved predictive ability and clear physical meaning, which can quantify the contributions of six geological actions and three stress sources. The findings of this study can help with understanding the formation mechanism of in situ stress fields for large, deep underground caverns in complex geological environments and provide a useful reference for optimizing excavation schemes and support designs.
Challenges associated with numerical back analysis in rock mechanics
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Numerical back analysis is a valuable tool available to rock mechanics researchers and practitioners. Recent studies related to back analysis methods focused primarily on applications of increasingly sophisticated optimization algorithms (primarily machine learning algorithms) to rock mechanics problems. These methods have typically been applied to relatively simple problems; however, more complex back analyses continue to be conducted primarily through ad hoc manual trial-and-error processes. This paper provides a review of the basic concepts and recent developments in the field of numerical back analysis for rock mechanics, as well as some discussion of the relationship between back analysis and more broadly established frameworks for numerical modelling. The challenges of flexible constraints, non-uniqueness, material model limitations, and disparate data sources are considered, and representative case studies are presented to illustrate their impacts on back analyses. The role of back analysis (or “model calibration”) in bonded particle modelling (BPM), bonded block modelling (BBM), and synthetic rock mass (SRM) modelling is also considered, and suggestions are made for further studies on this topic.
Empirical approach for reliability evaluation of tunnel excavation stability using the Q rock mass classification system
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The critical strain concept has been widely used in analytical or numerical approaches to evaluate the stability of underground excavations. Analytical, empirical, and numerical procedures are usually used to determine the critical strain values. This paper presents a reliability assessment procedure for evaluating excavation stability using the empirical approach based on the rock mass classification Q and the first order reliability method (FORM). In contrast to deterministic critical strain values, a probabilistic critical strain, which considers uncertainties in rock mass parameters, was incorporated in a limit state function for reliability analysis. Using the rock mass classification Q, the empirically estimated tunnel stain was included in the limit state function. The critical strain and estimated tunnel strain were probabilistically characterized based on the rock mass classification Q-derived rock mass properties. Monte Carlo simulations were also conducted for comparing the reliability analysis results with those derived from the FORM algorithm. A highway tunnel case study was used to demonstrate the reliability assessment procedure. The effects of the input ground parameter correlations, probability distributions, and coefficients of variation on tunnel reliability were investigated. Results show that uncorrelated and normally distributed input parameters (intact rock strength and elastic modulus) have generated more conservative reliability. The reliability analysis results also show that the tunnel had relatively high reliability (reliability index of 2.78 and probability of failure of 0.27%), indicating the tunnel is not expected to experience instability after excavation. The tunnel excavation stability was assessed using analytical and numerical approaches for comparison. The results were consistent with the reliability analysis using the FORM algorithm's Q-based empirical method.
A plastic-damage approach to the excavation response of a circular opening in weak rock
2022, Tunnelling and Underground Space Technology
Citation Excerpt :
Hence, it is suggested that Rs = (Rs1 + Rs2_1)/2, which leads to the error caused by neglecting the confining stress dependence is limited to 15%, as shown in Table 3. Sakurai (1997) and Cui et al. (2017) pointed out that the residual failure zone formed in surrounding rock is directly related to the opening instability, because it may lead to large deformation and significant displacement of the surrounding rock due to large-scale deterioration of mechanical parameters. Therefore, reasonably design of the support force to avoid the creation of residual failure zone plays an important role in maintaining opening stability.
During opening construction, the accumulated wall displacement and the developing excavation damaged zone are the results of stress redistribution and inelastic behavior induced by both the microcracking damage and the irreversible plastic deformation. The current approach to predict ground response to excavation disturbance is usually based on the Convergence-Confinement Method, which fails to consider the rock damage and the plastic-damage coupled mechanism, resulting in inaccurate estimation of rock deformation, particularly in the case of the weak rock with high deformability. This study establishes a plastic-damage theoretical approach for a circular opening by a kinematic decomposition of strains into an elastic, plastic and damage parts within the framework of finite strain using a hypoelastic–plastic theory, in order to investigate the inelastic behavior-induced excavation response. In this regard, the damage model provides the effective stress to the plasticity model defining the yield criterion, in combination with the strength-stiffness degradation and damage evolution. The numerical implementation is given and two examples are considered for validation. Extensive works are then carried out to clarify some issues, including the capacity of the model to characterize the inelastic behavior, the role of rock damage and confining stress dependence on ground response, and the preliminary critical support pressure for the residual failure zone.
Empirical correlations for prediction of tunnel deformation in squeezing ground condition
2022, Tunnelling and Underground Space Technology
The Himalaya has undergone various tectonic activities which resulted in variant geology in the region. Complex geology and weak geological structures along with ingress of water give rise to a high degree of fragility and instability in the whole mountain system. Moreover, the high mountainous topography causes high overburden pressure and stresses in the underground structures causing squeezing and other stability problems.
There are several parameters which do affect the squeezing behavior of rock. Parameters like rock mass strength, in-situ stresses, overburden pressure, support stiffness, joint orientation and water play a significant role in influencing the squeezing behavior of rock.
In this paper three dimensionally correct empirical correlations have been developed using regression analysis for prediction of tunnel deformation. In the study, data collected from 75 numbers of squeezing tunnel sections from Himalayan region have been included. Rock mass quality index (Q), rock mass number (N), ratio of horizontal stress (σ_h) to vertical stress (σ_v) that is K₀, joint factor (J_f), support stiffness (K), vertical stress (σ_v), tunnel radius (a) and observed deformation (U_obs) are the governing parameters considered for developing the correlations.
The developed correlations with regression coefficient of 0.87, 0.79 and 0.78 have been compared with existing correlations with main parameters as joint factor (J_f), rock mass quality index (Q) and rock mass number (N). In addition comparisons with existing correlations have been done on the basis of coefficient of accordance and linearity. The proposed correlations are definitely an improvement over the reported ones.
Critical shear strain and sliding potential of rock joint under cyclic loading
2022, Transportation Geotechnics
A new concept of critical shear strain $({ε_{τ}}_{critical})$ of rock joint under cyclic loading is presented, and the role of ${ε_{τ}}_{critical}$ in evaluating the sliding potential of rock joint is highlighted. A series of cyclic triaxial tests was conducted on a cylindrical rock joint specimen with a replicated rough surface representing a joint roughness coefficient $(J R C)$ value of 12.6 oriented at 60° with respect to the horizontal plane. The experimental results indicate that the onset of instability of rock joint is suppressed with increase in confining pressure and number of loading cycles $(N)$ until the normalized shear deformation increases beyond a threshold value of ${ε_{τ}}_{critical}$ . Generally, the critical strain of most rock types is considered in the proximity of 1% under small strain conditions [36], [37], however, in this study, the critical strain concept is extended to the domain of rock joints, and a semi-empirical model to more rigorously quantify the critical shear strain $({ε_{τ}}_{critical})$ of rock joint is suggested considering the effect of joint roughness coefficient $(J R C)$ , cyclic loading amplitude, and the number of loading cycles $(N)$ . Also, a rational classification of Joint Sliding Potential (JSP) based on the ${ε_{τ}}_{critical}$ and normalized total shear strain $({ε_{θ}}_{N})$ of rock joint is proposed to characterize the cyclic loading induced sliding instability of a rock discontinuity.

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Tunnels and deep spaceLessons learned from field measurements in tunnelling

Abstract

Direct strain evaluation technique in construction of underground openings

Displacement measurements associated with the design of underground openings

The assessment of tunnel stability on the basis of field measurements

Tunnels and deep space
Lessons learned from field measurements in tunnelling