Interpretation of Core Extrusion Measurements When Tunnelling Through Squeezing Ground

Squeezing intensity in tunnelling often varies over short distances, even where there is no change in the excavation method or lithology. Reliable predictions of the ground conditions ahead of the face are thus essential to avoid project setbacks. Such predictions would enable adjustments to be made during construction to the temporary support, to the excavation diameter and also to the final lining. The assessment of the behaviour of the core ahead of the face, as observed by means of extrusion measurements, provides some indications as to the mechanical characteristics of the ground. If the ground exhibits a moderate time-dependent behaviour and the effects of the support measures are taken into account, the prediction of convergence is feasible. If the ground behaviour is pronouncedly time-dependent, however, convergence predictions become very difficult, because core extrusion is governed by the short-term characteristics of the ground, which may be different from the long-term properties that govern final convergence. The case histories of the Gotthard Base Tunnel and of the Vasto tunnel show that there is a weak correlation between the axial extrusions and the convergences of the tunnel. By means of the case histories of the Tartaiguille tunnel and Raticosa tunnel, it is shown that to identify potentially weak zones on the basis of the extrusion measurements, careful processing of the monitoring data is essential: the analysis of the data has to take account of the effects of tunnel support and time, and has to eliminate errors caused by the monitoring process.     

Approximate time-dependent analysis of tunnel support structure considering progress of tunnel face

The pressure acting on tunnel support structures generally increases with lapse in time. The pressure increase may be caused by not only the time-dependent mechanical properties of the underground medium, but also by the progression of the tunnel face. In order to clarify these two effects, three-dimensional analysis is required. In this study, however, a method which takes into account the three-dimensional effects of the tunnel face progression in two-dimensional plane strain analyses is proposedby introducing the ‘equivalent intial stress’. The proposed method can be easily applied to time-dependent analyses of the behaviour of tunnel support structures installed in a visco-elastic medium. The tunnels considered here are circular in shape, being driven in homogeneous isotropic linear visco-elastic media having hydrostatic initial stresses. In the first part of this paper, detailed discussions of the proposed equivalent initial stress are given. The second deals with the mathematical formulations for obtaining closed-form solutions for the pressure acting on tunnel support structures. In the third, numerical results and discussions are given, and special attetion is paid to the effects of both the time-dependent mechanical properties of the material and the tunnel face progression. Finally, the theoretical results obtained here are utilized for interpretation of field measurements.     

Time-dependent behaviour of deep clays

The time-dependent behaviour of deep saturated clays is related both to the effects of hydraulic diffusion and of viscosity. In this paper, we present first, by means of an experimental approach, the main features of the mechanical properties of deep clays. Then, we study the effects of these specific behaviours on the time-dependent behaviour of underground structures. We present theoretical and numerical investigations of the effects of pore pressure diffusion resulting from a tunnel excavation in a poroplastic or poroviscoplastic medium. Finite-element calculations show that the time-dependent convergence of the tunnel wall is non negligible. Then, we focus on the behaviour of a cylindrical thermal source buried in a deep clay.

Coupling effects between thermal, hydraulic and mechanical behaviours are very important in soft and low permeable deep clays (saturated compressible clays with high porosity). We show in particular that the displacements and the stresses are very sensitive to the constitutive model. Irreversible behaviour may be traduced by appearance of residual stresses within the rockmass.     

Manuel Rocha Medal Recipient Simulating the Time-dependent Behaviour of Excavations in Hard Rock

Summary Although hard rock is not usually associated with large creep deformation, data collected from the tunnels and stopes of the deep South African gold mines illustrates significant time-dependent behaviour. Apart from application in mining, a better understanding of the time-dependent behaviour of crystalline rock is required to analyse the long term stability of nuclear waste repositories and to design better support for deep civil engineering tunnels in these rock types. To illustrate the subtle problems associated with using viscoelastic theory to simulate the time-dependent behaviour of hard rock, a viscoelastic convergence solution for the incremental enlargement of a tabular excavation is discussed. Data on the time-dependent deformation of a tunnel developed in hard rock further illustrates the limitations of the theory, as it is unable to simulate the fracture zone around these excavations. To simulate the rheology of the fracture zone, a continuum viscoplastic approach was developed and implemented in a finite difference code. This proved more successful in modelling the time-dependent closure of stopes and squeezing conditions in hard rock tunnels. A continuum approach, however, has limitations in areas where the squeezing behaviour is dominated by the time-dependent behaviour of prominent discontinuities such as bedding planes. To overcome this problem, a viscoplastic displacement discontinuity technique was developed. This, combined with a tessellation approach, leads to more realistic modelling of the time-dependent behaviour of the fracture zone around excavations. Received January 15, 2002; accepted June 3, 2002 Published online September 2, 2002     

盾构隧道注浆对既有隧道影响的离心模拟研究

随着地铁网络不断完善,越来越多的新建盾构隧道近距离穿越既有隧道,然而对于盾构隧道近距离穿越既有隧道影响的研究尚不够完善。以上海典型软弱地层为背景,通过离心模型试验,研究了不同注浆率下的盾构上穿越施工对既有隧道以及周围地层的影响。选用排液法在离心场中模拟盾构施工,在不停机状态下成功模拟隧道开挖卸载、地层损失和注浆效应。分析了在不同的注浆率条件下,既有隧道在上穿越施工期和工后长期的位移、周围孔压和纵向应力的变化规律。试验结果表明,新建隧道近距离上穿越既有隧道时,隧道开挖的卸载效应等会导致既有隧道的隆起,但随着注浆率增大,既有隧道的隆起量减小。但过高注浆率对周围土体扰动较大,从而导致工后既有隧道的沉降也越大。     

Practical Estimate of Deformations and Stress Relief Factors for Deep Tunnels Supported by Shotcrete

Summary. Even though ground-support interaction in the vicinity of the tunnel face is a typical 3D problem, tunnel support design is usually based on simplified plane strain models, which are strongly dependent on the assumed degree of ground stress relief at the time of lining installation. The paper focuses on tunnels supported by shotcrete close to the face, where the interaction between the loading process and progressive hardening of the green shotcrete makes the problem time-dependent. A constitutive law characterized by the time-dependent stiffness and strength of the shotcrete is employed herein. The results of an extensive parametric study based on 3D axisymmetric models are presented in the form of non-dimensional design charts, which can provide guidance to a preliminary evaluation of convergences and support loadings. Moreover a strategy is proposed to enhance the capability of simplified design methods (2D models, Convergence-Confinement Method). This consists in a “guided estimate” of stress relief factors, which again is based on the results of 3D time-dependent analyses. Finally, by way of example, the proposed method is applied to two well-documented case-histories.     

New Viscoplastic Model for Design Analysis of Tunnels in Squeezing Conditions

Summary  This paper is intended to describe the SHELVIP (Stress Hardening ELastic VIscous Plastic) model, a new viscoplastic constitutive law which has been developed to incorporate the most important features of behaviour observed in tunnels excavated in severe to very severe squeezing conditions. This model couples the elastoplastic and time-dependent behaviour by using a plastic yield surface, as frequently adopted in tunnel design analysis, and the definition of a state of overstress referred to a viscoplastic yield surface. The model is formulated in all its detailed aspects. The related analytical closed-form solution for representing triaxial creep deformations is developed. Also developed is an incremental numerical solution for describing the triaxial stress–strain behaviour under constant strain rate conditions. The model is shown to fit very satisfactorily the results of creep tests on clay shales and relaxation tests on coal specimens, as recently performed for design analysis of tunnels in squeezing conditions. Correspondence: D. Debernardi, Department of Structural and Geotechnical Engineering, Politecnico di Torino, Torino, Italy     

Time‐dependent modelling for soils and its application in tunnelling

Monitoring of the progressive convergence of a tunnel shows that deformations occurring in the soil surrounding a tunnel exhibit a strong evolution with time. This time‐dependent behaviour can be linked to three essential factors: the distance from the point of interest to the working face over time, the distance of unsupported tunnel to the working face and the viscous properties of the soil. The objective of this paper is to propose a constitutive model of the time‐dependent behaviour of soil which has been developed within the framework of elastoplasticity–viscoplasticity and critical state soil mechanics. The consideration of viscoplastic characteristic sets the current model apart from the CJS (Cambou, Jafari and Sidoroff) model as the basic elastoplastic model, and introduces an additional viscous mechanism. The evolution of the viscous yield surface is governed by a particular hardening called ‘viscous hardening’ with a bounding surface. The proposed constitutive model has been applied in the analysis of tunnelling. Two kinds of numerical calculations have been used in the analysis, axisymmetric analysis and plane strain analysis. Monitoring of the progressive convergence of a tunnel conducted in the railway tunnel of Tartaiguille (France), has been used to describe the calculation procedure proposed and the capability of the model. The finite difference software, fast Lagrangian analysis of continua (FLAC), has been used for the numerical simulation of the problems. The comparison of results shows that the observed deformations could have been reasonably predicted by using the constitutive model and calculation strategy proposed. Copyright © 2004 John Wiley & Sons, Ltd.     

A cyclic elastoplastic-viscoplastic constitutive model for soils

An elastoplastic-viscoplastic constitutive model for soils is presented in this study, based on an original approach concerning viscous modelling. In this approach, the viscous behaviour is defined by internal viscous variables and a viscous yield surface. The model has been developed from a basic elastoplastic model (CJS model) by considering an additional viscous mechanism. The evolution of the viscous yield surface is governed by a particular hardening called ‘viscous hardening’. This model is able to explain the time-dependent behaviour of soils such as creep (primary, secondary and un-drained creep rupture), stress relaxation and strain rate effects in static and cyclic loadings. The existing problems in the classical elasto-viscoplastic models related to the plasticity failure, the rapid loading and the cyclic loading are solved in the proposed model. The physical meanings and the identification strategy of model parameters are clearly given. The validation on certain triaxial test results and the simulation of cyclic triaxial test indicate the capacity of this model in prediction of time-dependent behaviour of clayey soils.     

Viscoplastic model for geologic materials with generalized flow rule

The general yield function in the hierarchical approach for constitutive modelling of materials is used with Perzyna's theory to characterize viscoplastic behaviour of geologic materials: a sand and rock salt. Particular attention is given to determination of the constants from laboratory quasistatic or short term, and creep tests. The proposed model is verified with respect to observed laboratory response of the sand and salt. It is implemented in a non-linear finite element procedure and applied to analyse time-dependent behaviour of a cavity in the rock salt.     

Time-dependent Behaviour of Deep Level Tabular Excavations in Hard Rock

Summary Although hard rock is not usually associated with large creep deformation, significant time-dependent behaviour is observed in the tabular excavations of the South African gold mines. Time-dependent closure data was collected in stopes of the Ventersdorp Contact Reef and Vaal Reef. This data typically consists of a primary closure phase after blasting, followed by a steady-state closure phase. This closure behaviour is the result of the rheology of the fracture zone around these excavations and the time-dependent extension of this zone following a mining increment. An elasto-viscoplastic approach was developed to simulate the time-dependent nature of the fracture zone. This model proved successful in simulating the experimental closure profiles. It appears that the closure data may provide useful diagnostic information of the stress conditions in the fracture zone ahead of the face. This may possibly be used to identify hazardous conditions such as areas prone to face bursting. The effect of preconditioning on the time-dependent closure behaviour is also illustrated.     

Progressive degradation of rock properties and time-dependent behavior of deep tunnels

Time-dependent response of deep tunnels is studied considering the progressive degradation of the mechanical properties of the rock mass. The constitutive model is based on a rock-aging law for the uniaxial strength of the rock and for the Young’s modulus. A semi-analytical solution is developed for the stresses and displacements around a deep circular tunnel taking into account the face advance. The evolution of the plastic and damage zones over time is determined. Numerical examples are presented for the case of Saint-Martin-La-Porte access adit in France of the Lyon–Turin Base Tunnel. The computed results which are compared with the field data in terms of the convergence of tunnel wall and of the displacements inside the rock mass monitored by multi-point extensometers show the efficiency of the approach to simulate the time-dependent deformation of a tunnel excavated in squeezing ground. Simple relationships are proposed to evaluate the parameters of the constitutive model directly from those of the empirical convergence law presented in previous work.     

An effective combined framework for modelling the time-dependent behaviour of soft structured clays

A constitutive framework for the simulation of the time-dependent behaviour of soft structured clays has been developed from the combination of a number of existing modelling approaches. The formulation accounts, in a natural way, for time-dependent phenomena such as loading rate dependency, stress relaxation, ageing, and creep deformations under constant load. In addition, the effect of structure is accounted for. To demonstrate the potential of the proposed approach, a specific model based on this framework is described and applied to the simulation of a number of relevant laboratory tests from the literature.

     

Numerical modelling of in situ stress conditions as an aid in route selection for rail tunnels in complex geological formations in South Germany

The paper describes three-dimensional numerical modelling studies of in situ stress distributions in complex geological conditions. The modelling was intended to augment and generalise extensive hydraulic fracturing stress measurements carried out to assist in selecting the optimum alignment of an approximately 14 km long tunnel, part of a proposed new rail link between Stuttgart and Augsburg, Germany. The numerical model includes specific representation of seven different geological layers and six geological faults with throws of up to 30 m. Results indicate complex and variable three-dimensional in situ stress conditions along the tunnel routes. This is confirmed by the field measurements. Stress conditions are characterised by strong inhomogeneity and anisotropy with a maximum to minimum principal stress ratio of up to 4:1. The numerical model indicates a large change in orientation of the quasi-horizontal maximum principal stress direction along the tunnel route. This is also observed in the measurement results. Based on the stress profiles from the model, the tunnel routes can be subdivided into four and five sections in each of which the stress conditions are approximately uniform. An initial assessment has been made of the necessary support measures and problems that may be anticipated during tunnel construction by determining a factor of safety for a circular tunnel of a certain diameter in each of the sections defined above.     

Brittle Rock Failure in the Steg Lateral Adit of the Lötschberg Base Tunnel

Summary  During the crossing of brittle rock formations at the L?tschberg base tunnel, failure phenomena have been observed both at the tunnel face and at the walls. A detailed analysis has been undertaken to explain these behaviours, based on the recent developments of Canadian research on brittle failure mechanisms. At the tunnel walls, a very good agreement is found between the calculated and observed damage and between two prediction methods, i.e. a semi-empirical failure criterion and elastic calculations with the “brittle Hoek-Brown parameters.” Near the face, due to the 3D nature of the stress conditions, some limitations of these approaches have been highlighted, and the growth of wall failure has been analysed. This research allowed a better understanding of the brittle rock mass behaviour at the L?tschberg base tunnel and showed that brittle failure processes dominate the behaviour of deep, highly stressed excavations in massive to moderately jointed rock. It also illustrates where improvements to the adopted approaches are required.     

Visco-Plastic Behaviour around Advancing Tunnels in Squeezing Rock

Summary  The visco-plastic behaviour of rocks plays a relevant role in the tunnelling works, especially for deep tunnels subjected to large initial stresses for which squeezing conditions may develop. A rheological model is discussed that accounts for visco-elastic (primary) and visco-plastic (secondary) contributions to rock creep. The effects of tertiary creep are included in the model by way of a gradual mechanical damage governed by the cumulated visco-plastic strains. The parameters of the intact rock are first identified based on laboratory test results presented in the literature. Then, after scaling them to those of the rock mass, the potential applicability of the model is tested through axisymmetric and plane strain finite element analyses of the full face excavation of a deep circular tunnel. The results are discussed with particular reference to the short term redistribution of stresses around the opening and to its convergence. The analyses show the relevant influence of tertiary creep on the tunnel closure. In addition, those based on an axisymmetric scheme turn out to be crucial for the correct long term prediction of the interaction between the rock mass and the supporting structure of the opening.     

THE ‘NEW IMPLICIT METHOD’ FOR TUNNEL ANALYSIS

Tunnel excavation is a coupled three-dimensional problem dealing with two different structures: lining and rockmass. For a simple application it is useful to develop simplified methods by treating the problem as plane strain. If the problem of tunnel face advance presents an axisymmetric geometry, then we show that the major parameter governing the ground–interface–lining interaction is the convergence of the tunnel U₀ at the moment of the lining installation. The ‘New Implicit Method’ (NIM) presented in this paper makes use of principles similar to those of the ‘convergence–confinement’ method, but it provides a better appreciation of the coupled behaviour of rockmass and lining. For independent time constitutive laws (elasticity and plasticity), we point out that the convergence U₀ depends not only on the mechanical behaviour of the rockmass and on the distance from the tunnel face, as predicted by the ‘convergence–confinement’ method, but also on the stiffness of the lining previously set. We present the ‘NIM’ for elastic and perfect elastoplastic rockmasses without dilatancy for many criteria. The development of this new method is based on the results of tunnel calculations with an axisymmetric FEM numerical model that takes into account the three-dimensional aspect of the problem. Using this method is simple and its results agree well with the FEM numerical results. Its accuracy is highly satisfactory for a geotechnical study.     

Modelling the behaviour of a retaining wall in residual soils for a cut and cover construction of a deep station in Metro do Porto

This paper presents a re-appreciation of the ground characterisation and the criteria to select the most representative geomechanical parameters to consider in a numerical model to predict the behaviour of a retaining wall of a deep excavation in highly weathered granite rock masses and residual soils. This study was focused in the construction of a deep station of Metro do Porto, which involved a cut and cover solution, with unusual proportions (in plant and in depth), built in the typical Oporto's granite weathered profiles, being the excavation conducted with retaining walls consisting of multi-anchored concrete piles. Specific sections were carefully instrumented, due to the presence of historic buildings in the vicinity. The definition of representative model parameters was based on precise laboratory tests over high quality soil samples, including oedometer and high-precision triaxial tests. Geotechnical and geological characterisation of all the area for the original design, was initially based on in situ tests, such as SPT and rock masses classification, and on the local experience on this type of ground. After this construction, the assumptions of parameterisation, using a constitutive modelling based on new laboratory tests over high quality block samples, allowed a reanalysis of the assumptions on the design phase. A back-analysis of monitored displacements and forces during the construction was made, assuming the designed structural solutions, which were in fact implemented in construction, but considering the new approaches on the definition of the geomechanical parameters for the prevailing weathered rock masses, necessary for the numerical simulation based on the commercial software Plaxis®, using the Mohr-Coulomb and “Hardening-Soil” models. Some specific changes of the constructive sequence during the excavation and activation of supporting system were attained, by looking at the information found during the construction. The results of this parametrical re-approach and analysis of the singularities of highly weathered granite and corresponding residual soils masses for modelling of retaining walls of large excavations are discussed.     

Modelling of a tunnel excavation in a non-cohesive soil improved with cement mix injections

The paper presents the results of a numerical analysis of the excavation of a tunnel in a non-cohesive soil. In situ test data recorded before and during the works are presented first. The constitutive law adopted and the procedure followed for modelling the injection phase is discussed next. It is shown that, in order to match the movements recorded within the soil mass, a rather complex procedure, allowing for either uniform diffusion of the mix or formation of mix lenses (‘claquage’), should be adopted. Calculated and observed data during the various phases of the excavation are then finally compared.     

平行小净距盾构与CRD法黄土地铁隧道施工力学研究

西安轨道交通工程是目前首例在我国黄土地区修建的地铁隧道,一号线枣园北路站至汉城路站K12+792.744～K12+889.899区间隧道为同时满足双线正常行车和右线停车线扩大断面的功能需要,选取了左线小断面隧道为盾构法与右线大断面隧道为CRD法相结合的施工方案。针对该地铁隧道的施工过程,进行了三维动态数值模拟和施工力学分析,通过分析施工引起的地表变形、中间土体应力和围岩塑性区的特征和规律,从而研究得出CRD法与盾构法隧道先后施工相互影响的规律性成果：先行大断面隧道采取CRD法施工对后行小断面盾构隧道上方地表沉降的影响较后者对前者的影响大;后行隧道的贯通使得先行隧道开挖形成的地表变形轴线向后行隧道侧偏移了约0.5倍净距,并且地表变形的横向影响范围和地表沉降量均有增大,主要表现在靠后行隧道一侧;先行大断面隧道的开挖较后者对中间土体应力影响大,对相邻洞土体的影响在同掌子面处最为显著。结合西安地铁隧道工程实践开展的数值模拟分析研究,可为今后在黄土地区修建地铁隧道提供具有指导意义的研究成果和宝贵的工程实践经验。