Simulation of progressive fracturing processes around underground excavations under biaxial compression

Fractures that develop progressively around underground excavations can be simulated using a numerical code called RFPA (rock failure process analysis). This code incorporates the mesoscopic heterogeneity in Young’s modulus and rock strength characteristic of rock masses. In the numerical models of a rock mass, values of Young’s modulus and rock strength are realized according to a Weibull distribution in which the distribution parameters represent the level of heterogeneity of the medium. Another notable feature of this code is that no a priori assumptions need to be made about where and how fracture and failure will occur – cracking can occur spontaneously and can exhibit a variety of mechanisms when certain local stress conditions are met. These unique features have made RFPA capable of simulating the whole fracturing process of initiation, propagation and coalescence of fractures around excavations under a variety of loading conditions. RFPA is used herein to simulate progressive fracturing processes around three common shapes of underground excavations – circular, elliptical and inverted U-shaped. The results of the simulations show that the code can be used not only to produce fracturing patterns similar to those reported in previous studies, but also to predict fracturing patterns under a variety of loading conditions. Based on these fracturing patterns, failure mechanisms are identified for various loading conditions.     

地铁车站地下连续墙接缝施工质量控制

本文分析了地铁连续墙分幅施工的接缝质量问题及成因,并提出了质量控制措施。     

Infinite elements for numerical analysis of underground excavations

Infinite elements were developed to overcome an inherent limitation of the Finite Element Method. The technology of infinite elements has been refined to such an extent that the modeling of an unbounded medium is no longer a limitation of the Finite Element Method. This paper shows that the remaining problem associated with a non-uniform and non-zero far-field decay of any one of the problem variables can also be resolved without having to create nodes at infinity. A combined finite-infinite element analysis now matches the power of the Boundary Element Method in handling the unbounded analysis domains while retaining the versatility of the Finite Element Method.     

异形幅地下连续墙施工质量控制

本文分析了地铁连续墙施工中的关键部位异形幅的主要质量问题及成因,提出了有效的质量控制措施。     

Numerical simulation of a jointed rock block mechanical behavior adjacent to an underground excavation and comparison with physical model test results

Mechanical behavior of a jointed rock mass with non-persistent joints located adjacent to a free surface on the wall of an excavation was simulated under without and with support stress on the free surface using approximately 0.5 m cubical synthetic jointed rock blocks having 9 non-persistent joints of length 0.5 m, width 0.1 m and a certain orientation arranged in an en echelon and a symmetrical pattern using PFC^3D software package. The joint orientation was changed from one block to another to study the effect of joint orientation on strength, deformability and failure modes of the jointed blocks. First the micro-mechanical parameters of the PFC^3D model were calibrated using the macro mechanical properties of the synthetic intact standard cylindrical specimens and macro mechanical properties of a limited number of physical experiments performed on synthetic jointed rock blocks of approximately 0.5 m cubes. Under no support stress, the synthetic jointed rock blocks exhibited the same three failure modes: (a) intact rock failure, (b) step-path failure and (c) planar failure under both physical experiments and numerical simulations for different orientations. The jointed blocks which failed under intact rock failure mode and planar or step-path failure mode produced high and low jointed block strengths, respectively. Three phases of convergence of free surface were discovered. The joint orientation and support stress played important roles on convergence magnitude. The average increment of jointed block strength turned out to be about 10, 7.9 and 6.6 times the support stress when support stresses of 0.06 MPa, 0.20 MPa and 0.40 MPa were applied, respectively. The modeling results offer some guideline in support design for underground excavations.     

Stress, instability and design of underground excavations

When the stress-induced risks to a projects warrant it, in situ stress must be measured. However, as the stress-induced risks increase, i.e., the stress magnitudes approach the rock mass strength, the confidence in commonly used stress measurement techniques decrease. The design of underground openings at depth requires knowledge of the in situ stress state, yet it is for these design conditions where our confidence in stress measurement techniques is at its lowest. To quantify the stress state for these conditions, elements of the Observational Design Method have to be used. These elements rely on the development of a geological site model, documented observations of over stressed rock in pillars or near the boundary of underground openings, and iterative two- and three-dimensional numerical modelling calibrated with observations. Examples are provided to illustrate how the philosophy of Observational Design Method can be used to infer the in situ stress state.     

Influence of wavelength-to-excavation span ratio on ground motion around deep underground excavations

Accurate estimation of ground motion around excavations is important for dynamic rock support design in deep civil tunnels and underground mines. Among the influencing factors, the wavelength-to-excavation span ratio (λ/D) has a large effect on ground motion. Using an advanced wave propagation simulation tool, we performed two series numerical experiments to study the effect of the λ/D ratio on ground motion near excavation boundaries. The modeling results reveal that the wave field becomes more complex as the λ/D ratio decreases. The absolute PPV (Peak Particle Velocity) values around an excavation are closely related to the intensity of the seismic source but the relative PPV value depends on the λ/D ratio. Amplification factors, defined as the PPV in the excavation model to the PPV in the background model without any excavation, are calculated for each case. The amplification factor around the excavation increases significantly as the λ/D ratio decreases. When the λ/D ratio is greater than 30, the wave amplitudes are less affected by the excavation and a seismic wave loading can be considered as “quasi-static.” When the λ/D ratio is less than 20, significant wave interaction occurs and the wave loading needs to be considered as “dynamic.” The numerical results provide additional insights into the ground motion behavior around excavations under both “quasi-static” and “dynamic” loading conditions.     

Blast-induced rock damage control in Fangchenggang nuclear power station,China

In the process of blasting excavation, stress wave propagation and gas expansion can basically induce damage to surrounding rocks, which is detrimental to rock mass integrity and engineering safety. In this case, evaluation and control of blast-induced effects are essential to the safety of nearby buildings and integrity of bedrock in blasting field. In Fangchenggang nuclear power station of China, the drill-and-blast method was employed for bedrock excavation. In order to reduce the blast-induced damage zone, the wave propagation and associated damage to rock mass should be carefully investigated. In this paper, the wave propagation regressively obtained from field monitoring data was presented based on empirical formula (e.g. Sadovsky formula). The relationship between the peak particle velocity (PPV) at a distance of 30 m away from the charge hole and charge per delay in blast design was derived. Meanwhile, the acoustic tests before and after blasting were conducted to determine the damage depth of rock mass. The charge per delay in blast design was then calibrated based on the blast-induced wave propagation regularity. The results showed that a satisfactory effect was achieved on blast-induced damage control of rock mass. This could be helpful to rock damage control in similar blasting projects.     

Tensile strength and abutment relaxation as failure control mechanisms in underground excavations

Classical assessment of instability potential in underground excavations are normally based on yield and rupture criteria for stress driven failure and on limit equilibrium analysis of structurally controlled failure. While it is true that ultimate failure and falls of ground can be an eventual consequence of stress fracturing and unfavourable structure within the rock mass, the timing of such failure is often controlled by the presence of residual tensile capacity, in the form of rock bridges separating joint segments and fractures and by the mechanisms of clamping and relaxation. Using crack and rock-bridge analogues in conjunction with an updated voussoir beam model, this paper explores the influence of residual tensile strength and boundary parallel relaxation on the failure process. The impact on support design is also examined. In underground hard rock mines with complex geometries and interacting openings, relaxation is identified as a key controlling factor in groundfall occurrence. Empirical stability assessment techniques for underground tunnels and for mining stopes are updated to account for relaxation.     

桩锚支护体系在深基坑支护中的应用

针对秦皇岛嘉远SOHO大厦深基坑工程的工程概况和地质条件进行了阐述,对桩锚支护方法在该工程中的应用进行了介绍,分析了基坑支护技术方案的技术要点,对同类工程的设计和施工具有借鉴意义。     

Probabilistic estimate of rock mass static and dynamic demands for underground excavation stabilisation

Excavation damage under high in situ stress depends largely upon the potential block size associated with any violent ejection. The size and shape of the dynamic instability are largely controlled by the location, orientation and extent of the pre-existing geological discontinuities. A new methodology is presented in which the rock mass demand can be expressed in terms of the mass in tonnes of unstable rock that is ejected per unit area of the excavation surface where failure occurs. A probabilistic approach has been implemented to estimate the potential rock mass instabilities and their associated static and dynamic demands. The new methodology considers that the strain energy released by the rock mass during violent stress-driven failure is largely converted into kinetic energy of ejection for blocks. The estimated dynamic demand has been favourably compared with observations of rock mass damage in a number of underground excavations.     

城市地下隧道爆破震动的特性及控制措施

城市地下隧道往往修建于人口稠密区,且周边建筑物、地下管线复杂,这给爆破施工增加了很大困难。通过工程实践,本文介绍了在厦门机场路隧道工程施工中爆破震动的特点及采取的控制措施,提出了在隧道正上方的"极震效应",可作为今后同类工程借鉴。     

Damage mechanics and its application for the design of an underground theater

This paper presents a damage mechanics method for the evaluation of the elastic damage of jointed rock mass and its application in predicting damage distribution in the jointed rock mass in the excavation for an underground structure in the framework of continuum damage mechanics. The damage variable is a second-order damage tensor which is assumed to describe the damage effect of jointed rock mass. The elasticity tensor defined in the model is verified by employing laboratory cement mortar specimens with various cases of joint distribution. The same specimen is analyzed using the developed damage mechanics method as well as the normal FE method, which employs joint elements to simulate jointed cracks. In order to apply the developed damage elasticity theory to three-dimensional damage analysis in underground structure excavation, a three-dimensional grouted rockbolt element is introduced, which considers the resistance of the grout to the relative shear displacement between the bolt and the rock mass around it in the axial direction. Plastic behaviors of the steel bar and the grout of the grouted bolt are considered in the model by using various constitutive laws. Finally, an engineering example for damage analysis is presented, namely an underground theater assumed to be excavated in a geological environment of jointed rock.     

基坑工程与地下工程安全及环境影响控制

随着城市中基坑工程与地下工程的建设规模和建设难度的增大及环境条件的日益复杂化,对基坑工程与地下工程的安全以及其施工过程对环境的影 响控制提出了更高的要求。回顾了近年来国内外基坑工程与地下工程技术的发展;对软弱土、高水位、富水粉、砂土层中的基坑工程和地下工程施工造成的 环境影响及引发的灾害进行了分类;在此基础上对我国基坑与地下工程施工变形控制及工程安全控制策略进行了系统总结。     

软土地区基坑工程变形控制方法及工程应用

基坑变形控制是软土地区基坑工程的核心内容,不仅与自身工程安全密切相关,更涉及到对周边环境的影响.随着城市地上、地下各类建(构)筑物越来越密集,基坑工程施工产生的变形对环境影响的控制愈加成为基坑工程的焦点问题.首先,从基坑施工全过程控制的视角,分析了基坑施工全过程各阶段的变形特征、机理以及对环境的影响.进而,将基坑变形及...     

厦门市地下交通系统的发展策略

对厦门市地下交通空间开发现状的分析及地下空间资源的评估,借鉴国内外城市地下交通空间的开发经验,结合厦门城市发展的需要,提出厦门市地下交通系统发展的方向及网络体系。     

Present state,problems and development trends of urban underground space in China

The status, problems and trends of China’s underground space development are presented in this paper. In association with the features of use of China’s underground space, such as the large scale, rapid development, high speed subway, various forms are summarized. Five problems in the use of underground space in China are put forward, including the uncoordinated solutions, unintegrated management system, incomplete regulations, incomplete planning, and unclear policy. Four use trends of underground space are also presented, together with (1) the integrated planning, construction and management relating to the development of underground space in central business district (CBD) of Beijing, after World Expo in Shanghai and the business district in south Ningbo; (2) introducing the urban underground complex with an integrated transport hub as its center relating to the cases of Hongqiao Transport Hub and Beijing South Railway Station; (3) the development of underground subway, road and logistic system; (4) the scientific utilization of underground water storage space and aeration of “sponge city”. In this study, some institutional perspective on the use of underground space is also given.     

Numerical simulation of tensile damage and blast crater in brittle rock due to underground explosion

The prediction of blast crater in brittle rock due to an underground explosion has gained importance in recent years due to the great number of accidental events that affected engineering safety. This paper uses the Taylor–Chen–Kuszmaul (TCK) continuum damage model to analyze dynamic fracture behavior of rock in tension due to blast loading. The TCK damage model, together with an erosion algorithm, was implemented into the explicit FE code, LS-DYNA, as a constitutive augmentation. The damage pattern around the blasthole and the formation of blast crater near a free surface were subsequently simulated using the developed numerical tool. It is shown that the free surface is vitally responsible for the blast crater. Furthermore, the size and shape of the blast craters can be reasonably predicted if the erosion criterion of critical tensile damage is well calibrated. The effects of common charge modes on blast craters were also investigated numerically, and the mechanisms of the coupled, air-decoupled and water-decoupled charge mode are compared and presented.     

Utilization of underground space in Japan

This paper discusses the utilization of underground space in Japan. Firstly the classification of underground space is indicated and then the utilization of underground space under private lands and public lands is explained separately. Finally the characteristics of Japanese underground shopping malls and the future use of underground space is analyzed. Many people must contribute diverse ideas including measures for planning and using the space under public areas and private properties in an integrated manner, streamlining the relationships between occupying properties and systems for rationally managing aboveground and underground spaces.