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Chapter 1
Introduction
The stability of underground and surface geotechnical structures during and after excavation is of great concern to designers, as any kind of instability may result in damage to the environment, as well as high repair costs and time consumption (Figs. 1.1–1.4). The rock in nature is not always continuous and may have numerous discontinuities that vary in scale. As a result, the safety evaluation of a structure under consideration is a highly complex problem and requires very careful investigation. Accordingly, it is always necessary to examine the most likely forms of instability in relation to the physical nature of the rock mass and the geometry of the structure and its site, as well as the pre-existing state of stress. The forms of instability and their mechanism and the factors and conditions associated with them must be clearly understood to correctly stabilize the structure.
Figure 1.1
Various underground structures in rock.
Figure 1.2 Tunnels in rock.
Figure 1.3
Foundations on rock.
In addition to the stability problems, the environmental requirements and functional duties of structures may need to be carefully evaluated. All these factors together with those related to the stabilization procedure will result in setting the conditions for the selection of support members that satisfy mechanical as well as environmental and functional requirements.
The design of support members and the evaluation of the stability of structures are not possible unless one understands what rock mass really is. Most of the available approaches are either mechanically orientated without proper consideration of rock mass or geologically orientated without paying proper attention to the mechanics. In this respect, the present volume attempts to bridge the two approaches and bring a unified approach for the design of support and reinforcement systems for rock engineering structures, from not only the mechanical engineering but also the geological engineering point of view.
Rockbolts of various types (i.e. mechanically anchored, grouted, etc.) have recently become one of the principal support members in the civil and mining engineering fields. This probably results from the ease of their transportation, storage, and installation and their rapidly developing reinforcement effects as compared with other support members, such as steel sets and concrete liners. Their superior reinforcement effects in securing the stability of geotechnical engineering structures excavated in various types of ground and states of stress are very well known qualitatively in engineering practice. However, the first fundamental study for quantifying the reinforcement effects of rockbolts has been carried out by Aydan (1989) in his doctorate study. Subsequent studies by Pellet (1994); Moosavi (1997); Marence and Swoboda (1995) and Ebisu et al. (1994a, 1994b) have made further contributions on the behavior of rockbolts under different conditions. The studies on rockbolts, cable rockbolts, and rockanchors are now orientated towards their response under dynamic conditions (e.g. Aydan et al., 2012; Owada et al., 2004; Owada and Aydan, 2005; Li, 2010).
In the last decade, the use of shotcrete has rapidly increased, particularly in tunnel construction, and shotcrete has become an important element of modern tunnel-support techniques. The development of the early age strength of shotcrete is a decisive factor, because the excavation cycle and attainable excavation speeds are significantly influenced by it. The first fundamental study on the characteristics of shotcrete and its representation in numerical simulations was undertaken by Sezaki (1990) and his colleagues (Sezaki et al., 1989, 1992; Aydan et al., 1992).
Steel ribs or steel sets have long been used in many rock excavations. Their design concept is based as a moment-resisting structure under uniform or concentrated loads, and their load-bearing capacity is evaluated by assuming moment resistance capacity or buckling failure.
Despite decades of use of concrete liners in rock excavations, the supporting effects of concrete liners is not well understood. This is due to a poor understanding of how they interact with the surrounding rock mass, together with the incorporation of other support and reinforcement members and in relation to the installation stage in the overall construction scheme. The concrete liners are auxiliary support members rather than main load-bearing structures. Therefore, there is a strong debate whether they are necessary support members. In this book, various aspects of concrete liners are also presented and discussed.
The present book has been undertaken to highlight the reinforcement functions of rock-bolts/rockanchors and support systems consisting of shotcrete, steel ribs, and concrete liners under various conditions and to evaluate their reinforcement and supporting effects, both qualitatively and quantitatively.
The book consists of 12 chapters. The contents of 10 chapters out of 12 are described briefly as follows:
Chapter 2 is devoted to the mechanism and influencing factors of failure phenomena in rock engineering structures. The rock and types of discontinuities encountered in natural rock are briefly described, and their combined effects on the mechanical response of rock mass as a structure are discussed together with the implications on real rock structures. Then, classifications on the forms of instability in underground openings, slopes, and foundations, under both compressive and tensile stress fields, are described in relation with the structure of rock mass.
Chapter 3 is concerned with the present design philosophy of support and reinforcement for rock engineering structures. A brief description of available design approaches, such as empirical, analytical, and numerical methods, are given and discussed. The approaches, which are used independently of each other, are presented in a unified manner. The presently available support members and their functions are briefly described and discussed, with an emphasis on rockbolts and rockbolting.
Chapter 4 describes experimental studies undertaken on the mechanical behavior of the rockbolt system. First, the behavior of the bolt material used in practice is given, then the experimental study undertaken for the anchorage performance of rockbolts in push-out and pull-out tests and subsequent shear tests on the mechanical behavior of interfaces within the system and grouting material are described. In this chapter, the constitutive laws for the rockbolt system are described. A constitutive law for the bar is derived based on the classical incremental elasto-plasticity theory, as bar materials such as steel exhibit a nondilatant plastic behavior. On the other hand, the constitutive law for the grout annulus and interfaces is derived based on the multi-response theory proposed by Ichikawa (Ichikawa, 1985; Ichikawa et al., 1988), as the grout annulus and interfaces exhibit a dilatant plastic behavior. Then, procedures to determine the parameters for the constitutive laws from the experimental data are described and several examples are given. Evaluation of the contribution of rockbolts/rockanchors for improving the properties of rock mass is described and the shear reinforcement effect of rockbolts on rock discontinuities is presented in view of some theoretical and experimental findings. A detailed presentation of estimation of pull-out capacity of rockbolts/rockanchors under various conditions are described. Furthermore, the evaluation of reinforcement effect of mesh bolting on rock masses subjected to tensile stresses are presented.
Chapter 5 describes the characteristics of various support elements, such as shotcrete, concrete liner, and steel ribs/sets. The constitutive laws of each support member and various experimental studies on their characteristics are presented. Furthermore, the concepts for their mechanical modeling are also explained.
Chapter 6 describes the models representing reinforcement and support systems in numerical analyses, particularly in finite element studies. Details of rockbolt elements, shotcrete, and beam elements are presented.
Chapter 7 is concerned with the analytical and numerical methods for evaluating support and reinforcement systems and their effects in underground excavations. Analytical methods for evaluating the ground-response-support reaction, which incorporates various support members, rockbolts, and rockanchors, and the face effect are presented, and several examples of applications are given. Furthermore, a theoretical formulation of the effect that mesh bolting has for compressed air energy storage schemes is given, and several examples of excavations are presented. A series of finite element simulations are presented to show the effects of various conditions for the effective utilization of reinforcement and support systems for underground structures. The effect of rockbolting with other support members is investigated in relation to some practical situations. Several examples are analyzed on the response of rockbolts in discontinuum, and their implications for interpreting field measurements of rockbolt performances are discussed. Furthermore, the presently available proposals on the suspension effect, the beam building effect, and the arch formation effect of rockbolts are re-examined and more generalized solutions are presented. In addition to covering the reinforcement effect of rockbolts against the sliding type of failure, solutions for the reinforcement effect of bolts against the flexural and columnar type of toppling failure are given.
Chapter 8 describes the effect of support and reinforcement systems for the stabilization of rock slopes. Procedures for stabilizing the rock slopes against some typical failure modes are presented, along with several examples of applications. Furthermore, the chapter presents applications of the discrete finite element method, incorporating the effect of rockbolts to rock slope stability problems. In addition, model experiments on the effect of rockbolting against planar sliding and block-toppling modes are given and compared with estimations from the limit equilibrium technique.
Chapter 9 is concerned with the stabilization of the foundations of bridges, pylons, and dams subjected to tension or compressive forces. Examples of applications include the potential use of rockanchors as foundations of pylons and of tunnel-type anchorage for suspension bridges. The use of rockanchors for the stabilization of bridge and dam foundations under compression is also presented and discussed.
Chapter 10 deals with dynamic issues such as rockburst, earthquakes, and blasting, which cause dynamic loads on rock support and rock reinforcement. Theoretical, numerical, and experimental studies on rockbolts and rockanchors under shaking are presented, along with several examples of applications.
Chapter 11 describes the mechanisms and techniques for evaluating corrosion in steel and iron materials in relation to the long-term performance and degradation of reinforcement and support systems and provides site examples. Furthermore, some procedures are presented for non-destructive evaluation of support and reinforcement systems.
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Chapter 2
Mechanism of failure in rock engineering structures and its influencing factors
This chapter deals with natural rock, the types of discontinuities encountered in it, rock mass, and the mechanism of the modes of instability in underground and surface structures and associated factors and conditions.
The first part of this chapter is devoted to the geological description of rocks and of the formation and types of discontinuities in rocks and rock mass. Then, the mechanical behavior of rock mass is discussed, considering the behaviors of intact rock, discontinuities, and the structure of the rock mass.
In the secon...
