Engineering geological evaluation for the geotechnical analysis and design of tunnels, with special emphasis to tunnel behaviour appraisal.
Analysis and computational methods are fields in which significant progress has been made in almost all stages of geotechnical design in tunnels. Nevertheless, regardless of the capabilities offered by the numerical tools, the results can still involve uncertainties when parameters are used directly without considering the actual failure mechanism of the rock mass in tunnelling. The steps of the engineering geological assessment towards the geotechnical tunnel design are presented with special emphasis to the appreciation of the tunnel behaviour types as a key information for the design of excavation and temporary support measures. Several examples from tunnelling in a variety of geological conditions will be presented. Nowadays, knowledge and understanding of the role of the geological material and its implication in design is reinforced with advances in site investigation methods, the development of the geotechnical classification systems and the consequent quantification of the rockmasses. RMR (Bieniawski,1976) and Q (Barton et al.,1974) systems were developed in order to provide tunnel support estimates through a rating of rockmasses. The development of powerful microcomputers and of user-friendly software prompted a demand for data related to rockmass properties required as input for numerical analysis or close form solutions for designing tunnels. This necessity preceded the development of a different set of rockmass classifications and the Geological Strength Index (GSI), connected with the Hoek and Brown failure criterion (Hoek et al.,2002; Hoek and Brown, 2018), is one such classification. These classifications should not be used though as a “cookbook” (Hoek, 1999) and as a cure-all. The current work argues that use of the classification ratings must be accompanied by an understanding of the actual rockmass behaviour in tunnelling. The behaviour in tunnelling may differ from one rockmass to another, even if they have the same classification rating under similar insitu conditions. The assessment of ground behaviour in tunnelling is based on the identification of the rockmass type and the engineering geological characteristics - “keys” that dictates the stability or instability of the tunnel. Having defined the most critical failure mechanism, the temporary support philosophy and principles for every behaviour type are discussed in order to contain and control the specific failure mechanism upon excavation. The appropriate design parameters can then be selected. These proposals allow an early assessment of the principles for the choice of appropriate support measures and their basic dimensioning, as dictated by the ground behaviour and the associated mode of failure. Within this framework, a classification scheme for assessing the rockmass behaviour in tunnelling is presented. This study is based on a large set of data from the design and construction of 62 tunnels through a wide variety of geological conditions. The experience from the design and construction of 62 mountain tunnels of the Egnatia Highway in northern Greece, a major European infrastructure, many of which have been constructed under difficult geological conditions in weak rockmasses, is presented. The weak and complex rockmasses under analysis with multi heterogeneity and members of extremely low strength have most of the times undergone highly tectonized disturbance or weathering resulting in the destruction of their initial structure. The presented examples are rockmasses that can be developed in flysch, molassic sediments, limestones, and ophiolites. The characterization for the tunnel behaviour assessment in these examples is based on the geological model and on the appreciation of the rockmass characteristics controlling the potential for instability of the rockmass.
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Engineering geological evaluation for the geotechnical analysis and design of tunnels, with special emphasis to tunnel behaviour appraisal.
Abstract
Analysis and computational methods are fields in which significant progress has been made in almost all stages of geotechnical design in tunnels. Nevertheless, regardless of the capabilities offered by the numerical tools, the results can still involve uncertainties when parameters are used directly without considering the actual failure mechanism of the rock mass in tunnelling. The steps of the engineering geological assessment towards the geotechnical tunnel design are presented with special emphasis to the appreciation of the tunnel behaviour types as a key information for the design of excavation and temporary support measures. Several examples from tunnelling in a variety of geological conditions will be presented. Nowadays, knowledge and understanding of the role of the geological material and its implication in design is reinforced with advances in site investigation methods, the development of the geotechnical classification systems and the consequent quantification of the rockmasses. RMR (Bieniawski,1976) and Q (Barton et al.,1974) systems were developed in order to provide tunnel support estimates through a rating of rockmasses. The development of powerful microcomputers and of user-friendly software prompted a demand for data related to rockmass properties required as input for numerical analysis or close form solutions for designing tunnels. This necessity preceded the development of a different set of rockmass classifications and the Geological Strength Index (GSI), connected with the Hoek and Brown failure criterion (Hoek et al.,2002; Hoek and Brown, 2018), is one such classification. These classifications should not be used though as a “cookbook” (Hoek, 1999) and as a cure-all. The current work argues that use of the classification ratings must be accompanied by an understanding of the actual rockmass behaviour in tunnelling. The behaviour in tunnelling may differ from one rockmass to another, even if they have the same classification rating under similar insitu conditions. The assessment of ground behaviour in tunnelling is based on the identification of the rockmass type and the engineering geological characteristics - “keys” that dictates the stability or instability of the tunnel. Having defined the most critical failure mechanism, the temporary support philosophy and principles for every behaviour type are discussed in order to contain and control the specific failure mechanism upon excavation. The appropriate design parameters can then be selected. These proposals allow an early assessment of the principles for the choice of appropriate support measures and their basic dimensioning, as dictated by the ground behaviour and the associated mode of failure. Within this framework, a classification scheme for assessing the rockmass behaviour in tunnelling is presented. This study is based on a large set of data from the design and construction of 62 tunnels through a wide variety of geological conditions. The experience from the design and construction of 62 mountain tunnels of the Egnatia Highway in northern Greece, a major European infrastructure, many of which have been constructed under difficult geological conditions in weak rockmasses, is presented. The weak and complex rockmasses under analysis with multi heterogeneity and members of extremely low strength have most of the times undergone highly tectonized disturbance or weathering resulting in the destruction of their initial structure. The presented examples are rockmasses that can be developed in flysch, molassic sediments, limestones, and ophiolites. The characterization for the tunnel behaviour assessment in these examples is based on the geological model and on the appreciation of the rockmass characteristics controlling the potential for instability of the rockmass.
