INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Geotechnical engineering considerations for the analytical design of an adequate tunnel support system Joan Ongodia, Denis Kalumba, Civil Engineering, University of Cape Town, Graduate student & Senior Lecturer, South Africa, [email protected] Harrison Mutikanga, Brajesh Ojha Civil Engineer, Uganda Electricity Generation Company Limited, Uganda & Civil Engineer, Navayuga Engineering Company, India ABSTRACT: Tunnel excavation impacts the ground both at the surface and underneath thereby affecting the natural in-situ stability. Failure results from stress redistribution, stress relaxation and deformations around the tunnel. Ultimately, tunnel failure occurs when the loads exceed the value of the bearing resistance. Therefore, a support system comprising unique structural components is needed to resist the weight of unstable rock wedges thereby ensuring that the tunnel does not fail. Nevertheless, stability depends on an effective rock- support interaction, specifically with the rock bolt which is the main support member. In order to assess and improve stability conventional methods to design are borrowed from geology which assesses material properties by visual observation, physical identification, using rudimentary geological handy tools, charts and graphs to estimate tunnel support ranges. For stability, the specific value of the required capacity to prevent failure should be established. This paper presents the background to the engineering design of an adequate tunnel support based on the peak rock load which must be resisted to ensure the tunnel remains stable. RÉSUMÉ: L’excavation de tunnels impact le sol au niveau de la surface et en profondeur, en affectant ainsi la stabilité in situ naturelle. Des défaillances surviennent du fait de la redistribution des contraintes, le relâchement des contraintes, et des déformations autour du tunnel. En fin de compte, la défaillance du tunnel se produit lorsque les charges dépassent la valeur de résistance de soutènement. Donc, un système de support comportant des composantes structurales uniques est nécessaire pour résister au poids des rocs instables pour ainsi s’assurer contre l’écroulement du tunnel. Néanmoins, la stabilité dépend de l’interaction adéquate entre la roche et le support, particulièrement avec les boulons d’ancrage dans le roc qui est l’élément de support principal. Afin d’évaluer et d’améliorer la stabilité, les méthodes de concept conventionnelles sont empruntées à la géologie qui évalue les propriétés des matériaux par l’observation visuelle, l’identification d’aspect physique, en utilisant des outils géologiques rudimentaires, et des tableaux et graphiques pour estimer les portées souterraines. Concernant la stabilité, une valeur spécifique de la capacité requise doit être établie pour éviter la défaillance. Cet article présent la connaissance technique en concept d’ingénierie pour un support adéquat de tunnel basé sur les charges maximales des rocs instables à soutenir, et ainsi d’assurer l’intégrité du tunnel. KEYWORDS: Tunnels, Geotechnical engineering design, Rock mechanics theory, Rock load, Support 1 INTRODUCTION Presently, the accuracy of a tunnel design mainly depends on experience and geological methods used (Mohammed 2015). The methods employed by tunnel practitioners including geotechnical engineers often involve visual observation, physical identification, rudimentary hand-held tools and estimation of corresponding support parameter ranges from charts (Hoek et al. 1995). Geological materials identified are described, assessed and categorised, in order of reducing strength, as Class I, II, III, IV and V based on the Rock Mass Figure 1. Continuum scale (adapted from Kanji 2014) Rating (RMR), Q and Rock Quality Designation (RQD) (Palmström 1995, Brown et al. 1983). RMR, Q and RQD systems give an indication of the level of deformation and (1) ability of the rock to support loads. Alternatively, rock can be classified on a continuum scale as shown in Figure 1 although where σ'1 and σ'3 are major and minor effective principal the methods are generally subjective thereby giving ranges of stresses at failure, σci is the UCS for the intact rock pieces, mb is the support parameters (Kanji 2014, Ongodia 2017). the Hoek-Brown rock mass constant, s and a are rock mass Guiding standards include the tunnel lining design guide characteristic constants. (TLDG 2004), the New Australian Tunnelling Method (NATM), the Chinese HC standard for tunneling, the Indian tunneling standard and the adapted Czech NATM (Spackova 2012, Sousa 2 KEY CONSIDERATIONS FOR TUNNEL SUPPORT 2010, PRC 2008). Further, underground stability problems are The main geotechnical engineering considerations for analysed using mostly the non-linear Hoek-Brown failure estimation of rock loads and the corresponding tunnel supports criterion (see Eq. 1) which approximates jointed rock comprise 1) geology, 2) material properties, 3) geotechnical characteristics better than the linear Mohr-Coulomb failure parameters, and 4) the construction process (Palmström 1995). criterion because it approximates isotropic behaviour for intact Stability of the surrounding rock mass depends on geological rock (Greer 2012). conditions, residual strength, in-situ stresses, burial depth, tunnel dimensions, excavation disturbance factor and material properties which influence stress magnitudes and deformation - 1737 - Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017 such as the Poisson’s ratio, Young’s modulus, shear modulus groundwater interactions (Sharp 2007). Discontinuities provide and stiffness (Greer 2012). Figure 2 illustrates the network of void spaces through which water permeates the rock mass. significant factors which control stability. Porewater pressures widen the discontinuities in the structure thereby aiding physical disintegration and eventually failure. Therefore, drainage provision is an important aspect of the support system (Bondarchuk et al. 2012). The rate of hydrogeological flows and extent of widening of the discontinuities is controlled by the minerals present both in the rock mass structure and void space infilling (Wahlstrom 1973). Clay minerals have a high affinity for water and cause swelling and squeezing of the mineral structure when wet and dry, respectively (Kanji 2014). Infillings have strong anisotropic mechanical properties thus they significantly reduce rock strength (Palmström 1995). Typical unstable wedges and potential tunnel failure Figure 2. Web of ground factors influencing stability (Jones 1989) resulting from peak rock loads in the roof, sidewalls and floor are illustrated in Figure 4. Rockbursts or popping conditions 2 .1 Geology usually occur in the roof and sidewalls resulting in collapse or The ground type and its integral structure are important to for slide, respectively, whereas unstable floor wedges usually heave stability as they directly provide anchorage for the support. Key (Viggiani 2012). Furthermore, squeezing is a severe tunnel features include the rock mass structure, type, degree of construction challenge besides mudflows and face collapses weathering, interlocking, types and orientation of discontinuity (Sousa 2010, TLDG 2004, Hoek et al. 1995). sets, persistence, roughness and width of the discontinuity Tunneling strain and squeezing behaviour can be predicted (Jones 1989). Intact rock is strong although as weathering, using charts (see Figure 5 below) in which strains above 2.7% which is a significant natural earth process, takes effect the rock indicate mandatory support (Hoek et al. 1995). An equivalent mass begins to disintegrate thereby forming discontinuities such value of the laboratory measured oedometer swell pressure is as joints and fault lines (Brown et al. 1983). Discontinuities aid applied onto the rock in the field to ensure stability the agents of weathering including erosion such that the rock (Mohammed 2015). mass eventually degenerates into soil over geological periods. The five rock classes correspond with the varying degrees of Roof wedge collapses weathering (Palmström 1995). Weak discontinuous rock is generally unstable and requires mandatory support (Bieniawski 1992). Sidewall wedge slides or displaces Geological prediction gives insight on the material condition, state, properties and responses of the ground to the construction process such that advance support is provided where necessary and the excavation sequence adjusted timely to avoid excessive Floor wedge heaves damage of the overall rock mass structure (Hoek et al. 1995). Figure 3 illustrates how probes are used to predict geology ahead of tunnel advance such that sudden failure during Figure 4. Typical unstable wedges and failure (Ongodia 2017) excavation is avoided. However, actual material characteristics and engineering parameters of the excavation should be evaluated to determine the resistance forces to be counteracted (Mohammed 2015). Figure 5. Strain and squeezing (TLDG 2004 after Hoek et al. 1995) 2.3 Geotechnical parameters Mechanical rock