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This research focuses on evaluating the rock masses geotechnical properties and recommending the optimal support design for an adit tunnel being constructed in the North Western Himalaya, India. Predicting rock mass behaviour with precision is dependent on accurate characterisation of in-situ rock mass properties. Tunnel performance in discontinuous rock masses is determined by geometric properties of discontinuities, resistance properties of the intact rock, and the influence of water. Difficulties emerge because the factors that describe these features are distributed rather than distinct. During the last 50 years, there has been significant progress in the stochastic description of discontinuous rock masses and the statistical distribution of their geometric properties. The tunnelling quality index (Q) and rock mass rating classifications were used to characterize the rock mass and based on rock mass categories; methodology of excavation was finalised. New Austrian Tunnelling method was utilised for the recommendation of tunnel support and primary support measures executed included forepoling, shotcrete, rock bolts, lattice girder, and steel ribs. The 2D finite element method was used to estimate the effectiveness of support design, plastic zone size, and deformations. The input geotechnical parameters which were required for modelling were derived/estimated. On the crown and walls, maximum displacement observed was 13.0 mm and 18.0 mm respectively. Using the MN curve, the capacity of a reinforced shotcrete section to resist bending and cracking is determined. Modelling indicated that support recommended was sufficient for adit tunnel stability point of view and deformation was within the permissible limit. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive geotechnical and geological investigation of the rock masses along 1024-m long adit-6 of the rail line tunnel between Rishikesh and Karnprayag. The adit lies in the tectonically active Garhwal Himalaya, over North Almora Thrust (NAT), which has experienced two disastrous earthquakes in the last two decades. NAT in the study area, characterized by five principles and numerous smaller faults, presents an unprecedented risk to the stability of the adit. Geotechnical conditions have been assessed using rock mass rating (RMR) and geomechanic classification (Q) system for assessment of rock mass qualities along the adit alignment, and the result was correlated using NATM for the performance of support design and excavation method based on empirical rock mass classification, stability was measured, and corresponding tunnelling support system is proposed and verified by numerical modelling. Results reveal that the adit is highly unstable due to the existence of faults and fold, and therefore strong support is recommended for poor and very poor rock class, especially on the portal. [ABSTRACT FROM AUTHOR]