| dc.description.abstract | Underground structures have played a major role in supporting infrastructure development such as water supply systems, storage facilities, transportation, and others. Furthermore, the construction of underground facilities has been rapidly increasing as free surface space is decreasing. Thus, the stability of these facilities is an important aspect to be maintained. Generally, while designing underground excavation, seismic loading is not considered because of the conservative notion stating underground structures will have a negligible effect due to seismic loading. But this notion has been proven invalid as the cases of underground structure failures during earthquakes like the 1995 Kobe earthquake, 1999 Chi-Chi earthquake, and others have been observed. Such failure cases have motivated this study to evaluate rock supports, which are integral in providing stability for underground openings.
In this thesis work, Melamchi Water Supply Project (MWSP) is taken as a case study. The MWSP project site lies in the lesser Himalaya region of Nepal, where the active movement of the Indian tectonic plate and Eurasian tectonic plate exists. Thus, making the project site highly vulnerable to earthquake activity. In such a case, the water tunnel of MWSP should be assessed for static loading conditions and dynamic (seismic) loading. Furthermore, young geology in lesser Himalaya makes it crucial to assess rock supports (specifically, Reinforce Ribs of Shotcrete and Lattice Girder) implemented in weak rock mass conditions in both static and dynamic conditions. Thus, this thesis work focuses on evaluating and comparing these two support systems.
To proceed with assessment in this study, three sections along MWSP tunnel alignment are considered, namely, sections with chainage 9200 meters, 14100 meters and 16750 meters. The selection of sections are carried out rationalizing the fact that rock mass quality should be low (that is Q-value between 0.1 to 0.01) where RRS are implemented and depth of overburden less than 300 meters (where the effect of earthquake loading can be observed, according to (Sharma and Judd, 1991)).
In this thesis, three rock supports are considered for study purposes. These are WRS-5a (Reinforced Ribs of Shotcrete (RRS) based rock support), WRS-5b (Lattice Girder based rock support) and RRS-II (obtained from Q-support chart NGI (2015)). First two supports are based on MWSP design and latter one is designed using standardized Q-support chart design. For evaluation purpose of support system in static condition, support pressure calculation are conducted using empirical, semi empirical, analytical and internal force calculation of support system using numerical modelling. While for dynamic modelling, only numerical modelling is conducted using RS2 software from Rocscience. Furthermore, dynamic modelling is conducted with three different analyses approach, namely, pseudo-static, simplified dynamic and full-dynamic analysis. For seismic loading inputs, peak ground acceleration (PGA) obtained from 2% probabilistic seismic hazard analysis conducted by Rahman and Bai (2018) is utilized. While for simplified and full-dynamic analysis, the 2015 Gorkha earthquake time series data is taken. Results obtained from numerical modelling of seismic loading on support systems are extracted in term of axial force, bending moment, hoop (ring) stress and deformation. Empirical, semi-empirical, analytical approach and comparison of internal forces using numerical modelling suggest RRS-II being a flexible support and Lattice girder being stiff one. Furthermore, comparison of WRS-5b support with WRS-5a support results obtained from aforementioned analysis approaches shows WRS-5b is slightly stiffer than WRS-5a with very less difference. This study also recommends use of full dynamic analysis approach, among three analysis approaches used, to better understand support response during seismic loading. Support systems assessment by full dynamic analysis approach conducted in this study suggests all three support sustains failure; hence recommending for increase in load capacity of ground itself via using rock reinforcement such as spilling bolts, grouting and other. | |