Ambika Srivastav received her doctorate in Civil Engineering (CE). Her research work was supervised by Prof. K S Rajan. Here’s a summary of her research work on Seismic response of rock tunnels in mixed Himalayan geology:
Tunnels are the most important components of infrastructure, as they play an important role in public transport, water transport, and hydroelectric power generation. The tunnel industry has long believed that tunnels are safe from any seismic activity. Recent rigorous damage to tunnels has proven the, unlike scenario. Several case histories raised the concern for the safety of underground structures. Examples include Chi-Chi Earthquake in Taiwan (1999), Wenchuan Earthquake in Sichuan (2008), Nepal Earthquake (2011, 2015) and Sikkim earthquake (2011). At this point, it will be prudent to understand the seismic performance of rock support systems while designing tunnels. The continuous variability of geological settings influences the integrity of the tunnels which can be accomplished using innovative methods of rock reinforcement. The need for more power plants has led to significant amount of hydroelectric tunnelling work in the Himalayan region. The challenges for tunnelling in Himalayan region are enormous owing to the heterogeneous geology and the high seismic activity. The analysis and understanding of the prevalent rock mass in such areas are a challenge and necessary for the successful execution of tunnelling project. A tunnel construction typically consists of the ground support system and the tunnel lining construction. The effect of earthquake activity is measured only after there is a static design available. One of the most effective methods is by providing flexible rock support system for tunnels to withstand earthquakes. Hence it is very important to develop an integrated design tool to facilitate the cost effective and aseismic construction of tunnels in mixed Himalayan Geology.
The main aim of this thesis is to provide a simplistic design approach as an effective mechanism for engineers to design tunnel support. This study explores the effects of
earthquakes on the tunnel lining. The analyses are carried out to understand the effect of rockmass quality ‘Q’, size of tunnels, effect of discontinuities on the seismic performance of tunnel lining. In this thesis an attempt is made to improve the current design approach for the support design of tunnel using Q system and expected PGA values at the tunnel site. For this, case study of three hydropower tunnels from India has been selected: 1) (4111 MW) Vishnugarh Pipalkoti hydropower project (Uttarakhand), 2) 450 MW Shongtong Karcham hydroelectric project (Himachal Pradesh), and 3) 37.5 MW Parnai hydroelectric project (Jammu and Kashmir). Seismic parameters have been evaluated by carrying out probabilistic seismic hazard assessment (PSHA) for these specific sites.
To understand the seismic impact on tunnels’ support various methods are used which are static method, pseudo-static method, and dynamic method (Zou et al. 2017). The pseudo-static method is used in this study to understand the tunnel behavior in static and seismic cases. The increase in axial force in the lining is studied as a function of rock mass quality Q and tunnel dimension. Three sedimentary rock mass classes with different Q values representing from “very poor” to “very good” rock masses are modelled with different geo-mechanical parameters determined through empirical relations. The rock mass surrounding the tunnel was modelled using two different approaches i.e., continuum and continuum with interfaces. The results determined using both these approaches are analysed and compared for the static and seismic scenario. The study has been conducted using Phase2 version 8.0 software which has been used frequently to understand various case scenarios related to seismic response of tunnel. In this study, a tunnel model with specification (diameter-6m, 12m, 18m, and 24m) at a depth of 100 m surrounded by rock masses of Q (rock mass quality) value ranging from 1 to 30 was
used to examine the effect of Q and the joint orientation on the seismic response of tunnels. The results from both models were analysed and compared to understand the variation from using continuum and continuum with discontinuity post-failure characterization. Such an analysis will help the researchers and field engineers to also consider the case of a continuum with the interface while considering the seismic response of the tunnel. Sections of tunnels situated in a fragmented geological setting with high overburden are more vulnerable to seismic disruption, so a parametric study was conducted to assess the seismic activity of tunnels with discontinuities. In the case of jointed rockmass, numerous pseudo-static finite element analyses is conducted to determine the influence of joint orientation, spacing, friction, stiffness on the dynamic forces developed in the liner. In the tunnel response, the function of joint stiffness and shear strength was found to be crucial. Stability of the tunnel was improved significantly by the reduction of the joint dip angles which is an accepted field observation. Neither the seismicity nor the joint orientation had a noticeable impact on tunnels for competent rocks with joints. On the other hand, the weak rock mass deforms and is affected, regardless of the position of the joints. The results were found to be compatible with the recommendations for the Norwegian Rock Index System (Q) and Bhasin et al. (2006) earlier tests.
It is necessary to modify the current Q-system tunnel design chart to use Qseismic instead of Qstatic to achieve the new support system. Barton (1984) advised halving the Q value. However, here it is suggested that Qseismic = Qstatic, where ‘’ is a coefficient mostly reliant on the PGA and the rock mass quality Q, respectively. It is hypothesized that ‘’ depends on other qualities of the rock mass, the size of the tunnel, and the parameters of the shotcrete liner. There is a strong correlation
between tunnel deterioration and the Peak ground Acceleration (PGA). Increases in PGA put more strain on the tunnel’s liner. There is a correlation developed between ‘’ and PGA for the three case study locations with varying seismic stress. For the optimistic design of the tunnel support system, the proposed research observation is also used to design a key tool for evaluating the support system for Himalayan tunnel projects.
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