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The geological condition of the Himalayan region is very complex and challenging. So far, empirical and analytical approaches for rock mass characterization have been a common practice in the Himalayas. Due to the limitations of input parameters and governing equations in design practices, rock mass characterization in tunnel boring machine (TBM) excavated tunnels is crucial. This research introduces robust machine learning (ML) approaches to predict rock mass quality conditions in complex geological environments, leveraging a large database of TBM parameters and rock mass rating (RMR) values. To do so, a total of 6879 stable phase TBM cycle data were collected from 12 km long tunnel in Nepal. The pre-processed parameters were randomly split into a training set (80%) and a testing set (20%). Seven individual classifiers consisting of logistic regression (LR), support vector machine (SVM), decision tree (DT), random forest (RF), k-nearest neighbor (KNN), extreme gradient boosting (XGBoost), and bagging, and stacking ensemble classifier were exploited with optimal hyperparameters. The comprehensive assessment carried out has shown that the ensemble classifier gave highest overall accuracy as compared to other individual classifiers. More importantly, the synthetic minority over-sampling technique (SMOTE) performs better to handle the imbalanced database, while the RF and stacking classifier demonstrated the best prediction performance with accuracy of 92%. Moreover, for the minority rock mass class, the RF shows better performance compared to stacking classifier. The authors emphasize that the effective application of ML-based data-driven approach shows substantial potential for rock mass characterization in TBM tunnelling. [ABSTRACT FROM AUTHOR]

Frequent pressure transients are identified as the cause of block failures in many unlined hydropower tunnels. The primary design objective of such tunnels is to prevent hydraulic jacking at design static pressure and mass oscillation but neglects the effect of short transients, i.e., water hammer. The issue has not been studied from the perspective of hydro-mechanical interactions due to frequent pore pressure changes in the rock mass. This article mainly focuses on the effect of pressure transients at different static heads, or different effective normal stresses across the joints and the effect of time period of pressure transient. Further, the change in such behaviour due to different mechanical properties of rock joints, such as stiffness, friction angle and dilation, is investigated. Numerical simulations of observed pore pressure response in the rock mass during a pressure transient are carried out using distinct element code 3DEC. The results show that relative joint deformation due to short pressure transients are the highest when joint normal stresses are 1.5–2.5 times higher than static water pressure in the tunnel and thus the vulnerability to weakening of such joints by hydraulic fatigue is higher. Further, results show that water hammers can travel up to 4 m into the rock mass even in stiff joint conditions and sufficiently high normal stresses. Results further indicate that the hydraulic impact due to water hammer is smaller as compared to mass oscillation. It is concluded that water hammers, wherever applicable along the waterway, can still contribute to hydraulic fatigue of rock joints in addition to the effect of mass oscillation and cannot be neglected when pressure transients occur frequently. Tunnel filling/dewatering and mass oscillations cause macroscopic joint displacements or block movements over long-term operation which is the major cause of block falls in unlined pressure tunnels. [ABSTRACT FROM AUTHOR]

The use of unlined/shotcrete lined pressure tunnels and shafts are cost-effective solutions for a hydropower project and are being implemented worldwide. To implement this concept, the ground conditions at the area of concern should be favorable regarding minimum principal stress magnitude, which should be higher than hydrostatic water head acting on the tunnel periphery. In addition, the rock mass should be relatively unjointed or joints in the rock mass should be relatively tight. Among the most important issues in the design of unlined/shotcrete lined pressure tunnels is the extent of hydraulic jacking and water leakage out of the tunnel during operation. This manuscript first presents fluid flow and potential hydraulic jacking assessment of two selected locations of the headrace tunnel of Upper Tamakoshi Hydroelectric Project (UTHP) in Nepal using the UDEC. It is noted here that the 7960 m long headrace tunnel will experience a hydrostatic water head that will vary from 2.9 to 11.5 bars (0.29–1.15 MPa). The headrace tunnel is supported by sprayed concrete (shotcrete) in combination with systematic rock bolts in the tunnel walls and crown. The invert of the tunnel and few hundred meters downstream end (at surge shaft area) of the headrace tunnel is being concrete lined after the completion of all other works. The qualitative fluid flow assessment carried out using UDEC indicated considerable pressure built-up in the joint systems suggesting potential hydraulic jacking. This was especially the case at the downstream segment (downstream from chainage 7100 m) of the headrace tunnel. The manuscript further presents the quantitative results of water leakage estimation from the headrace tunnel carried out using Panthi (Panthi KK (2006) Analysis of engineering geological uncertainties related to tunnelling in Himalayan rock mass conditions. PhD Thesis, NTNU, Trondheim, Norway;Panthi, Note on estimating specific leakage using Panthi's approach, NTNU, Trondheim, 2010;) approach. The leakage assessment carried out indicated an average specific leakage of about 2.5 l/min/m tunnel, which may result in over 210 l/s leakage from the headrace tunnel. The evaluation also indicated that the outer reach (860 m downstream segment) of the headrace tunnel after chainage 7100 m seems extremely vulnerable and over 80 l/s water leakage may occur only from this headrace tunnel segment during operation of the hydropower plant. [ABSTRACT FROM AUTHOR]

Keywords: Rock swelling; Cyclic moisture change; Oedometer test; Water tunnel; Rock texture EN Rock swelling Cyclic moisture change Oedometer test Water tunnel Rock texture 463 476 14 02/09/21 20210101 NES 210101 Introduction Swelling of rocks is often forecasted by the content of swelling clay minerals or anhydrite; however, the intensity of the expansion and subsequent swelling pressure cannot be attributed only to these rock constituents (Ruedrich et al. [10]). It is interesting to note that the intact sample Flysch E shows much higher swelling pressure than the powder sample, suggesting that the structural features of this rock have a strong effect on the swelling behavior. The measured swelling pressures of the pulverized samples are higher than the swelling pressures of the intact rock samples. The location of the swelling minerals within the rock texture seem to play an important role and underlines the importance of performing swelling tests on intact rock and to analyze the structure of the rock to assess the swelling characteristics of rocks containing even low amounts of swelling minerals. [Extracted from the article]

Load changes in hydropower plants result in significant pressure transients and unsteady flow in the waterway. It has been observed that instances of block falls in tunnels have increased in unlined pressure tunnels subjected to frequent load changes. To examine this problem, field instrumentation was conducted in the 3.5 km long unlined headrace tunnel of 50 MW Roskrepp hydropower plant in southern Norway. This article describes the methodology of instrumentation, presents the observations and findings. The monitoring clearly demonstrates that frequent load changes have a considerable effect in the rock mass consisting of system of joints. The observations show that pressure transients can travel deep into the rock mass irrespective of their time period. Moreover, pressure transients with longer time periods, i.e. mass oscillations, are seen to induce a higher hydraulic gradient between the rock mass and the tunnel itself. A delayed response from the rock mass is observed during pressure transients, which is the main cause of development of hydraulic gradient and additional pore pressure acting on the rock blocks. Hence, it is evident that the cumulative impact of small but frequent pressure gradients is significant and is responsible for increased instances of block falls over a long period of operation of the unlined tunnels of hydropower plants with frequent start–stop sequences. The overall impact is governed by pore pressure response of the jointed rock mass which depends on the conditions of joint geometry and joint wall properties. [ABSTRACT FROM AUTHOR]

Water tunnels built for hydropower passing through weak and heterogeneous rock mass pose challenges associated to slaking and disintegration, as they are first exposed to dry condition during excavation and are then filled with water to produce hydropower energy. Over the period of operational life, these tunnels are drained periodically for inspections and repair leading to drainage and filling cycles. The weakening of rock mass caused by cycles of drying, saturation and drainage may lead to the propagation of instabilities in the tunnels. Therefore, it is important to study the slaking and disintegration behavior of the weak rock mass consisting of clay and clay-like minerals. This paper assesses the mineralogical composition of flysch and serpentinite from the headrace tunnel of Moglicë Hydropower Project in Albania. Further, to determine the slaking and disintegration behavior of these rocks, extensive testing using both the ISRM, Int J Rock Mech Min Sci Geomech Abstr 16(2):143-151, (1979) suggested test method and a modified variant of this test are performed. Finally, comprehensive assessments, discussions and comparisons are made. It is found that the modified slake durability test better suits for the tunnels built as water conveying systems such as hydropower tunnels. [ABSTRACT FROM AUTHOR]

The complex topography, geology, and tectonic environment prevailing in the Himalaya are the main challenges while applying the unlined or shotcrete lined pressure tunnel concept for hydropower projects. In addition, rock masses in the Himalayan region are influenced by faulting, folding, schistosity, and jointing to a varying degree representing geological complexity. Similarly, frequent occurrence of large scale earthquakes changes in situ stress dynamics. In spite of these challenges, an unlined/shotcrete lined pressure tunnel is being constructed at the headrace tunnel system of Upper Tamakoshi Hydroelectric Project (UTHP) in the Nepal Himalaya. This article studies the unlined or shotcrete lined pressure tunnel in terms of the topographical conditions, in situ stress state, and overall rock engineering aspects. First, the pressure tunnel alignment is assessed using the Norwegian confinement criteria. Second, the minimum principal stress state is assessed using numerical simulation by validating measured in situ stress conditions. Finally, a comprehensive assessment on the rock engineering aspects of the headrace tunnel is carried out. It has been found that if there exist good quality rock mass with tight joints, it is possible to apply the unlined/shotcrete lined pressure tunnel system in the Himalaya provided that the stress requirement is fulfilled. It was also found that the Norwegian confinement criteria are too optimistic for direct use in the design of high pressure headrace tunnel alignment. The detailed rock engineering assessment and stress state analysis indicated that there are some critical locations along the Upper Tamakoshi headrace tunnel alignment. This is specially the case for an about 700 m downstream stretch of the headrace tunnel from where there is a risk of hydraulic jacking, which may possibly lead to excessive water leakage during power plant operation. [ABSTRACT FROM AUTHOR]

Reliable estimation of in situ stress state is very important in implementing unlined/shotcrete-lined pressure tunnels and shafts. The topography, local tectonic setting and geological environment greatly influence the magnitude of in situ stress level. This paper aims to evaluate in situ stress state at the Upper Tamakoshi Hydroelectric Project (UTHP), where unlined/shotcrete-lined headrace tunnel with considerable hydrostatic head is being implemented. Initially measured minimum principal stress indicated much lower values than the hydrostatic pressure at the downstream end of the headrace tunnel, which led to shift the alignment at the upper elevation with reduced hydrostatic pressure. In order to explore the reason behind much lower stress level as expected, a comprehensive assessment is carried out by developing a full rock stress model so that the minimum principal stress along the unlined pressure tunnel is evaluated. To address the complex geotectonic and topographic environment of the UTHP project area, a final rock stress model (FRSM) concept as suggested by Stephansson and Zang (2012) has been utilized. The FRSM concept considers stepwise evaluation of the in situ stress state analysis integrating the best estimate stress model (BESM), stress measurement methods (SMM) and integrated stress determination methods (ISD). The analysis carried out revealed that the in situ stress state at the project area has high degree of spatial variation even at the similar overburden due to the presence of complex topography and the presence of local shear and weakness zones. The analysis further demonstrates that a presence of local shear/weakness zone has considerable de-stressing effect, which leads to the reduction of in situ minimum principal stress magnitude. The reduction in the minimum principal stress along the pressure tunnel increases the risk for the potential hydraulic jacking and leakage if static water pressure is higher than the magnitude of minimum principal stress. [ABSTRACT FROM AUTHOR]

Total plastic deformation in tunnels passing through weak and schistose rock mass consists of both time-independent and time-dependent deformations. The extent of this total deformation is heavily influenced by the rock mass deformability properties and in situ stress condition prevailing in the area. If in situ stress is not isotropic, the deformation magnitude is not only different along the longitudinal alignment but also along the periphery of the tunnel wall. This manuscript first evaluates the long-term plastic deformation records of three tunnel projects from the Nepal Himalaya and identifies interlink between the time-independent and time-dependent deformations using the convergence law proposed by Sulem et al. (Int J Rock Mech Min Sci Geomech 24(3):145-154, 1987a, Int J Rock Mech Min Sci Geomech 24(3):155-164, 1987b). Secondly, the manuscript attempts to establish a correlation between plastic deformations (tunnel strain) and rock mass deformable properties, support pressure and in situ stress conditions. Finally, patterns of time-independent and time-dependent plastic deformations are also evaluated and discussed. The long-term plastic deformation records of 24 tunnel sections representing four different rock types of three different headrace tunnel cases from Nepal Himalaya are extensively used in this endeavor. The authors believe that the proposed findings will be a step further in analysis of plastic deformations in tunnels passing through weak and schistose rock mass and along the anisotropic stress conditions. [ABSTRACT FROM AUTHOR]