Construction and stabilization of underground openings in complex geological terrain are a challenging work. Opening created for any purpose provides avenues for the release of large amount of pre-existing stress and causes the material to deform elastically. Further, if the stresses are sufficiently high, rocks start to behave inelastically, causing fractures in rock mass and overall reduction in the bearing capacity (Ewy and Cook, 1990). Analysis of in-situ measurements and analytical modeling of excavations show that an area of 2D (D is the diameter) is mostly affected in terms of stress redistribution and resulting strain (Brown et al., 1983, Kontogianni et al., 2008). Singh et al. (2004) observed that anisotropy in deformational behaviors of rocks is induced if the number of joint set is not very large, and modeling such behaviors of intact rocks as well as joint properties should be incorporated. The deformation and failure of surrounding rocks are widespread and the associated deformation mechanism has been a matter of great concern to researchers (Singh et al., 2011, Kainthola, 2015, Zou and Yan, 2015).
Schubert and Schubert, 1993, Schubert, 1996, and Steindorfer (1998) have studied the effect of geological structure on deformational behavior of rocks surrounding tunnel using Alpine tunnels’ data. The deformation behaviors of rocks surrounding tunnels in varying conditions have been also studied, and different opinions and classifications are proposed accordingly. Five geomechanical modes of classes of rock deformation and failure were proposed by Zhang et al. (1981). The classes were creep–crack, slip-pressure-induced crack, bending-crack, plastic flow–crack, and slip–bending. Furthermore, Wang et al. (1984) analyzed and summarized the proposed classification in actual underground engineering basis and discussed rock deformation mechanisms, structures, methods and characteristics of the classification. Hu and Zhao (2004) recommended three types (roof falling stones, dome transverse tensile collapse and sidewall tangential squeeze slide) of deformation and failure of caverns in low stress condition. Variation of block sizes and shapes not only changes the failure mode, but also leads to considerable changes in the stress distribution around the tunnel (Solak and Schubert, 2004). Pan and Brown (1996) carried out research on the effects of out-of-plane stress and dilation on the convergence and stability of the surrounding rocks and found these parameters to be the major parameters for understanding the failure mechanism around tunnel surroundings. The size of underground excavation and types of rocks also influence the type and size of deformation. Hatzor et al. (2010) observed the following parameters to be responsible for the stability of shallow karst caverns against collapse, which are listed in order of importance: (1) height of rock cover; (2) span of the opening; (3) intensity of jointing characterized by the number of principal joint sets, mean spacing, and mean persistence discontinuities; (4) orientation of discontinuities; (5) shear strength of discontinuities; (6) strength of intact rock; and (7) groundwater conditions.
Clearly, a number of variables are associated with the deformation of tunnel and many researches have been done to understand them (Wang et al., 2004), but only few can be found on the effect of structural control of joint (mainly joint persistence and spacing) and shear strength of the rock joint. This research focuses on the observation of problems related to these three parameters by taking a case study from a tunnel of Kulekhani III hydroelectric project, Nepal.
The study area lies partly in the Lesser Himalaya and partly in the Higher Himalaya Zone of the Nepal Himalayas. The tunnel of Kulekhani III passes through five geological formations and eight rock types (Fig. 1). The geological setting of the study area is proposed by Stöcklin and Bhattarai (1977). The present study site lies in the southernmost part of the Mahabharat synclinorium, which consists of Kathmandu complex and Nawakot complex differentiated by the varying metamorphic grade (Table 1). Bhainsedhovan marble comprises coarse-crystalline marble, thickly-to-thinly well bedded, massive with subordinate schist intercalations. The Raduwa formation comprises coarse-crystalline, highly garnetiferrous mica-schist, locally gneissic schist, some quartzite, abundant segregation quartz with green chlorite-schist at base. Benighat slate comprises dark-grey slates, whereas Malekhu limestone is composed of white to grey, siliceous, fine crystalline limestone and dolomite. Robang formation comprises blue–green chloritic phyllites, partly with inter-bedding Dunga quartzite beds.
Rock samples were collected from each of the eight rock units for the determination of seven geotechnical properties, i.e. unit weight (γ), uniaxial compressive strength (σci), tensile strength (σti), Young’s modulus (Ei), Poisson’s ratio (ν), angle of internal friction () and cohesion (ci), following the ISRM standards. The observed values of intact rocks are given in Table 2. The strength values show a large range in the measured data and notably mean values are close to the upper range.
The presence of quartz content increases the tensile strength of rocks considerably; however substantial reduction in tensile strength is observed in slate due to the absence of quartz and marked lamination. The intact rock elastic modulus is the highest in dolomite and the lowest in slate as expected. Using Roclab, a Rocscience package, rock mass strength was estimated based on geological strength index (GSI), intact rock properties and depth of overburden (Hoek and Brown, 1997, see Table 3). A remarkable reduction in rock mass strength is observed which is possibly due to the disposition of joints and overburden load. The tunneling quality index (Q-system) proposed by Barton et al. (1974) was used to classify the rock mass of study area as well as for support design. Q-system was preferred over rock mass rating in this study because of the past experiences and successful application of Q-system in the Himalayan condition. The observed values of classification and the maximum, minimum, average values, standard deviation (SD) and coefficient of variation (CV) are given in Table 4.
A number of intact rock strengths and elastic moduli were obtained by rigorous laboratory testing, followed by estimation of rock mass strength and deformation modulus using the intact rock properties. These properties can be used for similar rock conditions and environments with a very high degree of reliability.
Application of high-end numerical tools has become an important part in the design and construction phases of many engineering structures. These methods are proven useful in simulating the behavior of rock mass and estimate of other parameters such as distribution of stresses, zones of stress and strain localization, etc., due to imposition of any engineering load. A number of researchers have used different models depending upon the conditions to simulate rock mass behaviors in the vicinity of the openings (Jing and Hudson, 2002, Verma and Singh, 2010, Verma et al., 2011, Qiu et al., 2013).
In this study, finite element method (FEM) is used to model the influence of rock joint persistence, spacing and shear strength on the stability of tunnel and subsequent estimation of parameters that are responsible for creation of maximum zone of overbreak to resemble the field condition. FEM has been used previously by various researchers, showing versatility of the method towards successful implementation in various rock engineering problems (Eberhardt, 2001, Vermeer et al., 2003, Lee, 2009, Kainthola et al., 2012, Singh et al., 2013, Singh et al., 2.
The tunnel has been modeled using shear strength reduction (SSR) technique, commonly used in various rock engineering environments. The rock mass is allowed to deform elastoplastically to converge to the final solution. Equivalent Mohr-Coulomb shear strength parameters (c and ϕ) have been obtained from linear curve fitting method, using generalized Hoek-Brown failure criterion. This criterion allows incorporation of GSI into the model, and the benefit of which is the fact that GSI includes rock mass deformation parameters in addition to disturbance factor (Sonmez and Ulusay, 1999).
Geometry, boundary condition, and meshing
The tunnel serves the purpose of headrace tunnel of a hydroelectric project. The excavation is horseshoe-shaped with a diameter of 3.8 m. The in-situ vertical and horizontal stresses are obtained from the overlying column of rock and the lateral stress ratio (K, a ratio of horizontal to vertical stress) was calculated from the values of Poisson’s ratio (Table 5). Since the main emphasis of this study is to observe the deformation around the opening, the far-end boundaries have been restrained in both horizontal and vertical directions (Fig. 2). The discontinuities were then incorporated into the model based on field conditions. The meshing used in the model is graded 3-node triangle, which is further refined near the opening.
Results and discussion
From the tunnel logs, different types of deformations can be observed owing to different geological and geotechnical conditions (Fig. 3). The nature and occurrence of joints have a major effect on the failure of specific rock types. Irrespective of rock types, overbreak was frequently observed along the tunnel section. However, block failure, roof collapse and side wall failure, which are all a type of overbreak, were also frequently observed. In situations where rock masses are consistently disturbed due to excavation processes, persistent joints may be exposed, enabling kinematic feasibility. There will be very little internal deformation for kinematic release in the cases where discontinuities are relatively persistent and adversely oriented. On the other hand, if the kinematic release of the blocks does not occur through the pre-existing planar discontinuities, the failure mechanism will require sufficient internal deformation, and in such cases rock mass is degraded by localized slip along the joints (Eberhardt et al., 2004). Within the same rock type, the persistence and spacing are frequently varied along with variable confining stress. Barton et al. (1974) performed detailed experimental analysis of shear strength of rock and rock joints, and suggested that at very low effective normal stresses, rock joints exhibit a wide range of shear strength owing to surface roughness and variable rock strength.
Eight different rock types are encountered along the tunnel alignment. Almost all rock types have three sets of discontinuities. Discontinuity distribution in slate, quartz-schist, phyllite, psammatic schist and dolomite are more regularly distributed in comparison to quartzite, garnetiferous schist and marble (Fig. 4). As a result, the joint parameters like spacing persistence vary randomly in hard rocks in this region. The tunnel is excavated by drill-and-blast method and supports are given based on Q-system. The Q-value along the tunnel is also varying because of variable discontinuity characteristics. The intact strength of rocks is relatively high but some parts show various degrees of alteration due to water percolation. It is interesting to note substantial reduction in rock mass strength owing to joint alteration, nature and occurrence of joints and change in confining stress condition.
Various ground conditions were modeled using FEM and the results were obtained in the form of deformation behavior under different geological conditions. Parametric study was done by varying joint persistence, spacing, and shear strength parameters under low to medium level stress condition. The maximum and minimum ranges of stress levels correspond to the overburden depth observed at each section.
In numerical modeling, joint conditions like spacing and persistence were varied as observed in ground condition till the rock failure like overbreak resembled ground observation, followed by estimation of the optimum stability conditions (no overbreak) due to these parameters. The limit of optimum stability conditions is presented as “marginal stable condition” in Table 6.
The persistent joints are the most vulnerable in terms of stability. Einstein et al. (1983) suggested that joint persistence is among the parameters which significantly affect the rock mass strength. In the present study, all the sections in different rocks have joints which are highly persistent along which variable spacing is observed in rocks. Out of the eight rocks, garnetiferous schist, phyllite and slate show the most consistent joints with very less spacing. This causes the formation of small block size as is observed during tunnel excavation. While in the other rocks, the block size is relatively large which in this study directly influences the overbreak zone except in slate and garnetiferous schist where persistent joints are arranged in such a way that kinematic sliding initiates along that plane. This is also a result of reduction in joint shear strength values. Tsesarsky and Hatzor (2003) observed that multi-joint rocks around the openings are stable when joint shear strength is higher. The present result also shows as joint cohesion and friction angle increase, the total displacement decreases which leads to the stability of the tunnel surrounding rocks. The observed displacement conditions and their corresponding tunnel logs are presented in Fig. 3, Fig. 4, Fig. 5. Rose (1982) studied the deformation characteristics in terms of degree of jointing based on RQD and classified the deformation of rocks on the basis of Terzaghi’s classification. According to his findings, the expected overbreak is
This study correlates well for the failure condition when RQD is 30–75, overbreak is (0.2–0.6) (B + ht), but the result contradicts his observations on situations where marginal stable conditions (critical or limiting stability) are observed. In such conditions, deformation is observed but there were no overbreaks observed in all rock types.
From the above analyses, it is found that with the exception of garnetiferous schist and slate, the extent of overbreak is less than 1 m. No squeezing conditions were observed but block fall either from roof or from side walls is common as was also observed in the field (Fig. 3, Fig. 4, Fig. 5). The maximum probability of rock block fall condition was found at the lowest values of joint shear strength along with low joint spacing and low persistence like in quartzite and marble. Therefore, it is established that joint strength, spacing and persistency are very sensitive parameters for block failure in tunnel surrounding for all analyzed rock types. However, Tsesarsky and Hatzor (2006) found that the extent of loosening above excavation in blocky rock masses is predominantly controlled by joint spacing and only to a lesser extent by joint shear strength. Yeung and Leong (1997) studied the effects of joint attributes on tunnel stability using discontinuous deformation analysis (DDA) and found that block volume based parameter may be more appropriate than joint spacing as a measure of the effects of block size on tunnel stability, and the tunnel excavated in a blocky rock mass is likely to be more stable if joint spacing is larger with higher joint friction angle. The present study also confirms that with an increase in joint friction and spacing, the stability of the rock mass around tunnel increases.
This paper attempts to address some of the issues observed during tunneling in low stress conditions by taking a case of Kulekhani III hydroelectric project, Nepal. A detailed field study was done to collect rock joint parameters, mostly persistence and spacing, which have a major influence on stability of openings. The following observations are drawn.
(1) Stability analysis was done using FEM by parametrically varying rock joint persistence, spacing and shear strength parameters until the condition of overbreak was achieved. Another case of marginally stable condition was also obtained based on the same parameters.
(2) The purpose of FEM is to obtain the optimum condition for the formation of overbreak similar to ground conditions. It was observed that the size of overbreak is controlled by joint persistence, spacing and shear strength of rock joints.
(3) Garnetiferous schist and slate having high persistence show the development of large plastic zone but smaller block size owing to low joint spacing, whereas low persistence and low spacing in marble and quartzite create rock block fall condition and large block size along with significant reduction in the overbreak zone.
(4) Squeezing condition was not observed during FEM simulations as well as field condition; however block failure and side wall collapse were observed in almost all the rock types.
(5) The present study gains huge significance for the Himalayan region. A number of infrastructure-related activities are being undertaken in the fragile Himalayan terrain and the results obtained from this study can be used as markers for excavation in similar litho-tectonic units. The result can come in handy for design and execution of the similar works.
Control of rock joint parameters on deformation of tunnel opening
AuSuman Pantheea, P.K.Singh, Ashutosh Kainthola, T.N.Singh
Journal of Rock Mechanics and Geotechnical Engineering