Indian experiences with Q and RMR systems

The Q-systemir of Barton et. al. (1974, 1976) and the RMR-system of Bieniawski (1973) have been evaluated on the basis of measured tunnel support pressures from 26 tunnel sections, 2 to 14 m wide, covering both squeezing and non-squeezing ground conditions. The comparison shows that the Q-system is unsafe for large tunnels under squeezing ground conditiona. A new correlation has been developed considering tunnel depth, tunnel radius, tunnel closure, and Rock Mass Number—i.e., “stress free Q”—to obtain reliable estimates of tunnel support pressures. Changes suggested by Sheorey (1991) for satisfactory application of the Q system to coal-mine roadways on the basis of 44 case histories are presented. Unal's (1983) correlation for coal-mine roadways is shown as overly safe for large tunnels under non-squeezing ground conditions, and unsafe for all sizes of tunnels under squeezing ground conditions. Correlations between tunnel support pressure, tunnel depth, tunnel closure, and Bieniawski's RMR have been developed to provide reliable tunnel support pressures for all sizes of rock tunnels under varying ground conditions. The correlations between RMR and Q proposed by Bieniawski (1976) and by Rutledge and Preston (1978) are not reliable, because RMR and Q are not truly equivalent. Therefore, an acceptable correlation between rock mass number N and RMR_mod, i.e., RMR without joint orientation and intact rock strength, has been presented for a better interrelation.     

Geotechnical considerations in an unlined high pressure tunnel at Lower Kihansi in Tanzania

The Lower Kihansi unlined high-pressure tunnel is the first of its kind to be constructed in Tanzania. The pressure tunnel consists of a 500 m vertical shaft and a 2.195 km inclined headrace tunnel. The cross sectional area of the shaft is 25 m² and that of the headrace tunnel is 30–37.5 m². The headrace tunnel slopes 1:7 towards the powerhouse cavern. The pressure tunnel acts as waterway towards the underground hydroelectric power generation plants with a maximum generating capacity of 180 MW. The Kihansi River has been deviated through the shaft and headrace tunnel from an elevation of 1,146–300 m above sea level. The maximum water pressure created by this deviation is 8.5 MPa.The decision not to steel line the pressure tunnel was reached after the excavation and documentation of the underground rock mass. The hydraulic jacking and hydro-fracturing tests confirmed the rock to have a minimum acceptable confining stress of 9.6 MPa, capable of withstanding the expected water pressure in the tunnel. The permeability of the rock mass is relatively low and any poor zones were sealed by grouting.The discontinuities had a favourable orientation with respect to the tunnel axis such that rock bolts and steel fibre reinforced shotcrete could be used to provide the necessary support. No failures occurred and the decision not to line the Kihansi high-pressure tunnel has proved both technically acceptable and economical.     

Effects of steel fiber reinforced high-strength shotcrete in a squeezing tunnel

In order to quantitatively clarify the effects of steel fiber reinforced high-strength shotcrete (SFRS) applied to a squeezing tunnel, a non-linear numerical analysis is carried out, in which the stress–strain–time constitutive relationships of SFRS and the time-dependent movement of the ground surrounding the tunnel are taken into account. Through a comparison of field measurement and analytical results, it is recognized that SFRS can be applied as a reasonable primary lining for tunnels excavated in grounds with severe geological conditions. In particular, high strength during the early stages of the execution and ductility after its peak strength both contribute to the safe construction of squeezing tunnels.     

Effect of adverse geological condition on TBM operation in Ghomroud tunnel conveyance project

With the planned length of 36 km, Ghomroud tunnel is one of the longest tunnels under construction in central Iran. About half or 18 km of this tunnel was excavated by a double shield TBM. Several adverse geological conditions encountered, consisting of ground squeezing and face collapse, hindering TBM performance, and caused several TBM stoppages and jamming. This paper presents the impact of ground conditions on machine performance based on the information obtained from field observations and geotechnical site investigations. As built geological conditions are described while the method and results of tunnel convergence measurements and their impacts on tunneling operation is examined. Based on the detail study of the available geological information and tunnel convergence measurements, it was evident that the existence of weak structures in rock mass resulted in high rate of the convergence, which was the dominant factor in the TBM jamming. Since it was not possible to make observation and measurements of geological parameters when working in a lined tunnel built by a shielded machine, an attempt was made to correlate TBM operational parameters and ground convergence. The preliminary result of the analysis has indicated a good correlation among machine’s operational parameters and tunnel convergence. If the system is fully developed, these parameters can be used as an indicator of the potential for high rates of convergence. An early warning on ground convergence is essential for taking precautionary measures to avoid TBM from getting jammed by squeezing ground.     

Predicted versus actual rock mass conditions: A review of four tunnel projects in Nepal Himalaya

The Himalayan region possesses enormous potential for hydropower development. However, there exists great challenge for the successful underground excavation due to complex geological set up of the region. It is generally accepted that the cost and time are of main concerns in any tunnelling project as these two factors influence greatly on the economic viability of the hydropower projects. Consequently, the accuracy of predicted geological conditions during planning phase plays an important role during its implementation. The degree of accuracy in predicting, evaluating, and interpreting the quality of rock mass along the tunnel alignment is thus a key for the successful completion of any hydropower project. This paper assesses and compares the predicted and actual rock mass conditions of the four recently constructed hydro-tunnels in Nepal Himalaya. Further, an evaluation is made on the extent of pre-construction phase engineering geological investigations. The effect on the overall cost and construction time caused by the variations in rock mass quality of these tunnel projects are discussed. A recommendation is given on the minimum level of geological investigation that may help improve the predictability of geological conditions and reduce the discrepancy between predicted and actual rock mass conditions to an acceptable level.     

The use of stress-strength relationships in the assessment of tunnel stability

A method of predicting probable ground behaviour considering stress-strength relationships is presented. The influence of overburden pressure and rock mass strength which are implicitly used in calculating the Q value (Q-system of Barton et al. 1974) are explicitly considered in forecasting stability problems in tunnels. In particular, squeezing, spalling and loosening of the rock mass in tunnels constructed through the complex geological set-up of the Himalayas have been analysed. A new Q-system correlation which takes into account the dimension of an opening is suggested for evaluating support pressures in squeezing ground.     

Estimation of large-scale mechanical properties of a large landslide on the basis of seismic results

Knowledge of large-scale mechanical properties is essential for estimating the stability of rock slopes being deformed by deep creep and especially for predicting a transition to rapid sliding. Standard geomechanical testing procedures may not result in significant values as the volume of the mass movements under consideration exceeds the volume of even the largest in situ tests by several orders of magnitude. Seismic measurements may help to estimate some of the relevant mechanical properties by direct correlation with the seismic velocities. However, not all parameters necessary for a stability analysis correlate closely with seismic velocities. An alternative approach could be the geomechanical modelling of the structures and properties of the creeping or sliding mass, as determined from seismic exploration. In this paper, we present an application of this approach to the giant landslide of Koefels. The development of the creeping rock mass, which represents the initial phase of the mass movement, has been successfully modelled by assuming a transition of the originally compact rock mass to “soft” rock, controlled by a Mohr–Coulomb and no tension yield criterion. Geomechanical parameters are determined by fitting the geomechanical model to the seismic results. An apparent angle of internal friction has been determined in the range 20–24°. These low values are compatible with the fact that a transition to rapid sliding took place in the past.     

Preliminary analysis of the dynamic interaction between Norra Länken and a subway tunnel for Stockholm, Sweden

A large infrastructure project called “Ring road Around Stockholm” is presently being planned. A large proportion of the roads will be constructed underground. All kinds of hazardous materials; even explosive gases and liquids, will be allowed to be transported. At Roslagstull, which is located at the northern boundary of the city, five underground road tunnels will cross over an existing subway tunnel, with only about 3 m of rock between them. A scenario in which a truck loaded with explosive gas or liquid starts to leak and then explodes has been proposed by Stockholm's Street and Traffic Administration. Consequently, the effect on rock and the reinforcement surrounding the subway tunnel must be evaluated. This paper presents the results of a study based on two-dimensional distinct element modelling of the rock mass and reinforcement response to a load caused by an exploding truck as described above. Two different assumptions regarding the explosion load have been simulated—namely, static load and fully dynamic load. The study (i) provides general information about the rock mass and reinforcement response for a specific assumption regarding the rock mass conditions, and (ii) compares the results from the two different assumptions regarding the representation of the explosion load. Both unreinforced and reinforced conditions have been modelled for each loading condition (i.e., a total of four models). The results indicate that for the unreinforced models, the differences in the model response are not dramatic when compared to the models with static and dynamic representation of the explosion load. On the other hand, the reinforced models indicate major differences, particularly in the reinforcement response. All numerical analyses presented in this paper were performed using the two-dimensional distinct element code UDEC (Itasca 1992).     

Theoretical prediction of wear of disc cutters in tunnel boring machine and its application

Predicting the cutter consumption and the exact time to replace the worn-out cutters in tunneling projects constructed with tunnel boring machine (TBM) is always a challenging issue. In this paper, we focus on the analyses of cutter motion in the rock breaking process and trajectory of rock breaking point on the cutter edge in rocks. The analytical expressions of the length of face along which the breaking point moves and the length of spiral trajectory of the maximum penetration point are derived. Through observation of rock breaking process of disc cutters as well as analysis of disc rock interaction, the following concepts are proposed: the arc length theory of predicting wear extent of inner and center cutters, and the spiral theory of predicting wear extent of gage and transition cutters. Data obtained from 5621 m-long Qinling tunnel reveal that among 39 disc cutters, the relative errors between cumulatively predicted and measured wear values for nine cutters are larger than 20%, while approximately 76.9% of total cutters have the relative errors less than 20%. The proposed method could offer a new attempt to predict the disc cutter's wear extent and changing time.     

A numerical approach to the hyperstatic reaction method for the dimensioning of tunnel supports

The hyperstatic reaction method is particularly suitable for the dimensioning of support structures. This method simulates the interaction between the support and rock surrounding the tunnel through independent “Winkler” type springs. It requires the definition of the active loads that are applied directly to the support structure by the rock mass. Further passive loads are due to the reaction of the rock mass to the displacement of the support structure.A numerical approach to the hyperstatic reaction method is presented in this paper. The parameters that condition the calculation and the dimensioning techniques of the support structures, on the basis of the results of the method, are also presented. A specific code, named FEMSUP, was developed using a FEM framework, to perform calculations with the HRM. This code is able to consider the effective geometry of the support and the horizontal active loads that are different from the vertical ones; it is therefore able to analyse the rock mass–structure interaction in detail.An extensive parametric analysis performed using FEMSUP has made it possible to define the necessary support structures (steel sets incorporated into a shotcrete lining) in a wide number of cases that cover the conditions that are generally encountered in excavation practice. Ten design tables were drawn up to summarise the results of the parametric analyses. From an examination of the tables, it is possible to verify the influence of the various calculation parameters on the dimensioning of the support structure and to obtain rough indications on the entity of the support structures in the preliminary stages of a project.     

Ground reaction curves for tunnels excavated in different quality rock masses showing several types of post-failure behaviour

Rock mass behaviour model selection and, in particular, rock mass post-failure behaviour are key issues in analysing tunnel stability, in particular in terms of the correct application of design techniques such as the convergence–confinement method and also numerical modelling. Three different quality rock masses (good, average and poor) were defined in which simulated standard tunnels were excavated. Different behaviours – including elastic perfectly plastic, elastic brittle and three strain softening behaviours – were modelled for each type of rock mass and increasingly realistic parameters were calculated, along with the corresponding ground reaction curves. The results obtained demonstrate the importance of adequate post-failure behaviour model selection for tunnel analysis. Also assessed are the effects of the standard support and reinforcement.     

Methodology for tunnel and portal support design in mixed limestone, schist and phyllite conditions: a case study in Turkey

The purpose of this study is to present a methodology for tunnel and support design in mixed limestone, schist and phyllite conditions through investigating two highway tunnel case studies that are located along the Antalya–Alanya Highway in southern Turkey. The main lithologies of the project area are regularly jointed, recrystallized limestone and the weak lithologies of the schist unit (i.e., pelitic schist, calc schist, graphitic phyllite and alternations of these lithologies). A detailed geological and geotechnical study was carried out in the project area, and the tunnel ground support types and categories were determined according to the Q-system, rock mass rating method and New Austrian Tunneling Method (NATM). The shear strength parameters and geomechanical properties of the rock masses were obtained by using the geological strength index (GSI). The deformation moduli and post-failure behavior of the rock masses have been determined. Slope stability analyses were performed at the portal, side or cut slope sections. Kinematic and limit equilibrium analyses incorporating the effects of water pressure were performed for the regularly jointed failed rock slopes. Circular failure analogy was used for the slope stability analyses of irregularly jointed, highly foliated lithologies. Slope support system recommendations were made. A back analysis on a failed slope was performed. The results of the back analysis compared well with the results obtained through the GSI method. The tunnel grounds were divided into sections according to their rock mass classes. The deformations and stress concentrations around each tunnel section were investigated and the interactions of the empirical support systems with the rock masses were analyzed by using the Phase² finite element software. The regularly jointed rock masses were modeled to be anisotropic and the irregularly jointed, highly foliated and very deformable soil-like lithologies were modeled to be isotropic in the tunnel finite element analyses.     

Deformational characteristics of weak sandstone and impact to tunnel deformation

In northern Taiwan, a tunnel under construction along a segment where weak sandstone, the Mushan sandstone, was encountered and an excess crown settlement (14–30 cm) has been reported. This paper studies the deformational characteristics of Mushan sandstone and its impact on tunnel deformation. To distinguish the volumetric and the shear deformation of the sandstone, experiments with controlled stress paths, including hydrostatic compression, pure shearing and conventional triaxial compression, were conducted. The measured deformations were then decomposed into elastic and plastic components further exploring the stress–strain behavior of weak sandstone. The results indicate that, similar to other soil-like geo-materials, this sandstone has plastic strain before the stress path reaches the failure envelope and significant shear dilation is induced, especially when approaching the failure envelope. Meanwhile, the distinct features of deformation have also been highlighted by comparing the experimental results to the prediction, derived from existing constitutive models that were originally developed for other geo-materials. These features include significant plastic volumetric strain at low levels of confining stress, suppression of plastic volumetric strain at higher levels of confining stress, and the fact that the actual amount of shear compression is less than that predicted by the model. Numerical analysis indicates that the weak rock leads to the greatest inward displacement, which results from the shear dilation prior to failure state.     

Influence of the slope in the ventilation semi-transversal system of an urban tunnel

The covering of a section of the Inner Belt roadway (“Ronda del Mig”) in Barcelona gives rise to an urban tunnel of great length (1535 m). The tunnel is divided into two independent parallel galleries and its orientation is North–South, with a 2% upward slope towards the North. Although normal ventilation is achieved with jet fans, between the two galleries there is an interior passage for smoke extraction, in case of fire, through exhaust openings on both sides of this passage. Therefore, the tunnel has a semi-transversal ventilation system for fire incidents.The behavior of the smoke generated during those possible fire incidents in the traffic galleries was simulated with a commercial code, FLUENT^®, which allows a three-dimensional multispecies Navier–Stokes unsteady simulation. The mesh of each tunnel was made with about 250,000 triangular base prismatic cells. The simulated fire had a thermal power of 30 MW and the time step was set to one second, while the simulation covered 15 min.Special emphasis was put on the influence of the tunnel slope on the smoke’s behavior in each gallery. Simulation results showed that the fans’ capacity established in the project specifications was not enough to extract the smoke of a fire with the simulated power. A significant percentage of the smoke was aspirated through the exhaust openings but the rest continued rising to the tunnel portal due to the slope. This created a great risk mainly in the descending gallery with opposite traffic direction. For a more efficient extraction it was determined that the exhaust sections should be opened upward of the fire’s location. The standard opening, at both sides of the fire, reduced the capacity to extract smoke due to clean air aspiration from the lower portal.     

Predicting tunnel squeezing using a hybrid classifier ensemble with incomplete data

Bulletin of Engineering Geology and the Environment - Tunnel squeezing occurs when time-dependent rock creep produces large tunnel convergence. The occurrence of tunnel squeezing may result in...     

Three-dimensional structural analyses of the shield-driven “Green Heart” tunnel of the high-speed line South

The tunnel lining generates a significant part of the bore tunnel project costs. This tunnel structure is one of the most important components of the whole tunneling process. The tunnel structure has to fulfill all necessary functional requirements during its lifetime. Because of this it is essential for engineers first to understand the realistic tunnel-lining behaviour and then to design a tunnel structure in a properway. The design of the lining structure is actually quite simple because of the wide range easy-to-use models now available. In contrast, predicting realistic tunnel-lining behavior is very difficult. The available numerical models for a segmented concrete lining cannot predict realistic structure behavior at all stages of excavation and during the tunnel lifetime. Conventional models ignore the influences of assembling processes, imperfections of segments, type of joints and variation in stress distributions in the concrete sections. This paper deals with three-dimensional finite element analyses of the tunnel structure, observations during the construction phase and in-situ measurements on the Second Heinenoord Tunnel (Bakker 1999) applied on the structural design of the shield-driven “Green Heart” Tunnel of the High Speed Line-South in the Netherlands.     

Study of squeezing pressure phenomenon in a tunnell—II

The phenomenon of squeezing pressures is quite common in the younger rocks of the lower Himalayas. Predicting rock pressure for the design of tunnel supports is an intricate problem due to numerous handicaps. This paper deals with the development of a criterion for predicting rock pressures under squeezing ground conditions. The rock pressure is very much influenced by the broken zone around the tunnel periphery. The paper also suggests a graphical procedure for estimating the radius of the broken zone.     

Evaluation of earthquake impact on magnitude of the minimum principal stress along a shotcrete lined pressure tunnel in Nepal

In situ stress condition in rock mass is influenced by both tectonic activity and geological environment such as faulting and shearing in the rock mass.This influence is of significance in the Himalayan region,where the tectonic movement is active,resulting in periodic dynamic earthquakes.Each large-scale earthquake causes both accumulation and sudden release of strain energy,instigating changes in the in situ stress environment in the rock mass.This paper first highlights the importance of the magnitude of the minimum principal stress in the design of unlined or shotcrete lined pressure tunnel as water conveyance system used for hydropower schemes.Then we evaluated the influence of local shear faults on the magnitude of the minimum principal stress along the shotcrete lined high pressure tunnel of Upper Tamakoshi Hydroelectric Project(UTHP) in Nepal.A detailed assessment of the in situ stress state is carried out using both measured data and three-dimensional(3 D) numerical analyses with FLAC~(3 D).Finally,analysis is carried out on the possible changes in the magnitude of the minimum principal stress in the rock mass caused by seismic movement(dynamic loading).A permanent change in the stress state at and nearby the area of shear zones along the tunnel alignment is found to be an eminent process.     

Effect of rock mass quality on construction time in a road tunnel

On the basis of the existing NTNU (Norwegian University of Science and Technology) advance rate model and field experiences, an estimation model which can analyze construction time for a broad range of works related to tunnel construction by drill and blast has been established. The model includes the time spent for excavation, rock support, various installations in the tunnel, and site preparation. The model is developed as a spreadsheet. Furthermore, by the use of the model, analyses on various effects of rock mass quality on construction time and advance rate for four Q-values of 0.01, 0.1, 1.0 and 10.0 on seven sizes of road tunnels applied in Norway were made. The results show that construction time increases up to 30–40% with cross sectional area varying from T5 (35.2 m²) to T12 (86.9 m²) under the assumption that the same Q-value is applicable to the entire tunnel length. Standard advance rate considering the effect of rock mass quality may be about 50% lower for Q = 0.01 than in the case of not considering the same effect in a 3 km tunnel. The gap between the two standard advance rates is gradually decreasing with increased Q-value.     

The new three-dimensional subsidence influence function denoted by n–k–g

This study presents a three-dimensional development of the n–k–g influence function with the aim of predicting subsidence phenomena and characterizing the shape and dimensions of the corresponding trough. The parameters “n” and “k” characterize the ground and “g” is related to the gravity. This function depends on two physical concepts: the first is gravity, which characterizes the forces acting on the ground, and the second, the convergence of the roof and floor of the mine workings due to the stress state of the ground. Caving in of the roof generates direct subsidence, and the swelling of the floor, indirect subsidence, which allow us to establish the shape of the trough.The physical concepts introduced are fundamental in the mathematical implementation of the n–k–g influence function, allowing a more intuitive interpretation of the subsidence trough and notably facilitating the work of calibration, validation and sensitivity analysis. These concepts likewise allow the scope of application of influence functions to be extended to non-horizontal seams, also taking into account the quality of the rock mass and the presence of preferential sliding directions, in both the roof and the floor of the seam.In the development of this paper, we shall first see the physical concepts considered, to then present the three-dimensional implementation of our n–k–g influence function. We shall see the results obtained when calibrating the proposed numerical model with real data obtained from subsidence measurements in a coalmine in the Coal Basin of Asturias, situated in the North of Spain.