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Paper presented at the ISRM International Symposium - 8th Asian Rock Mechanics Symposium, October 14–16, 2014
Paper presented at the ISRM International Symposium - 6th Asian Rock Mechanics Symposium, October 23–27, 2010
Paper presented at the ISRM International Symposium - 5th Asian Rock Mechanics Symposium, November 24–26, 2008
Paper presented at the ISRM International Symposium - 5th Asian Rock Mechanics Symposium, November 24–26, 2008

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Proceedings Papers

Paper presented at the ISRM International Symposium - 10th Asian Rock Mechanics Symposium, October 29–November 3, 2018

Paper Number: ISRM-ARMS10-2018-100

... key issues shall be addressed in accordance with Eurocode EC 7 (ӦNORM EN 1997–1): Design concept for determining excavation and support Determination of safety relevant parameters, including definition of expected behavior and

**criteria**for the assessment of the system stability on the basis of...
Abstract

Abstract Limited information on the ground structure and properties, as well as simplifications in the design lead to uncertainties in assessing ground and systems behaviors. Considering all expected and unexpected conditions in the a-priori design is acceptable only in special cases. For safe and economical tunnel construction the residual risk needs to be managed by appropriate monitoring and a safety management system. The paper addresses requirements for the geotechnical safety management, as well as state-of the-art monitoring and data evaluation techniques in the context of the observational approach. Experience with safety management systems over the last two decades shows that the risk can be significantly reduced. Case histories are used to demonstrate the potential of proper monitoring and safety management. 1. Introduction Uncertainty is unavoidable in underground projects. It originates from the impossibility to completely investigate the geological and geotechnical conditions, even if considerable effort has been made during the preparation phase. Upscaling of small scale laboratory test results to representative rock mass volumes and simplifications in our models and analyses further decrease the accuracy of the prediction of the ground and system behaviors. To allow for safe and economical construction of underground structures, sound preparation, as well as accompanying measures during construction are essential. When applying the observational approach, following key issues shall be addressed in accordance with Eurocode EC 7 (ӦNORM EN 1997–1): Design concept for determining excavation and support Determination of safety relevant parameters, including definition of expected behavior and criteria for the assessment of the system stability on the basis of the expected ground conditions Monitoring concept, allowing a continuous comparison of expected and observed behavior, including all organizational and technical requirements Management concept for cases where ground conditions and/or system behavior deviate from that predicted, both for favorable and unfavorable deviations. During construction attention should be paid to following issues: Appropriate organization of the site, and employment of qualified personnel Technical provisions for high quality observation of the system behavior Efficient information flow and reporting

Proceedings Papers

Paper presented at the ISRM International Symposium - 10th Asian Rock Mechanics Symposium, October 29–November 3, 2018

Paper Number: ISRM-ARMS10-2018-243

... other feasible method, drilling & blasting using high explosives along with the latest electronics blasting system would be the most practical and feasible way to remove the bedrock in the shortest time frame. Reservoir Characterization

**criteria**isrm international symposium asian rock...
Abstract

Abstract This paper will share the controlled rock blasting experiences in two underground MRT projects in Singapore. Dedicated rock blasting design elements including decoupled charging, suitable spacing/burden, time delay and loading density to promote forward movement of the rock with balanced blasting efficiency and impact to surrounding amenities and buildings are presented. The empirical formulae which was used to predict the blasting induced ground vibration was calibrated against the instrumentation data. Parameters such as the site specific constant and the attenuation coefficients in typical Bukit Timah granite are validated and recommended. Various measures which were implemented to minimize the vibration at nearby structures and mitigate fly rocks within blasting site will be shared and discussed. This includes the traffic management efforts at the heavily plied vehicular and pedestrian movements above the blasting area. Effectiveness of line drilling of relief holes to prevent the blasting induced damage to king post and ERSS are discussed. The appropriate blasting limit in the aspect of peak particle velocity (PPV) for ERSS, king post, decking and surrounding utilities and buildings are suggested. Since no blasting codes and guidelines are currently available in Singapore, the shared data could serve as reference for future projects. 1. Introduction Thomson-East Coast Line (TEL) (Fig. 1), as 6 th MRT line in Singapore, is currently under construction. TEL is 43km long, fully underground and adds 31 underground stations to the existing rail network, including 7 interchange stations, which will link to the existing East-West Line, North-South Line, North-East Line, Circle Line and the Downtown Line. It will run through a combined north-south and east-west corridor, starting from Woodland North to Gardens by the Bay, followed by east coast stretch that will connect commuters in the eastern part of Singapore. The TEL will strengthen the connectivity and resilience of the current rail network by providing alternative travel route for many more commuters on the existing lines. The line will open in five stages from 2019 onwards. When fully open, it is expected to serve about 500,000 commuters daily in initial years, rising to one million commuters daily in long term. Slated to open in 2021, TEL stage 3 (from Mt Pleasant to Gardens by the Bay) expects high rock crop above station formation level in 2 stations located at Bukit Timah formation, i.e. T217 Napier Station and T221 Havelock Station. The estimated rock volumes are 40,000m 3 and 57,000m 3 , respectively. With consideration of the difficulty in breaking and removing the bedrock by any other feasible method, drilling & blasting using high explosives along with the latest electronics blasting system would be the most practical and feasible way to remove the bedrock in the shortest time frame.

Proceedings Papers

Paper presented at the ISRM International Symposium - 10th Asian Rock Mechanics Symposium, October 29–November 3, 2018

Paper Number: ISRM-ARMS10-2018-206

... found that, overall, fitting results with Hoek-Brown (HB)

**criteria**are more accurate than those with the Mohr-Coulomb (MC) criterion for the dynamic compression test results. Finding in the test confirms that HB strength envelope under static conditions can be used to describe dynamic strength behaviors...
Abstract

Abstract In the present study, tests and empirical formulas were utilized to investigate the problem of determining the mechanical properties for igneous rocks considering seismic effect. The following points summarize the results obtained in the present note. The most concentrated frequency band of earthquake energy was determined by the platform segment of the design response spectra. With this frequency range, cyclic loading test was conducted for the rock sample, in which a strain rate range was determined for the rocks considering seismic effect. This strain rate range is lower than the traditional intermediate strain rate range. Dynamic triaxial compression tests were conducted for igneous rock specimens of two major underground rock engineering. A general trend of compression strength and the Young's modulus increase with the increasing strain rate can be observed. And it is found that, overall, fitting results with Hoek-Brown (HB) criteria are more accurate than those with the Mohr-Coulomb (MC) criterion for the dynamic compression test results. Finding in the test confirms that HB strength envelope under static conditions can be used to describe dynamic strength behaviors, and only need to change the intercept of the envelope, i.e. the uniaxial compressive strength (UCS). Thus, HB strength criteria considering seismic effect was obtained. It is practical to estimate the mechanical properties of rock mass considering seismic effect with the seismic HB strength criteria and Hoek & Diederichs equation based on the UCSs at various strain rates. The seismic modulus of the intact rock can be directly estimated with the UCS at different strain rates since the modulus ratio MR is strain rate dependent. The Baihetan (BHT) hydropower plant project was taken as a study case, its rock mass mechanical properties considering seismic effect were estimated with the proposed approach. 1. Introduction Various studies have been conducted to establish the well-known fact that the mechanical properties of rock materials are dependent on the strain rate (Green and Perkins 1968; Blanton 1981; Lajtai et al. 1991; Cai et al. 2006; Liang et al. 2011; Liu et al. 2012). Research investigations concerning the dynamic mechanical properties of rock materials have primarily involved laboratory tests, during which the rock mechanical properties at various strain rates have been explored. For example, the dynamic compression results of Li et al. (1999, 2000) and Zhao et al. (1999) concerning rock materials showed that the compressive strengths of rock materials generally increase with an increase in the strain rate. Similarly, the results of the experimental studies conducted by Zhao (1999) and Zhao and Li (2000) also showed that the tensile strength, shear strength and fracture toughness of rock materials all show increasing tendencies with increasing strain rates.

Proceedings Papers

Paper presented at the ISRM International Symposium - 8th Asian Rock Mechanics Symposium, October 14–16, 2014

Paper Number: ISRM-ARMS8-2014-211

... mechanics symposium arms8 disaster recovery work colluvial deposit iya district stability gp automatic monitoring system contingency planning

**criteria**slope failure large-scale colluvium 8th Asian Rock Mechanics Symposium ARMS8 14-16 October 2014, Sapporo, Japan Monitoring the Stability of a...
Abstract

Abstract Many large-scale slopes failed due to the heavy rainfall brought about by a typhoon in September of 2011 in a mountainous area of the Kii Peninsula, Japan. The colluvial deposit caused by the failures dammed up a major river and became unstable. Then, a debris flow occurred due to the erosion of the colluvial deposit. A GPS automatic monitoring system has been installed on the colluvial deposit to assess the validity of disaster recovery works aimed at stabilizing the deposit. The system is composed of three segments, namely, the measurement segment, the server segment and the user segment. The data received from satellites are transmitted to the server computer via a mobile phone. The server computer analyzes the data automatically and provides the three-dimensional displacements of the measurement points to the client over the Internet. The accuracy of the measurements is 1–2 mm for horizontal displacements and 2–3 mm for vertical displacements, although the accuracy of a conventional GPS is 5–10 mm. This paper describes the functions and the advantages of the GPS automatic monitoring system. The system was used to monitor displacements in a large-scale colluvial deposit due to slope failures brought about by heavy rainfall. The stability of the colluvial deposit is discussed on the basis of the measurement results, and disaster recovery works aimed at stabilizing the colluvial deposit were designed.

Proceedings Papers

Paper presented at the ISRM International Symposium - 8th Asian Rock Mechanics Symposium, October 14–16, 2014

Paper Number: ISRM-ARMS8-2014-019

...

**criteria**developed above, and hence the safe maximum withdrawal rate of the compressed-air can be determined. Upstream Oil & Gas asian rock mechanics symposium arms8 rock salt dilation Reservoir Characterization criterion shear strain Loading Rate reservoir geomechanics cavern...
Abstract

Abstract The objective of this study is to determine effects of loading rate on compressive strength and deformability of the Maha Sarakham salt under elevated temperatures. The effort is aimed at determining the safe maximum withdrawal rates for the compressed-air energy storage (CAES) in salt caverns. The constant axial stress rates range from 0.0001 to 0.1 MPa/s. The testing temperatures are maintained constant between 273and 373 Kelvin. The results indicate that the shear stress-strain curves monitored under various loading rates and temperatures show nonlinear relations, particularly under high temperatures. To incorporate the thermal and rate (time-dependent) effects into a strength criterion the distortional strain energy at dilation of the salt is calculated as a function of the mean strain energy density. Finite difference analyses (FLAC 4.0) are also performed to determine the stresses and strains at the boundaries of CAES caverns for various reduction rates of the internal pressures. The maximum stresses and strains obtained during retrieval period are used to calculate the strain energy density induced at the cavern boundaries. The results are compared against the criteria developed above, and hence the safe maximum withdrawal rate of the compressed-air can be determined.

Proceedings Papers

Paper presented at the ISRM International Symposium - 8th Asian Rock Mechanics Symposium, October 14–16, 2014

Paper Number: ISRM-ARMS8-2014-020

... Abstract The objective of this study is to determine rock mass strength and deformability in the laboratory by simulating joints in sandstone specimens. The results are used to assess the predictive capability of rock mass strength

**criteria**developed by Hoek-Brown, Ramamurthy-Arora, Yudhbir...
Abstract

Abstract The objective of this study is to determine rock mass strength and deformability in the laboratory by simulating joints in sandstone specimens. The results are used to assess the predictive capability of rock mass strength criteria developed by Hoek-Brown, Ramamurthy-Arora, Yudhbir and Sheorey. Empirical criteria developed by Goodman, Yoshinaka and Yamabe and Ramamurthy criteria are used to predict the deformation modulus of rock mass. Triaxial compressive strength tests have been performed on cubical sandstone specimens with nominal dimensions of 60×60×60 mm 3 and 80×80×80 mm 3 .A true triaxial load frame is used to apply confining pressures up to 12 MPa. The joints are simulated by saw-cut surface. The compressive strengths and deformation modulus of rock specimens decrease exponentially with increasing joint frequency. Under the same joint frequency the deformation modulus tends to increase with confining pressure. When the major principal stress is normal to the joint planes the specimen strength is greatest and the deformation modulus is lowest. Hoek-Brown strength criterion can effectively describe the specimen strengths under all configurations. The m and s parameters decrease rapidly with increasing joint frequency and joint set number. Ramamurthy criterion can best describe the deformation modulus of rock specimens under all configurations.

Proceedings Papers

Paper Number: ISRM-ARMS8-2014-275

... additional data between measurements and regulate the input to constant time intervals. Secondly, statistical techniques are used to weed out predicted data points that are outside preset parameters. Short-term predicted values that satisfy with clear statistical

**criteria**(low co-variance or low standard...
Abstract

Abstract Earth-fill structures such as embankments, which are constructed for the preservation of land and infrastructure, show significant amount of settlement during and after construction in lowland areas. The long term settlement of those structures is often measured. In this paper, we examined the applicability of a neural network model for settlement prediction using measurements in the early stage after construction. Simulations using a basic network model showed that when the measurement data used for teaching the neural network accumulated, the prediction was in good agreement with the measurement data, and the variance of predicted values was low. However, the basic model could not predict the settlement behaviour precisely, when the amount of teach data was limited as would be in the early stage after construction. Some improvement was necessary for this model to conduct early settlement prediction. To achieve a higher accuracy in long term settlement prediction from early stage measurements, several improvements to the model are proposed, which generate additional data points and improve the prediction accuracy. Firstly, a cubic spline interpolation technique is used to generate additional data between measurements and regulate the input to constant time intervals. Secondly, statistical techniques are used to weed out predicted data points that are outside preset parameters. Short-term predicted values that satisfy with clear statistical criteria (low co-variance or low standard deviation) are added to the network teach data. The improved network model simulations showed that the accuracy of settlement prediction based on early stage measurements improved significantly.

Proceedings Papers

Paper presented at the ISRM Regional Symposium - 7th Asian Rock Mechanics Symposium, October 15–19, 2012

Paper Number: ISRM-ARMS7-2012-001

... ABSTRACT In this paper, several new developments regarding the failure, fracture and fragmentation of rocks will be discussed. The first topic discussed is the development of true-triaxial failure

**criteria**that involve all three principal stresses. The next topic is a new approach to modelling...
Abstract

ABSTRACT In this paper, several new developments regarding the failure, fracture and fragmentation of rocks will be discussed. The first topic discussed is the development of true-triaxial failure criteria that involve all three principal stresses. The next topic is a new approach to modelling the propagation of cracks and fractures using accurate local calculations of the stress intensity factor. Lastly, a method for fracture-driven rock fragmentation with a velocity-dependent propagation law is discussed.

Proceedings Papers

Paper presented at the ISRM Regional Symposium - 7th Asian Rock Mechanics Symposium, October 15–19, 2012

Paper Number: ISRM-ARMS7-2012-117

... index and lining uniaxial compressive strength have the most significant effects on the convergence and lining load

**criteria**, respectively. Increase of 0.2 in the coefficients of variation of geological strength index and lining uniaxial compressive strength can result in about 30 and 8% increase in the...
Abstract

SYNOPSIS This paper has illustrated the application of reliability analysis to the problem of a lined circular tunnel excavated in a Hoek-Brown medium with isotropic in situ stress filed. The uncertainties in the values of in situ stress and mechanical properties of the rock mass and lining are considered in the analysis. Tunnel convergence and lining load are regarded as critical factors which indicate the acceptable or unacceptable performance of the system. It is shown that deterministic methods can lead to overestimation of safety level while reliability analysis can evaluate the system more realistically. The higher values of probability of failure using convergence criterion relative to those of lining load criterion indicated that the former criterion is more vulnerable to uncertainties. Changing the coefficient of variation of the parameters revealed that geological strength index and lining uniaxial compressive strength have the most significant effects on the convergence and lining load criteria, respectively. Increase of 0.2 in the coefficients of variation of geological strength index and lining uniaxial compressive strength can result in about 30 and 8% increase in the probability of failure, respectively. In addition, the observed relationship between the coefficient of variation of effective parameters and probability of failure was nonlinear in most cases.

Proceedings Papers

Paper presented at the ISRM Regional Symposium - 7th Asian Rock Mechanics Symposium, October 15–19, 2012

Paper Number: ISRM-ARMS7-2012-138

... point estimate method reservoir geomechanics reserves evaluation institute technology Kramadibrata soft rock slice method engineering department Rock mechanics Symposium Hoek FP value stability curve probability Reservoir Characterization drilling operation management

**criteria**...
Abstract

ABSTRACT In 2011 Kramadibarata et al. curve slope stability has been developed using dimensional analysis as applied to open pit coal. In principle, the stability curve has been able to answer the open pit slope stability conditions of coal in Indonesia are more influenced by the relatively high rainfall, resulting in decreased strength (deterioration) in the rock. Other factors are also considered in the stability curve is the change in dimensions, the orientation of the field discontinuities and shear strength characteristics of rocks. However, these curves have not been able to explain the failure failure probability (FP) slope due to the variation in the factors to be input parameters. The method used in determining the FP value of slope is a point estimate method (PEM). The results showed that FP values of the stability curve of 10.2% is more pessimistic than the curve Hoek & Bray (0.1%) and Bishop's slice method (0.4%). These results provide an indication that the slope stability is not just rely on the safety factor (SF), but should also look at the failure probability (FP).

Proceedings Papers

Paper presented at the ISRM International Symposium - 6th Asian Rock Mechanics Symposium, October 23–27, 2010

Paper Number: ISRM-ARMS6-2010-127

... fields), we have an upper bound for ultimate failure load. So that if this loads affect the structure, it will certainly collapse. It is quite difficult to directly introduce

**criteria**into the plastic limit analysis. A numerical technique based on the nonlinear mathematical programming will be developed...
Abstract

Abstract: Conventional calculations of ultimate bearing capacity of shallow foundations are based on linear Mohr-Coulomb failure criterion. However, experimental data show that the strength envelops of almost all types of rocks are nonlinear over wide range of normal stresses. In this paper, the strength envelope of rock mass is considered to follow Hoek-Brown failure criterion. Hoek-Brown failure criterion introduced into limit analysis theorem. The plastic dissipation power in terms of kinematically admissible velocity fields and a nonlinear optimization formulation is obtained. Then by implementing the acquired formulation into a using nonlinear finite element technique code, the failure mode and the ultimate bearing capacity of a strip footing on a rock mass is calculated. The numerical results are compared with existing limit analysis solutions. 1. INTRODUCTION Assessment of ultimate bearing capacity of shallow foundations is one of the most common problems in civil engineering. The methods used for assessing the ultimate bearing capacity of shallow foundations mainly fall within one of the four categories: the limit equilibrium method, the slip-line method, the limit analysis method, and the numerical methods. To obtain the maximum load of rock mass under static loading, a step-by-step method based on traditional elastic-plastic analysis is commonly used. However, this incremental approach is often too cumbersome to use in practice because it requires a complete specification of the stress-strain relation and the nonlinear material properties of a rock mass. Therefore, other direct methods such as the slip-line method, limit equilibrium method and limit analysis applied to determine the plastic limit state of a continuous media. [1] 2. LIMIT ANALYSIS BASED ON A GENERAL YIELD CRITERION For simplification the stability calculations, with using the plastic theorems of soils, we can ignore some of the equilibrium or compatibility equations. Then, instead of unique answer, we have a bound for ultimate loads. If we ignore the equilibrium equations (admissible stress fields), we have an upper bound for ultimate failure load. So that if this loads affect the structure, it will certainly collapse. It is quite difficult to directly introduce criteria into the plastic limit analysis. A numerical technique based on the nonlinear mathematical programming will be developed to perform kinematic limit analysis for these yield criteria. [2] 2.1. Mohr-coulomb criterion: The expression (1) can be regarded as a general yield criterion for frictional materials. 3. KINEMATIC THEOREM OF LIMIT ANALYSIS An upper bound to the plastic limit load of a structure can be obtained by using the kinematic theorem of limit analysis. The kinematic theorem states: among all kinematically admissible velocities, the real one yields the lowest rate of plastic dissipation power.[1] 4. APPLICATION AND NUMERICAL RESULTS Optimization of equation 5 is too difficult by means of variation calculus method therefore a numerical method is needed. The finite element method is applied to perform the numerical calculation for the kinematic limit analysis. 4.1. DEFINITION OF PROBLEM This problem involves finding bearing capacity of a strip footing on a homogenous rock mass.

Proceedings Papers

Paper presented at the ISRM International Symposium - 6th Asian Rock Mechanics Symposium, October 23–27, 2010

Paper Number: ISRM-ARMS6-2010-133

... decision-making process is inevitable with regard to many geological and geotechnical variables and uncertainties and also unpredictable ground conditions during tunneling operation. So, use of an efficient decision making technique which can take into account several

**criteria**in such conditions is...
Abstract

Abstract: Tunneling method selection is a critical point and a strategic issue in the planning stage of a tunneling project. Compatibility of a selected tunneling method with the geological and geotechnical conditions is crucial to success of a tunneling project. On the other hand, use of a decision-making process is inevitable with regard to many geological and geotechnical variables and uncertainties and also unpredictable ground conditions during tunneling operation. So, use of an efficient decision making technique which can take into account several criteria in such conditions is necessary. This paper develops an evaluation model based on the analytic hierarchy process (AHP) and the technique for order performance by similarity to ideal solution (TOPSIS), to help the engineers in tunneling industries for the selection of optimal tunneling methods in a fuzzy environment. The fuzzy-AHP is used to determine global weights of the criteria and fuzzy TOPSIS method is used to obtain final ranking of alternatives. This approach is applied to select optimal tunneling method among six tunneling techniques including open TBM, double shield TBM, single shield TBM, roadheader, drilling & blasting and hydraulic hammer in Zagros water transfer tunnel of Iran. Finally, double shield TBM was selected as optimal tunneling method in Zagros water transfer tunnel of Iran. 1. Introduction Selection of the proper tunneling methods for use in various ground conditions is a very delicate and important part of planning a tunneling project. This is because this decision is almost irreversible and if the selective tunneling method be not suitable for the ground conditions, it can cause major delays, could be detrimental to the safety of the crew and personnel, and ultimately could bring the project to a halt. Obviously, accurate characterization of the ground using surface and subsurface investigation will allow the designers to foresee the potential problems and select a method that can cope with the anticipated conditions. This often means selecting a method that can offer optimum performance in the given conditions, and sometimes adding special features and devices that could provide the flexibility and capability to mitigate adverse ground conditions. In brief, selection of a proper tunneling method with the right technical specifications and functionalities is important to ensure the speedy, safe, and successful completion of the tunnel project within the requirements of the contract. However, by its very nature, subsurface investigation and ground characterization can be all but certain. There are multiple levels of uncertainty involved in ground characterization starting from the location of the borings, formations or layers that could be missed in the drilling program, proper logging and characterization of the lithotypes, accurate measurement of the rock properties in the field and laboratory testing, issues of groundwater and their sources, in situ stresses, and so on [1]. This indicates the need for additional tools for assisting the decision making in the design and construction stage. Typical tools at the engineer's disposal are statistical methods, systematic risk assessment methods and, more recently, artificial intelligence (AI) approaches such as neural networks and fuzzy logic.

Proceedings Papers

Paper presented at the ISRM International Symposium - 6th Asian Rock Mechanics Symposium, October 23–27, 2010

Paper Number: ISRM-ARMS6-2010-103

.... Compared to the other two failure

**criteria**, the Inscribed Drucker-Prager and the Mohr-Coulomb are conservative as they predict higher well collapse pressure than the other two**criteria**, i.e. the Modified Lade and the Circumscribed Drucker- Prager. On the other hand, the latter provides very optimistic...
Abstract

Synopsis: Major factors influencing critical well bore pressures are summarized and used in a study in which well collapse pressure is calculated using the Mohr-Coulomb, the Modified Lade, and two versions of the Drucker-Prager criterion. Both linear and nonlinear rock models are considered. Compared to the other two failure criteria, the Inscribed Drucker-Prager and the Mohr-Coulomb are conservative as they predict higher well collapse pressure than the other two criteria, i.e. the Modified Lade and the Circumscribed Drucker- Prager. On the other hand, the latter provides very optimistic predictions that are significantly below predictions given by the other three criteria. Thus, the outer Drucker-Prager seems to be significantly underestimating well collapse pressure. In addition, the collapse pressure obtained using the modified Lade criterion is lower than the corresponding pressure obtained from the Mohr-Coulomb criterion, which – in turn – is below the critical well pressure obtained from the Inscribed Drucker-Prager criterion. Linear elastic collapse pressures are above the corresponding critical well pressures obtained when nonlinear material properties are taken into account. The effect of wellbore inclination on stability is also discussed. 1. INTRODUCTION Critical well bore pressures are commonly used in petroleum engineering design and monitoring of drilling and production operations to minimize the risk of excessive costs incurred as a result of wellbore instability incidents. Wellbore instabilities receive special attention due to their huge economic impact. The related downtime associated with well construction is about 10–15% of total well cost or about 50% of total nonproductive time [1]. Among many factors that typically enter wellbore stability studies such as in-situ state of stress, pore pressure regime, rock mechanical and constitutive properties, the wellbore direction and inclination, and – sometimes – thermal effects and chemical factors, the accuracy of critical wellbore pressure predictions will depend strongly on the correctness of the failure criterion used and the possibilities are many. Well drilling disturbs the natural stress state in the rock, causes stress redistributions and produces stress concentrations at or near the borehole wall. This may potentially lead to different types of hole problems such as stuck pipe, borehole collapse, etc. Mechanical wellbore stability is discussed in terms of critical wellbore pressures that are related to rock stresses around a wellbore: sz (axial), sr (radial), and sq (hoop), Fig. 1. It is defined by specifying critical well pressures or the mud weight window, i.e. the lower and upper limit of drilling mud weight that can be used for safe drilling of a new well. The lower limit is associated with shear failure (borehole breakouts, Fig. 1, are also evidence of shear failure) and calculated using the rock failure criteria. The upper limit corresponds to rock fracturing when tensile rock failure occurs, i.e. the tangential stress at the wellbore wall exceeds the tensile strength To of the rock, sq >To. Well fracturing pressure is calculated from this criterion. Exceeding the upper wellbore pressure may lead to lost circulation but sometimes is done intentionally as in well fracturing for stimulation or in-situ stress measurements [2].

Proceedings Papers

Paper Number: ISRM-ARMS6-2010-019

... all levels of σ3, showing the mean misfit = 15.4 MPa. The Coulomb and Hoek and Brown

**criteria**cannot describe the salt strengths beyond the condition where σ2 = σ3, as they cannot incorporate the effects of σ2. Both circumscribed and inscribed Drucker-Prager**criteria**severely underestimate σ1 at...
Abstract

Abstract: True triaxial compressive strengths of Maha Sarakham (MS) salt are determined by using a polyaxial load frame. The salt specimens are cut and ground to obtain rectangular blocks with a nominal dimension of 5.4×5.4×10.8 cm 3 . The load frame equipped with two pairs of cantilever beams is used to apply the constant lateral stresses (σ2 and σ3) to salt specimen while the axial stress (σ1) is increased at 0.5–1.0 MPa/s until failure occurs. The deformations induced along the three loading directions are monitored and used to calculate the tangent elastic modulus and Poisson's ratio of the salt. The results indicate that the elastic modulus and Poisson's ratio of the MS salt are averaged as 21.5±2.6 GPa and 0.40±.04. For the Coulomb criterion the internal friction angle determined from the triaxial loading condition (σ2= σ3) is 50°, and the cohesion is 5 MPa. The effect of σ2 on the salt strengths can be best described by the modified Wiebols and Cook criterion with the mean misfit = 3.5 MPa. The empirical (power law) Mogi criterion tends to underestimate the salt strengths particularly under high σ3 values. The modified Lade criterion overestimates the actual strengths at all levels of σ3, showing the mean misfit = 15.4 MPa. The Coulomb and Hoek and Brown criteria cannot describe the salt strengths beyond the condition where σ2 = σ3, as they cannot incorporate the effects of σ2. Both circumscribed and inscribed Drucker-Prager criteria severely underestimate σ1 at failure for all stress conditions, showing the largest mean misfit of 19.5 and 34.7 MPa, respectively. INTRODUCTION The effects of confining pressures at great depths on the mechanical properties of rocks are commonly simulated in a laboratory by performing triaxial compression testing of cylindrical rock core specimens. A significant limitation of these conventional methods is that the intermediate and minimum principal stresses are equal during the test while the actual in-situ rock is normally subjected to an anisotropic stress state where the maximum, intermediate and minimum principal stresses are different (σ1 ≠ σ2 ≠ σ3). It has been commonly found that the compressive strength obtained from conventional triaxial testing cannot represent the actual insitu strength where the rock is subjected to an anisotropic stress state [1–6]. From the experimental results on brittle rocks obtained from Haimson [2], Colmenares & Zoback [7], it can be generally concluded that in a σ1 - σ2 diagram, for a given σ3, σ1 at failure initially increases with σ2 to a certain magnitude, and then it gradually decreases as σ2 increases. The effect of σ2 is more pronounced under higher σ3. This states that the intermediate principal stress confines the rock in such a way that fractures can only be initiated and propagated in the direction parallel to σ1 and σ2. The effect of σ2 is related to the stress-induced anisotropic properties, and the end effect at the interface between the rock surface and loading platen in the direction of σ2 application.

Proceedings Papers

Paper presented at the ISRM International Symposium - 5th Asian Rock Mechanics Symposium, November 24–26, 2008

Paper Number: ISRM-ARMS5-2008-164

... [12–15] are to name a few. There are also some theoretical fracture

**criteria**for assessing the load bearing capacity of brittle cracked materials under mixed mode loading. Reservoir Characterization structural geology BD specimen Wellbore Design**criteria**hydraulic fracturing criterion...
Abstract

ABSTRACT Combined opening-sliding fracture or mixed mode I/II crack growth in rock materials is one of the main origins of collapse and catastrophic failure in rock structures. Some experimental and theoretical methods have been proposed in the past by researchers for investigating mixed mode fracture behaviour of rocks. Among these techniques, the cracked Brazilian disc (BD) specimen and the maximum tangential stress (MTS) criterion are the most commonly used experimental and theoretical methods, respectively for mixed mode fracture study of rock materials. However, the reported test data for mixed mode fracture toughness of rocks tested with the BD specimen do not comply with the theoretical curve of MTS criterion. The experimental fracture toughness data are always above the MTS curve. Therefore, a modified form of MTS criterion is employed in this paper for predicting the BD test results. The application of modified criterion is then examined for two rock materials namely: Italian light marble and Saudi Arabian limestone. It is shown that the modified criterion which uses the effects of non singular stress term along the boundary of fracture process zone can provide much better predictions than the MTS criterion. Introduction Investigation of strength and stability in cracked rock structures is an important task for rock mechanics designers and researchers. For example, rock burst or rock fragmentation is often considered as a process of crack formation and propagation in rock masses. Fracture toughness is the most important parameter for describing the material resistance against the fracture initiation and propagation. Therefore, evaluation of fracture toughness is essential for understanding the mechanical behavior of bodies containing cracks or flaws. Most of the previous investigations on fracture toughness of rocks have focused on mode I (or opening mode) cracks. Hence, the international society for rock mechanics (ISRM) has suggested some standard procedures and test methods for determining the fracture toughness ( K Ic) of rock materials under pure tensile loading [1,2]. However, in practice the pre-existing cracks and discontinuities in rock structures like dam foundations, tunnel walls and rock slopes are often subjected to complex loading. For such cases, due to arbitrary orientation of flaws relative to the overall applied loads, cracks experience a combined mode I and II deformation. Fig. 1, shows the mode I and mode II crack deformations schematically. Several studies have been conducted in the past to determine the mixed mode fracture toughness of rocks. Some experimental methods and different test configurations have been used for determining mode I, mode II and various mode I/II mixtures. The cracked beam specimen subjected to asymmetric four point bend loading [3–5], the compact tension- shear specimen [6–8], 1340 the edge cracked semi circular specimen subjected to three point bending [9–11] and the Brazilian disc specimen under diametral compression [12–15] are to name a few. There are also some theoretical fracture criteria for assessing the load bearing capacity of brittle cracked materials under mixed mode loading.

Proceedings Papers

Paper presented at the ISRM International Symposium - 5th Asian Rock Mechanics Symposium, November 24–26, 2008

Paper Number: ISRM-ARMS5-2008-016

... calculated for the AFPB specimens of different crack lengths. It is shown that the obtained mixed mode fracture data can be predicted well by means of the available fracture

**criteria**. The well known maximum tangential stress criterion gives better. 1. Introduction Cracks, flaws, natural fractures and...
Abstract

ABSTRACT In this research, mixed mode crack growth behaviour of Dehbid marble is assessed by means of the cracked beam specimen under asymmetric four-point bend (AFPB) loading. The specimen is a suitable configuration for testing rocks and geo-materials. The stress intensity factors are also calculated for the AFPB specimens of different crack lengths. It is shown that the obtained mixed mode fracture data can be predicted well by means of the available fracture criteria. The well known maximum tangential stress criterion gives better. 1. Introduction Cracks, flaws, natural fractures and inherent discontinuities are frequently found in rock materials and structures. In many practical applications, such as tunnelling, mining, excavation process, blasting, drilling the gas and oil wells and stability analysis of rock slopes, the evaluation of crack growth is an important task for rock mechanics engineers. While, most of the previous rock fracture studies have concentrated only on crack opening mode (tensile fracturing), the cracked rock structures in practice, are usually subjected to complex loading and hence the fracture of rock masses may occur under a combination of tension-shear (mixed mode I/II) loading. For example, cracks and natural fractures surrounding the mines or tunnel walls are usually located in arbitrary directions with respect to the far field applied loads. Consequently, these cracks may grow catastrophically under the activation of both crack opening and crack sliding modes of deformation. Therefore, it is important to investigate mode I, mode II and mixed mode fracture in rock structures and geo-materials. Based on the principals of fracture mechanics, fracture toughness which defines the resistance and the load bearing capacity of cracked materials is the most important parameter for characterizing the fracture process. There are some theoretical and experimental methods for evaluating the fracture behaviour under mixed mode I/II loading conditions. Fracture toughness of cracked materials is generally investigated by means of suitable laboratory test specimens. These specimens often are in the form of disc, plate and beam shapes having either edge or internal cracks and loaded by various types of fixtures [1–10]. Some of the mixed mode fracture configurations are shown in Fig. 1. The edge cracked rectangular beam subjected to asymmetric four-point bend (AFPB) loading is one of the favourite test configurations frequently used by researchers investigating mixed mode fracture in rock materials [e.g. 2, 3]. The simple geometry and loading set up, the convenient pre-cracking of specimen and the ability of introducing full mode mixities (from pure mode I to pure mode II) are among the advantages of AFPB specimen. Additionally, the specimen is loaded by a compressive type of load which is more suitable for testing brittle and quasi brittle materials like rocks. In this paper, a combined numerical and experimental study using the AFPB specimen is presented for mixed mode loading of an Iranian marble (Dehbid marble).

Proceedings Papers

Paper presented at the ISRM International Symposium - 5th Asian Rock Mechanics Symposium, November 24–26, 2008

Paper Number: ISRM-ARMS5-2008-017

... radius, the angle of pure shear deformation varies between 20o and 35o. The shear fracture test data obtained for a rock material using the proposed sample is in good agreement with the classical fracture

**criteria**. The ratio of shear fracture resistance to tensile fracture resistance in the tested...
Abstract

Abstract A hollow centre cracked disc specimen is proposed for determining shear fracture resistance of rocks. By setting the crack line in an appropriate direction relative to the applied load, pure shear is provided along the crack line. Depending on the crack length and the centre-hole radius, the angle of pure shear deformation varies between 20o and 35o. The shear fracture test data obtained for a rock material using the proposed sample is in good agreement with the classical fracture criteria. The ratio of shear fracture resistance to tensile fracture resistance in the tested specimen is about 0.84. This figure is in the range of 0.63 to 1 predicted by the three basic fracture criteria namely the maximum tangential stress criterion, the minimum strain energy density criterion and the maximum 1. Introduction The study of shear strength and deformability of rocks and rock like materials is among the favourite subjects for rock mechanics engineers and the earth science researchers. Shear deformation leads to mode II fracture that sometimes takes place in rock masses at great depth of earth which are usually under confined pressures and compressive forces. When other possible modes of deformation like opening or tensile rupture are suppressed, the in-plane sliding of rock mass layers may lead to large scale deformation and consequently large amounts of energy release. For example, in phenomena like earthquake and rock-burst the shear mechanism is the predominant cause of rupture in rocks. Meanwhile, in other engineering applications like the construction of dams, tunnels, abutments, mines and underground structures, the strength evaluation of rock structure against shear stresses becomes important. Therefore, it is necessary to study the shear failure behavior of rock masses using suitable theoretical methods and laboratory experiments. In rock mechanics, the shear characteristics of rocks are traditionally evaluated by means of Coulomb theory. In this criterion it is assumed that the loaded rock mass is primarily intact and the shear failure occurs along weak surfaces. However, rock masses very often contain a large number of joints, cracks, natural fractures, faults and other discontinuities. These cracks act as stress concentrators and hence can govern the fracture process. Therefore, for evaluating the failure behaviour of real rock structures, the use of more mechanistic methods like fracture mechanics approaches is expected to give more reliable results than traditional techniques. In fracture mechanics based methods, it is assumed that the overall failure of a cracked body initiates from the tip of pre-existing cracks. Accordingly, fracture resistance (or fracture toughness) of a cracked rock is the most important parameter for evaluating the critical load bearing capacity of cracked rocks. Based on the concepts of fracture mechanics, shear fracture resistance is the critical sustainable strength of a cracked body that is subjected to pure shear stresses along the crack line. Fig. 1 shows the pure shear crack deformation, schematically. Some laboratory test methods have been proposed in the past for determining shear fracture resistance of cracked rocks.

Proceedings Papers

Paper Number: ISRM-ARMS5-2008-060

... in situ stresses and in expressions involving induced stresses. The internal friction angle (φ) for rock masses can be determinable using Geological Strength Index - GSI (Hoek et al, [6]). Originally, the GSI was developed for the Hoek-Brown failure

**criteria**, but knowing the Hoek-Brown material...
Abstract

Abstract Knowing the Poisson's rate value of the rock mass is one of the an important rock mechanical and rock engineering parameters. This value has to be used for calculating the deformations of the tunnel or the dam, among the others. Unfortunately, measuring this value is very difficult and time consuming for rock masses. The goal of this paper is to present a simple method for estimating the Poisson's rate value for rock masses if it is known for the intact rock. A linear equation was found: decreasing the quality of the rock mass, the Poisson's rate is increasing. The presented calculation is also good, if the Poisson's rate of the intact rock is not determinable. Note, the suggested equation is not valid if any other horizontal stress is existing. 1. Introduction The Poisson's rate value of the intact rock can be relatively easily measured. It is used for several rock mechanics and rock engineering calculations, necessary to know it for the in situ measurements. In Figure 1, typical ranges of values are presented for Poisson's ratio of some rock types (after the collection of Gercek [1]). Generally, Poisson's ratio of intact rocks can be determined in the laboratory either indirectly by dynamic methods or directly by static tests. The behavior of rock masses are influenced by the mechanical behavior and properties of the discontinuities and those of the intact rock bounded by discontinuities. It has also been well known that structural features induce some degree of anisotropy in rock masses. It was found, that the value of Poisson's ratio for the rock mass was found to be about 20 % higher than the value for the intact rock [2]. Note, this paper did not classify the rock mass according to one of the widely used rock mass classification system. Unfortunately, measuring the Poisson's rate value of the rock mass is very difficult, so usually the intact rock's is used for the calculations. 2. Calculation method of the Poisson'svalue of the rock mass In rock engineering applications involving underground openings, Poisson's ratio of the rock mass is utilized for estimating in situ stresses and in expressions involving induced stresses. The internal friction angle (φ) for rock masses can be determinable using Geological Strength Index - GSI (Hoek et al, [6]). Originally, the GSI was developed for the Hoek-Brown failure criteria, but knowing the Hoek-Brown material constants, the Mohr-Coulomb parameters (namely the internal friction angle, φ and the cohesion, c) can be also calculated. 2.1. Geological Strength Index (GSI) The strength of a jointed rock mass depends on the properties of the intact rock pieces and also upon the freedom of these pieces to slide and rotate under different stress conditions. This freedom is controlled by the geometrical shape of the intact rock pieces as well as the condition of the surfaces separating the pieces.

Proceedings Papers

Paper Number: ISRM-ARMS5-2008-086

...

**criteria**). -Analytical methods (e.g. Duncan-Fama approach (1993) using Mohr-Coulomb**criteria**). In order to estimate the instability of each section of the tunnel, the first step is to determine that either the rock mass should be considered as continuum or dis-continuum material. Reservoir...
Abstract

ABSTRACT Beheshtabad Water Transmission Tunnel is going to be driven in the Iranian central plate with an approximate length of 65 kilometers. There are several zones of weak rocks in its rout. Some of these zones are situated at great depths so that the squeezing phenomenon may occur during the tunnel exaction period. In this paper the mechanical behavior of weak rocks are studied and their effects on the stability of tunnel are investigated. Several empirical, theoretical and numerical methods are considered to model the time dependence behavior of the weak rocks. Some of these methods are used to study the time dependent behavior of the rocks surrounding this tunnel. The computed results are compared for different zones of weak rocks with potential of squeezing. These results are tabulated in tables and graphically shown in some related figures of the text. Based on these results, it is concluded that there may be a potential of squeezing causing instability in some part of the tunnel rout during its excavation. 1. Introduction Population growth, decreasing the level of underground water resources and concentration of vital industries in the central part of Iran demands a high amount of water to be transmitted to this region. Therefore, projects of water transmission to Zayande-rood (the Zayand River) are some practical and effective ways to convey the water from the West and North West of Iran to its central regions. The Beheshtabad tunnel project is planning to drive a tunnel of about 64.93 kilometer in length in order to bring water from Karoon's water resources to the Iran's central plate. This tunnel (with a SW-NE trend) will start from Darkesh-Varkesh Valley near the Ardal city and finishes at the Se-Cham Asman. This paper briefly discusses about the squeezing phenomenon associated with some part of the tunnel. Squeezing describes the reduction of the cross section of the tunnel during its excavation stages. It's due to the time dependent deformation of the surrounding rock mass resulted from the redistribution of the stresses in an elasto-visco-plastic medium. Three methods are usually used to evaluate the squeezing behavior of the surrounding rocks in a tunnel excavation sequence: empirical method, Semi-analytical method and Theoretical- Analytical method. In this study, the following methods are used to estimate the squeezing of Beheshtabad tunnel and the corresponding results are compared with each other: -Empirical methods (e.g. Geol (1994)'s approach based on N value). -Semi-analytical methods (e.g. Hoek and Marinos (2000)'s approach). -Analytical methods (e.g. Carranza-Torres & Fairhurst approach (2000) using Hoek-Brown criteria). -Analytical methods (e.g. Duncan-Fama approach (1993) using Mohr-Coulomb criteria). In order to estimate the instability of each section of the tunnel, the first step is to determine that either the rock mass should be considered as continuum or dis-continuum material.