Rockburst laboratory tests database - Application of Data Mining ...

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Rockburst laboratory tests database - Application of Data Mining techniques

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He Manchaoa, L. Ribeiro e Sousaa,b, Tiago Mirandac & Zhu Gualonga

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State Key Laboratory for Geomechanics and Deep Underground Engineering, Beijing, China b

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University of Porto, Porto, Portugal

University of Minho, Guimarães, Portugal

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Abstract: Rockburst is characterized by a violent explosion of a certain block causing a sudden

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rupture in the rock and is quite common in deep tunnels. It is critical to understand the phenomenon

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of rockburst, focusing on the patterns of occurrence so these events can be avoided and/or managed

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saving costs and possibly lives. The failure mechanism of rockburst needs to be better understood.

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Laboratory experiments are undergoing at the Laboratory for Geomechanics and Deep

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Underground Engineering (SKLGDUE) of Beijing and the system is described. A large number of

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rockburst tests were performed and their information collected, stored in a database and analyzed.

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The statistical analysis of the tests permitted to better understand the phenomena of rockburst and a

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special rockburst index is proposed. Data Mining (DM) techniques were applied to the database in

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order to determine and conclude on relations between parameters that influence the occurrence of

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rockburst. Final considerations and conclusions are established.

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Keywords: Rockburst; Experimental tests; Data Mining; Rockburst index.

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1 Introduction

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A large numbers of accidents and other associated problems occur during construction and

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exploration of underground structures, and are very often related to uncertainties concerning ground

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conditions. It is essential to develop and implement risk analysis procedures to minimize their

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occurrence. Risk has a complex nature resulting from the combination of two sets of factors, the

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events and the corresponding consequences, and the vulnerability factors that determine the

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probability of an event having certain consequences. Risk analysis underlines the fact that decision-

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making must be based on a certain level of uncertainty (Einstein, 2002; Sousa, 2010; Feng et al.,

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2012a).

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Many researchers have collected analyzed and published reports on accident cases in tunnels

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during construction and exploration (HSE, 1996; Vlasov et al., 2001; Sousa, 2006; 2010). In the

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study conducted by Sousa (2010), data on accidents were collected from the technical literature,

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newspapers and correspondence with experts in the underground domain. The data were stored in a

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database and analyzed, and the accidents were classified into different categories, providing an

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evaluation on their causes and their consequences. The main goal was to determine the major

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undesirable events that may occur during tunnel construction, their causes and consequences and

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ultimately present mitigation measures to avoid accidents on tunnels during construction. Different

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types of events were identified and classified (Rock Fall, Collapse and Daylight Collapse,

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Rockburst, Excessive Deformation, Water Inflow, Fires and Explosions). The accidents can cause

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loss of lives, equipment damage and damage to the tunnel structure that may lead to collapse.

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In deep underground structures, rockburst is a frequent type of event caused by the overstressing

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of the rock mass or intact brittle rock, when stresses exceed the local compressive strength of the

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material. It can cause spalling or in the worst cases, sudden and violent failure of the rock mass.

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Rockbursts can cause serious, and often fatal, injuries. They are mainly dependent on the stress

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exerted on the rock, which increases with depth. Rockburst is also common in deep underground

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mines. This phenomenon can also occur in tunnels for transportation systems and hydroelectric

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projects (Sousa, 2012a).

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Underground construction works impose risks for all parts involved in the project and also to

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those not directly involved in the project. The nature of tunnel projects implies that any potential

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tunnel owner will be facing considerable risks when developing such a project. Due to the inherent

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uncertainties, including ground and groundwater conditions, there might be significant cost overrun,

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delays as well as environmental risks. Also, as demonstrated by tunnel collapses and other disasters

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in the recent past, there is a potential for large scale accidents during tunneling works. Furthermore,

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for tunnels in urban areas there is a risk of damage to third party persons and properties, which will

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be of particular concern where heritage buildings are involved. Finally, there is a risk that the

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problems caused by tunneling will give rise to public protests affecting the course of the project

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(Eskesen et al., 2004; Popielak and Weining, 2010).

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Traditionally, risks have been managed indirectly through the engineering decisions taken during

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the project development. Risk management processes can be significantly improved by using

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systematic techniques throughout the tunnel project development. By the use of these techniques

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potential problems can be clearly identified such that appropriate risk mitigation measures can be

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implemented in a timely manner. As a result, risk management became an integral part of most

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underground construction projects during the late 1990s, particularly in European countries.

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However, from discussion, it became clear that handling and management of risks were performed

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in many different ways, some more concise than others.

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Risk assessment is developed with the goal of avoiding major problems that can occur in the

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underground structures. There are many definitions for risk assessment. More generally for an

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undesirable event with different consequences, vulnerability levels are associated and the risk can

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be defined (Einstein, 2002; Sousa, 2010; He et al., 2011). For risk evaluation it is necessary to

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identify the models to be used to represent the existing knowledge and perform risk and decision

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analysis. Risk assessment and management requires an evaluation of the hazard and the likelihood

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of the harmful effects. It starts with the hazards identification, focusing on the likelihood of damage

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extent, followed by risk characterization, which involves a detailed assessment of each hazard in

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order to evaluate the risk associated to each one of them (Sousa, 2010).

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For deep underground engineering rockbursts is one of the most important accidents. They are

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not easy to predict. Rockburst danger assessment is therefore a very important task and the major

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topic of this paper. Section 2 analyzes the situation regarding the occurrence of rockburst and its

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particular relevance in mining activities, and in the Jinping II hydroelectric scheme, in China.

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Laboratory experiments are one of the ways to analyze the rockburst phenomenon. Section 3

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presents a description of a unique laboratory system developed at the SKLGDUE, Beijing, of China

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University of Mining and Technology, permitting to evaluate geomechanical properties of the rock

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related to rockburst phenomenon in deep underground projects. Several cases of rockburst tests

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performed in China and other countries were collected, stored in a database and analyzed. The

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analysis of the tests allowed one to list factors that interact and influence the occurrence of

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rockburst, as well as the relation between them. Section 4 presents the construction and the

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organization of the database, as well statistical results and a rockburst index is proposed and

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analyzed in detail. In section 5, the results of the application of DM techniques to the database are

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presented. The goal was to determine and conclude on relations between parameters that influence

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the occurrence of rockburst during underground construction. Final considerations about the

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rockburst phenomenon and the trends for risk assessment of this subject are presented at section 6.

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2 Rockburst occurrences

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Rockburst is characterized by a violent explosion of a block causing a sudden rupture in the rock

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mass and can be common in very deep tunnels. This phenomenon can occur in tunnels for

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transportation systems, hydroelectric projects and mining operations, and has been more associated

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with mine excavations for a long time. Therefore it is critical to understand rockburst, focusing on

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the patterns of occurrence so these events can be avoided and/or managed saving costs and possibly lives (Camiro, 1995; Kaiser, 2009; Tang et al., 2010).

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All rockbursts produce seismic waves and seismic disturbances. These seismic events have been

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recorded at seismological stations many times hundreds of miles away from the origin (Tang et al.,

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2010). Rockburst first became a recognized problem in the Kolar Fold Field of India in 1898 and by

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the end of 1903, 75 rockbursts had occurred with fatalities and serious injuries at a depth of about

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450 m below the surface. Investigations concluded that rockbursts were caused by great pressures

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on the mine pillars (Blake, 1972). Rockburst accidents were also reported in gold mines of

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Witwaterstrand, South Africa, in the early 1900s at lower depths damaging pillars and other mine

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workings. Rockbursts occur frequently in South Africa, and therefore long-term researches have

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been carried out systematically in the country on the mechanisms of rockbursts. In 1975, 680

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accidents took place in 31 gold mines which claimed a large number of fatalities and loss of

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production (Tang et al., 2010).

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Also in other types of underground structures rockburst have been reported for instance during

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the construction of the Simplon hydraulic tunnel in the Alpes region at depths greater than 2,200 m,

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in the Shimizu tunnel in Japan, for depths between 1,000 and 1,300 m and in the Kanestu tunnel

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constructed mainly in quartz diorite rockburst occurred at an overburden depth between 730 and

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1,050 m (Tang et al., 2010). Norwegian tunneling experience includes a significant number of

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tunnels subjected to high rock stresses. The majority of the problems are associated with spalling

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due to anisotropic stresses below steep valleys. This is found normally in road tunnels along or

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between the fjords under high overburden. An example is the 24.5 km long Laerdal tunnel where

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moderately intense spalling and slabbing was encountered most of the times. In some areas heavy

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rockbursts caused violent ejection of sharp edged rock plates (Sousa, 2010). Rockburst also

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occurred at the Gothard base railway tunnel in Switzerland.

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In China many rockbursts occurred during excavation of high pressure, drainage and auxiliary

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tunnels of the Jinping II hydropower scheme (Figure 1), (Wu et al., 2010; Feng and Hudson, 2011).

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This scheme is composed by four high pressure tunnels, each with 16.67 km in length, 60 m

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spacing between them, two parallel auxiliary tunnels A and B, 17.5 km long with a span of 6 m

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excavated by D&B and a drainage tunnel with about 16.73 km, with a diameter of 7.2 m excavated

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by a Robbins TBM and by Drill & Blast (D&B). The high pressure tunnels nos. 1 and 3 were

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excavated, respectively, by a Robbin TBM and by a Herrenknecht TBM both 12.4 m of diameter.

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The high pressure tunnels nos. 2 and 4 were excavated by D&B method with an equivalent diameter

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of about 13 m. They were excavated in marble, sandstone and slate strata, with a maximum

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overburden up to 2,500 m (Figure 2), (Wu et al., 2010; Wang et al., 2012).

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Fig. 1. Jinping II hydropower scheme.

Fig. 2. Simplified geological profile of the high pressure tunnel no. 1 of Jinping II project (Wang et al., 2012).

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A consulting Workshop took place in 2009 organized by the Chinese Society for Rock

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Mechanics and Engineering about the Jinping II long tunnels. Several reports were elaborated

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mainly focusing rockburst problem (He, 2009; Hudson, 2009; Kaiser, 2009; Qian, 2009; Sousa,

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2009). Conclusions were established and suggestions were made about TBM problems, the

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estimation of in situ stresses, the geometry of the excavations when using D&B method, and

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modelling specially for rockburst prediction. For rockburst the suggestions included the

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establishment of the Rockburst Vulnerability Index (RVI) to be calibrated with the rockburst

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experience (Hudson, 2009), the establishment of a database containing information about rockburst

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and the description of the events, application of DM techniques and development of Bayesian

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Networks (BN) for predicting the probability of occurrence of rockburst, its location, depth and

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width, and time delay for the different types of rockburst (Sousa, 2009).

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During construction different types of rockbursts were observed at Jinping II which permitted to

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describe the mechanism and to settle criteria for rockburst (Feng et al., 2012b; Wang et al., 2012;

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Yan et al., 2012). According to Wang et al. (2012), the statistics of rockburst events is represented

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in Table 1. The drainage tunnel was partly excavated by the TBM until around 6km, and after by

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Drill & Blast (D&B) due to the occurrence of very strong rockbursts (Liu et al., 2011). The

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classification of the rockburst is represented in Table 2, corresponding level I to light rockburst,

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level II to moderate, level III to intensive and level IV to very strong (Peixoto et al., 2011).

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Rockbursts along auxiliary tunnels mainly occurred in the strata T2z and T2b (marbles). The most

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intensive rockbursts in T2b are very strong, intensive in T3 (sandstones), and moderate in the other

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strata. Most rockbursts occurred within 6-12m from the face and 5-20 hours after excavation. For

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the high pressure tunnels, the number and intensity increase in spite a higher percentage of no

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rockburst length as it is presented in Table 1. The fractured face was rough or dome-shaped. Tensile

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and shear failure resulted in wedge or dome-shaped fracture surfaces with a depth that sometimes

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reached 1.6 m. Since the start of the high pressure tunnels, 77 rockbursts of several levels occurred

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at tunnel no. 1; about 200 happened at tunnel no. 2; about 100 at tunnel no. 3; and more than 100 at

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tunnel no. 4. Since the tunnels were excavated in marbles rockburst of levels III and IV occurred

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with the increasing depth. The maximum ejection distance of level IV rockbursts reached 5m and

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the depth of the crater ranged from 3 to 5m. Most intensive rockburst occurred within 10-30 m from

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the working face. The majority of the rockbursts occurred on the north spandrel (1-3 clock position)

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and at the south arch corner (7-10 clock position), mainly due to the principal stresses directions

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and the geological structures (Wang et al., 2012). Table 1 Statistics of rockburst occurrence at Jinping II. Tunnel

Auxiliary A Auxiliary B HP no.1 HP no.2 HP no.3 HP no.4 Total HP

No rockburst (%) 81.01 83.72 92.62 88.39 92.48 87.44 90.10

Level I

Level II

Level III

Level IV

(%) 12.54 10.32 6.76 7.62 7.20 10.52 8.04

(%) 4.13 4.67 0.62 3.36 0.32 1.78 1.62

(%) 1.73 1.12 0.00 0.50 0.00 0.26 0.21

(%) 0.09 0.17 0.00 0.12 0.00 0.00 0.04

HP – High Pressure tunnel

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Table 2. Classification of rockburst as used at Jinping II. Rockburst level I II

Description

Type of sound

Light Moderate (Mild)

Sound of cracking Clear sound of cracking

III

Intensive (Strong) Very strong (excess of loads)

Sound of a strong explosion Sound of an intensive explosion

IV

Characteristics of duration Sporadic explosion Long duration and not progressive with time Fast with increase of overburden Sudden with increase of overburden

Depth of the block (m) 2.0

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There are several mechanisms by which the rock fails, originating rockburst. The main source

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mechanisms are usually associated with local underground geometry of the cavities, structural

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elements like pillars and the existing geology (Ortlepp and Stacey, 1994; Camiro, 1995; He et al.,

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2012a). The representation of potential rockburst phenomena is indicated in Fig. 3. The rockburst is

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usually classified as strainburst, pillar burst or fault slip bursts (Castro et al., 2012; He et al., 2012a).

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These topologies normally occur in large scale mining operations, but, in civil works the most

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common phenomenon is strainbursting, although buckling and face crushing may also occur. Also

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impact-induced rockburst have to be considered for less stressed and deformed rock formations,

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created by blasting, caving and adjacent tunneling.

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Rockburst phenomena have been extensively investigated by many researchers based on in situ

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and laboratory tests and also by theoretical approaches. Laboratory tests play an important role in

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understanding rockburst mechanisms, calibration of numerical models, evaluation of mechanical

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parameters, and identification of the stress state where a dynamical event may be initiated. This

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event can be classified according to the potential damage, scale and violence (He et al., 2012a;

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Wang et al., 2012). In terms of damage the classification referred in Table 2 can be used. In terms

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of scale, rockbursts can be divided into sparse (length of rockburst L≤10 m), large-area

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(10