Rockburst laboratory tests database—Application of data mining techniques PDF

Preview

Summary

1 Rockburst laboratory tests database - Application of Data Mining techniques 2 He Manchaoa, L. Ribeiro e Sousaa,b, Tiago Mirandac & Zhu Gualonga a 3 State Key Laboratory for Geomechanics and Deep Underground Engineering, Beijing, China b 4 University of Porto, Porto, Portugal c 5 University of ...

Description

1

Rockburst laboratory tests database - Application of Data Mining techniques

2

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

3

a

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

4 c

5

University of Porto, Porto, Portugal

University of Minho, Guimarães, Portugal

6

7

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

8

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

9

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

10

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

11

Laboratory experiments are undergoing at the Laboratory for Geomechanics and Deep

12

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

13

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

14

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

15

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

16

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

17

rockburst. Final considerations and conclusions are established.

18 19

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

20 21

1 Introduction

22 23

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

24

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

25

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

26

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

27

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

28

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

29

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

30

2012a).

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

projects (Sousa, 2012a).

49

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

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

63

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

64

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

65

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

66

in many different ways, some more concise than others.

67

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

68

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

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

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

90

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

91

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

92 93

2 Rockburst occurrences

94 95

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

96

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

97

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

98

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

99 100

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).

101

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

102

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

103

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

104

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

105

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

106

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

107

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

108

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

109

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

110

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

111

production (Tang et al., 2010).

112

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

113

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

114

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

115

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

116

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

117

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

118

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

119

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

120

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

121

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

122

occurred at the Gothard base railway tunnel in Switzerland.

123

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

124

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

125

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

126

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

127

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

128

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

129

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

130

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

131

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

132

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

133 134

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).

135 136

137

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

138

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

139

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

140

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

141

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

142

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

143

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

144

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

145

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

146

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

147

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

148

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

149

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

150

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

151

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

152

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

153

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

154

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

155

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

156

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

157

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

158

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

159

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

160

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

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169 170

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

171 172

There are several mechanisms by which the rock fails, originating rockb...