Research on Faults Activity and Mining Dynamic

Phenomenon

Song Weihua1,2, Zhang Hongwei 1College of Resource and Environment Engineering, Liaoning Technical University, Fuxin, China 2Henan Shenhuo Group Co., Ltd, Yongcheng, China College of Resource and Environment Engineering, Liaoning Technical University, Fuxin, China

Email: ahua727@tom.com , kyzhw@263.net

Abstract: The accuracy and reliability of mining dynamic phenomenon prediction depend on the research level of regional tectonic stress field and active fault basically. Double shear friction experiments of sandstone have been made, and its slide criterion has been suggested considering the viewing of engineering. The geol-ogy structural model is built and tectonic stress field is made a back-analysis by applying finite element method. The calculating results fit with the analysis result of earthquakes mechanism and the distribution characteristic of the measurements. The high stress regional field locates discontinuous zone of Ⅰ level faults and is corresponding to underground earthquakes scene. From then it is certain that tectonic stress is the major origin and necessary condition of mine earthquakes. From the point of Coulomb Criterion and the result of Byerlee experiment, the relation between regional stress and fault’s activity has been discussed. It is believed that Beipiao fault has a dominate effect on other sub faults and tectonic stress area and is dynamical source of dynamic phenomena in the Beipiao mines.

Keywords: active fault; mining dynamic phenomenon; geo-dynamic division; tectonic stress field

1. Introduction

The dynamic phenomena in mines such as mine earth-quakes, rock burst, coal and gas outburst etc., whose mo-tive factors are regional tectonic stress field and hetero-geneity of its space distribution, has close correlation with faults activity[1]. At present, it is considered that dynamic phenomena is the synthetical result, and geo-logical structure especially active faults play a dominant role. The earthquake faults and outburst faults, which have correlation with dynamic phenomena, are active faults that are influenced by mining engineering in cur-rent in-situ stress[2-6]. Along with the development of the in-situ stress measurement technique and numerical simulation, the means of studying underground rock sta-bility and stress state have been improved[7][8]. For mine earthquakes, coal and gas outburst and tectonic stress type of rock burst in Beipiao mines, double shear friction experiments of sandstone have been made, and its slide criterion has been suggested. With the result of in-situ stress, the regional tectonic stress field has been simu-lated by multi-region coupled finite element method, also its variety and the faults unstable failure have been ana-lyzed.

2. Dividing Method of Active Faults

2.1. Dividing Blocks and Determining Active

Faults Using Drawing Method

Two criteria of dividing blocks should be followed. The highest portions of blocks are obviously the least portions touched by corrosion, and form jebels over standard plane in certain period. The borders of blocks can be traced according to difference topography signs of active-fault. Because the blocks are moving, ancient remains jebels of now lie different contour line levels, and can be regarded as the features of blocks’ relative movement. The main references of using Drawing Method are topog-raphy features. According to the principle that basic shape and main features of topography and physiognomy depend on geological newly tectonics structures, then the formation and development of regional active-fault are determined through researching the features of topogra-phy and physiognomy. Drawing Method is carried out on different maps, the most important one of them is the topographic map, which can reflect the three-dimension physiognomy shapes and provide some bases for physiognomy analysis. In the process of physiognomy analysis, the related fea-ture signs must be picked out, some typical signs are shown in Figure 1.

2.2. Determining Active Fault Using Trend Sur-

face Analysis Method Sponsored by the National Basic Research Program of China (2005CB221501), National Natural Science Foundation of China (50874058), Science Research Project of Liaoning Education Bureau (2008280)

Generally, two kinds of maps need be produced in the process of trend surface analysis, one is the trend map,

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438 398 296373 221

197395

220

191186 182

391

425

219

391

(a) (b) (c) (d)

(e) (f) (g)

1 2 3 4 Figure 1. Topography signs of active fault

(a)-variety of contour line; (b)-change of physiognomy and riverbed; (c)-canyon between two mountains; (d)-bend trend of river; (e)-surface features; (f)-river valley with special outline; (g)-lakes arranging like a straight chain. 1-contour line; 2-river; 3-lake; 4-faults related to physiognomy. which is made according to trend value; and the other is residual map, which is made according to residual value. Active-faults are found out through analyzing both kinds of maps. Trend surface and residual surface have differ-ent features, and are separately fit for different research purpose. The former reflects the whole change trend of regional surface (topography), and is fit for finding out the regional trend, the latter reflects local change com-paratively to the whole change trend, and is fit for finding out the local abnormity (such as active-fault).

Trend analysis of topographic altitude is simulated on computer, and can be expressed with algebraic multino-mial equation:

nm yaxayaxyaxayaxaaH ...3

62

542

3210 (1)

yxH ,, are surface coordinate; rep-resent surface distributing constant. According to least square method principle, equation coefficients are deter-mined, and square sum of distance between mathematics surface and peak surface are made to be minimum, namely:

maaaaaaa ,...,,,,,, 543210

MinyxfH iii )],([ (2)

Trend surface is looked as continuous surface, so the discontinuous components compose a kind of deviation to continuous surface, further become parts of residual components. A fault generally shows one line (or broken line), but in residual map, it generally shows slender belt-like positive abnormity (corresponding to perdu reverse fault), or negative abnormity (corresponding to perdu tension fault). Sometimes, it shows apposite positive and negative abnormity belt composed of large residual val-ues, or steep belt of great gradient. In a word, if only re-sidual map shows belt-like abnormity, it is reasonable that the abnormity means fault’s occurrence.

3. Stability Assessment of Active Faults

3.1. Double Shear Friction Eexperiments

The experiment sandstone was taken from Taiji mine of Beipiao mines area and the experiment equipment was small-scaled double shear equipment. Firstly, the smooth surface slide of sandstone sample is stable when normal stress is relatively small, then has minor amplitude stick-slip oscillation, but tends to the stability quickly. When normal stress is large, the more amplitude has been ap-peared after stable slide, the stick-slip stress is gradually decreased and come back the stable state finally with continue stick-slip. These samples that believed rough surface samples are further tested. The slide is stable firstly, then has stick-slip oscillation. In the main the de-scending of stick-slip is larger than smooth surface, then gradually disappeared and changed into stability after slide certain distance. Figure 2 is maximum friction slide criterion of sandstone smooth surface and rough surface.

rough surface

τg=0.75σ

n±0.36

τc=

0.81σn±

0.02

12

8

4

0

4

8

12

0 4 8 12 16

flat surface

n(MPa)

τ(M

Pa)

Figure 2. Maximum friction slide criterion for sandstone

flat surfaces and rough surfaces

3.2. Frictional Strength of Active Faults

Maximum shear surface orientation, fault stability and rock fracture are all decide based on Coulomb criterion. The slide condition is:

n 0max (3) Where τ0 is section bond strength, μ is section friction coefficient and σn is section normal stress. At present, due to the less understanding of natural fault deep structure and complexity of fault surrounding, the study on natural fault slide criterion has certain difficulty and is made mainly in laboratories. The most discussion used Byerlee law from various rock averages under low temperature[9].

n

n

6.050

85.0

(4) MPaMPa

MPaMPa

n

n

1700200

2003

Combined with previous achievements[10-13], the fault slide criterion of Taiji mine has been elicited according to the result of sandstone friction experiments. It is τ=(0.65～0.81)(σ-P0). Where P0 is pore water pressure.

3.3. Assessment of Faults Activities Hazard

If slide failure plane shear stress is , normal stress is n, and the failure condition can be expressed by Coulomb Criterion (Figure 3). When the three principal stress is

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known, average stress m and maximum shear stress m are:

m=1/2(1+3) (5) m=1/2(1－3) (6)

Normal stress n and shear stress of fault plane whose angle is between principal stress orientation and fault plane normal are:

n=m+m cos2 (7) =m sin2 (8)

The assessment criterion of faults activities is: max≥0+n=0+(m+mcos2)

That is: max≥0+/2[(1+3)+(1－3)cos2] (9)

Figure 3. Stress circle and destroyed limit

4. Application of Beipiao Mining Area

4.1. General situation of mining area

Beipiao mining area locates south slope zone of Inner Mongolia earth's axis east segment and compound posi-tion between Tianshan-Yinshan EW-direction tectonic belt and Beipiao-Jianchang rift zone. The coal-accumulating basin is cut three structure units by Tay-ingzi and Jianshanzi NNE faults.

4.2. Tectonic Stress Field Numerical Simulation

Analysis

The original data files are established according to the calculation model, then the distribution characteristics of tectonic stress field near measuring points are ascertained by calculating. Based on the above, the calculation of the maximum principal stress, the maximum shear stress and the strain energy is made with finite-element method (Figure 4). The maximum principal stress traces are E-W distribu-tion and have different steering near faults zone. There-fore, Beipiao mining area principal stress direction is nearly east-westward and consistent with earthquake activities reflective stress field, then bear different mode force according to occurrence difference. The 123 MPa contour through the two faults endpoint of level Ⅰ fault has delineated one nearly east-westward tectonic stress region, in which stress value has vitiated between 123

MPa and 165 MPa, average stress is 145 MPa and stress concentration factor is 1.5. The stress sites over 145MPa are almost coincident with ML≥2.0 earthquake source. The maximum shear stress traces are N-E and N-W dis-tribution and faults of the unified orientation are more sliding.

-2-2

-2

-3

-5

-6-7

-8

Figure 4. Contour of maximum principal stress

4.3. Influence of Beipiao Fault to Mining Dy-namic Phenomena

Beipiao faults influence ranges are 4 km owing to about 400 m relative displacement, which occupy partial Taiji and Guanshan mining area and generate tectonic stress Concentration field(Figure 5). Through in-situ stress measurement of Beipiao mining(Table 1), the maximum shear stress of 220° and 310° orientation has reacted be-cause the orientation faults are in max influence. Also, σn of unloading zone has began to decrease, and lead to decrease. When shear stress of rockmass exceeds , the mining dynamic phenomena has appeared. These are confirmed by the practical distribution situation of coal and gas outburst. The dense area of outburst is close to Beipiao faults influence ranges. And the ranges are influ-enced by faults tip extended effect and divided the most dangerous area in the mining area.

Figure 5. Beipiao fault tectonic stress field

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Unstable slide of the faults surface is the main manifest and the mining activity is the leading factor. Beipiao fault has a dominate effect on other sub faults and tectonic stress area and is dynamical fountain of dynamic phe-nomena in the Beipiao mines. The existence of tectonic stress is necessary condition of earthquake, and mining activity is induction factors of mine earthquake.

Table 1. Contrast between in-situ stress measurement and numerical results

1 3 2 value/MPa 53.5 21 25

Measured Value orientation angle /º N84ºE N10ºW

near-vertical

value/MPa 70 26 Calculated Value orientation angle /º N78ºE N12ºW

4.4. Stability analysis of faults References In tectonic stress area, the faults, which are in critical slide status, may lead to elastic energy release when are influenced by mining exploit, then earthquake is formed.

[1] Zhang Hongwei, “Study of active faults and forecast of dynamic phenomena in mines”, Journal of China Coal Society 1998, 23(2):113-118.

The order of magnitude of τ0 is very small and can be neglected. μ is comparatively stable at certain normal stress limit, that is, it has not large fluctuation with rock type and normal stress change. For Taiji mine, μ is 0.67 and σn= (σ1+σ3)/2. The slide criterion of the mine has the following form.

[2] Guo Deyong, Han Dexin, “Research on the types of geological tectonic controlling coal gas outbursts”, Journal of China Coal Society, 1998, 23(4): 337-341.

[3] Qi Qingxin, Shi Yuanwei, Liu Tianquan, “Mechanism of instability caused by viscous sliding in rockburst”, Journal of China coal society, 1997, 22(2): 144-148.

[4] Li Tie, Cai Meifeng, Zhang Shaoquan, Li Dacheng, “Mining-induced seismicity in China”, Seismological research of northeast China, 2005, 21(3): 1-26.

3131

max 33.02

(10)

[5] Zhang Hongwei, Duan Kexin, Zhang Jianguo, Lu Xin, “Study on the regional prediction of mining dynamic phenomenon”, Journal of china coal society, 1999, 24(4): 383-387.

At I level fault tectonic stress area, σ1=145MPa, σ3=45MPa, τ=50MPa. Thereby friction resistance τmax is 63 MPa. In general τmax is higher than τ, but their differ-ence is little, evenly the latter is more than the former in local location and produces the fault slide.

[6] Pan Yishan, Zhao Yangfeng, Ma Jin, “Discussion on rockburst influenced by regional stress field in China”, Chinese Journal of Rock Mechanics and Engineering, 2005, 24(16): 2847-2853.

[7] MA Yuanchun, LIU Changyi, WANG Fengjiang, “A discussion on the seismic activity of the northern segment of the Honghe fault in according of the slide criterion and in-situ stresses”, the Institute of Crustal Dynamics of China Earthquake Administration (ed.), Proc. of the 10th Crustal Tectonic and Stress, Beijing: Seismological Press, 1997, 154–161,

Especially when fault surface is filled with water and exists pore fluid pressure, the friction resistance will de-crease, which is important condition triggering fault slide. In addition, nearby the exploitation level, the change of stress state can cause fault slide as well as.

[8] HUANG Xingchun, XIA Xiaohe, SHEN Weiping, “Measurement and back analysis on the initial rock stress field around the faults”, Journal of Shanghai JIAOTONG university, 1998, 32(12): 55-59.

5. Conclusions

Motive factors influencing mine earthquakes etc. mining dynamic phenomenon are regional stress field and het-erogeneity of its space distribution. The accuracy and reliability of mining dynamic phenomenon prediction depends on the research level of regional stress field and active faults. The ascertainment of active faults and stress concentration area induced by them is precondition of predicting the phenomenon.

[9] J. D. Byerlee, “Friction of rocks”, Pure Appl Geopliys, 1978, 116: 615-626.

[10] ZHANG Bochong, MA Yuanchun, “The experiments on frictional behaviors rocks and a discussion on criterion of fault slide”, the Institute of Crustal Dynamics of China Earthquake Administration(ed.), Proc. of the 1st Crustal Tectonic and Stress, Beijing: Seismological Press, 1987, 136–145.

[11] C. B.Raleigh, J. H. Healy, J. D. Bredehoeft, “An experiment in earthquake control at Rangely, Colorado”, Science, 1976, 191: 1230-1237.

Rock stress calculating results fit with the distribution characteristic of the measurements. The mining area is absolutely dominated by horizontal stress. At I level fault tectonic stress area, NW or NE orientation faults are in slide critical state and are dangerous occurrence positions of shallow earthquakes. The high stress regional centers station locates discontinuous zone of I level faults and is orresponding to underground earthquakes scene.

[12] M. D. Zoback, A. H. Lachenbruch, G. Shamir, “New evidence on the state of stress of the San Andreas fault system”, Science, 1987, 238: 1105-1111.

[13] M. D. Zoback, Stephen Hichman, “In site study of the phsical mechanisics controlling induced seismicity at Monticello reservoir south Carolina”, J.Geophys.Res, 1982, 87: 6959–6974.

c

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