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第 22 卷 第 10 期 2003 年 10 月

岩石力学与工程学报 Chinese Journal of Rock Mechanics and Engineering

22(10)：1607～1612 Oct.，2003

PREDICTION AND CONTROL OF ROCKBURST DURING DEEP EXCAVATION OF A GOLD MINE IN CHINA*

Zhang Guang，Chen Jingxi，Hu Bin

(Institute of Rock and Soil Mechanics，The Chinese Academy of Sciences， Wuhan 430071 China)

Abstract The studies of prediction and control of rockburst are presented during deep excavation in a gold mine in China. Firstly，the stress-relief method is used to obtain a vast amount of data about initial geostress. Secondly，3D FEM analyses of large scale are performed to find out the law of geostress distribution at various excavation levels of the mining area. At the same time，as an equally important measure，six typical kinds of rock blocks are sampled for the experimental study of rockburst tendency. According to the synthesized results of the theoretical and testing results，the methods of brittleness coefficient，brittle index and stress，and so on，are adopted. Finally，the evaluation on the possibility of rockbursts is given that may take place at the deep mining area and some effective measures are put forward to prevent and control the rockburst. Key words mining engineering，rockburst，deep excavation，prediction and control，testing and computation CLC number TU 453 Document code A Article ID 1000-6915(2003)10-1607-06

the stability and the safety of both mining field and

1 INTRODUCTION

A given gold mine is located in the east Shandong province of China. At present，the excavation of the ore body goes down to the depth of – 435 m. Both mining field and the galleries are safe during excavation，with the exception of the phenomena of side falls and rock pillars′ scaling-off that took place in the middle segment of –385 m. With the increment of excavation depth， mechanical behavior of deep ore the body and rockmass and the stability of surrounding rockmass have been brought to great attention of the mine and the designer. It is doubtless that the excavation at depth will encounter more complex rock mechanics problems，such as high earth pressure and rockburst. Those potential factors create questions to

galleries subject to deep excavation and also set problems to the designer when he makes designing plan for the deep excavation. In order to guarantee the safety in production， is it needed to evaluate，prior to deep excavating，the possibility that the rockburst takes place in the deep galleries so as to take relevant measures. The occurrences of rockburst in rockmass have two i.e.， necessary conditions[1]， the existence of fairly high geostress and high tendency nature of rockburst in the mass. For this reason， studies have been carried out on those two aspects. A great number of field tests have been conducted to measure the geostress of the country rocks in the middle segments at the depths of – 435，– 485 and –535 m，and the analyses on the in-situ measured data have been made，which leads us

Received 17 April 2002，Revised 25 May 2002. * Natural Science Foundation of China under Grant No.59979026. Zhang Guang：Male，Professor of Institute of Rock and Soil Mechanics，The Chinese Academy of Sciences，Wuhan 430071 China.

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to obtain the distribution law of the stress field of the country rocks in the mining area and its mathematical expression as well. On this basis，3D FEM analyses of large scale were carried out to obtain the geostress distribution law of various deep segments at different elevation levels in the mining area. As an equally important measure，six typical kinds of rock blocks were sampled simultaneously from the mining area for the experimental study of rockburst tendency. By synthesizing the results of those two aspects，the final evaluation is given to the possibility of the rockburst that may take place at the deep parts of the mining area and some measures are put forward to prevent and control it.

any effect on production be not allowed. In the case of limited funds，it is impossible to excavate a special testing gallery， the boreholes for testing can only be say， arranged in the existing galleries. In the light of the aforesaid factors，eight positions were selected with the distributions of (1) one horizontal borehole near the dummy main shaft at the level of – 435 m， three (2) boreholes for 3D measurements perpendicular to each other in the position of water pool at the level of – 435 m， (3) two horizontal boreholes in the gallery branch on the opposite side of the transportation gallery at the level of – 534 m， two vertical boreholes respectively (4) at different levels of – 485 m and –535 m to know the changes of stresses with depths. 48 tests have been conducted altogether in the above arranged boreholes. Based on the in-situ measured testing results ， regression analyses were performed to obtain the following expressions of the initial geostress field of the mining area：

2

IN-SITU MEASUREMENTS OF ROCK STRESS AND INTERPRETATION OF RESULTS

In testing，the stress-relief method was employed，

i.e.，back calculation of the initial geostress of the rockmass through the rock deformation during the process of stress-relief caused by overcoming. The measuring equipment for testing is type 36-2 borehole deformeter (Fig.1) that was developed by Institute of Rock and Soil Mechanics，the Chinese Academy of Sciences and now is widely used in China.

σx σy σz τ xy τ yz τ zx

0 = σ x + λγ h ? ? 0 = σ y + λγ h ? = σ z0 + γ h ? ? ? 0 = τ xy ? 0 ? = τ yz ? 0 ? = τ zx ?

(1)

where 0 0 σ x = 8.95 MPa，σ y = 5.54 MPa，σ z0 = 1.06 MPa ? ? ? 0 0 0 τ xy = ?4.70 MPa，τ yz = 0.64 MPa，τ zx = 0 MPa ? ? (2) The lateral pressure coefficient is taken as 0.668 (according to earth shell stress) and the unit weight of

1—Steel-ring supporter 2—Stell-ring 3—Feeler 4—Shell 5—Direction finder 6— Cable

the rock mass is taken as γ = 27 kN/m3. In equation (1)，the compressive and tensile stresses are positively and negatively defined，respectively.

Fig.1 Diagrammatic sketch of type 36-2 borehole deformeter structure

The selection of testing points and the borehole position are of paramount importance because the field measurement of geostress is considerably expensive and expends long time. Proceeding from the angle of the study on the overall engineering stability，it is required that the testing points be well representative in geology and engineering so as to reflect the geostress state in the rockmass and that in testing，and

3

3D NON-LINEAR FEM NUMERICAL ANALYSES

In order to know the geostress field distribution

during deep excavation of the ore body of mine 2# and the relevant possibility of induced rockbursts，the 3D non-linear FEM numerical analyses were made on the brittleness of the rocks. According to the engineering

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第 10 期 Zhang Guang，et al. Prediction and Control of Rockburst during Deep Excavation of a Gold Mine in China ? 1609 ?

geological reports offered by the mine，three engineering geological regions of I，Ⅱ，Ⅲ were selected for the calculation. Region I is the ore body；region Ⅱ is on two sides of region I，consisting mainly of potassiumized migmatitic granite； region Ⅲ is located and in the outside of region Ⅱ，consisting mainly of mingmatitic granite. In addition，taken into consideration is fault F1 that has great effects on the stress distribution of the mining area. The calculation model as shown in Fig.2 has a Cartesian coordinate system with three axes of x， z， y， having due east ， due north and upward vertical directions，respectively；and the calculation domain measures 900 m long along both x and y axes and its height measures from the level of – 800 m to the ground surface. The whole model has 17 617 nodes and 16 766 elements altogether.

geostress field. In our calculation，the latter one is employed. In order to ensure the vertical geostress to be maintained coincident with that of equation (1) obtained from regression calculation of in-situ measured data，it also comprises two parts：the first one that relates the overburden depth is contributed through self-weight action by means of FEM

0 calculation；whereas the part of σ x is considered in

such a way in which it is transformed into the equivalent node force and exerted upon the system. In consequence，the calculation cannot only meet the expression for geostress in an average sense but also satisfy the equilibrium along z direction. The calculation results have shown that at the level of – 435 m，the average minor principle stress ranges between 12～14 MPa and 18 MPa，which is fairly close to the in-situ measured results. The stress of the rock mass increases with the increment of depths， say， the major principle stress in the country rocks increases gradually from 18 MPa at the level of – 435 m to about 25 MPa at the level of –735 m，which means that as the excavating depth goes down in the mining area， the threat caused by the earth pressure will be getting great. We should pay special attention to this problem.

o

Fig.2 Solid of calculation model

4 EVALUATION OF ROCKBURST TENDENCY

From the above mentioned field tests for the geostress and the 3D non-linear FEM analyses，it can be known that the geostress in the country rocks at the level of – 800 m is as high as 25 MPa. Even a low stress concentration coefficient of 2.0 is considered when we excavate galleries at this depth，the induced stress will be as high as 50 MPa， which may definitely result in rockbursts. Consequently，we should make evaluation on the possibility of rockbursts so as to take suitable measures to monitor and control them. In our studies，such methods as those of brittleness coefficient，brittleness index and the stress method have been employed to evaluate the tendency of rockbursts[2

～9]

As we known，there are a variety of methods for our selection when we make attempts to take into account the geostress for FEM calculation of a geotechnical project. One of those methods in common use is that in which the distributing curves of geostresses are drawn according to the in-situ measured data firstly and then the geostress values are sent directly into each element. However，this method has a disadvantage that the initial geostress field does not always meet the equilibrium conditions. The other method is also in common use，in which the geostress actions are taken into account along the models boundary and then the body force of the rock mass self weight is added to obtain the final FEM solutions and so obtained stress distribution is taken as the initial

.

4.1 Method of brittleness coefficient

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This method makes evaluations on the tendency of rockbursts through the brittleness coefficient R of rocks. This coefficient is defined as the ratio of Rc over Rt (Rc and Rt being the uniaxial compressive strength and the tensile strength of the rocks)， i.e.，R = Rc / Rt .

follows the criterion of

? ? K u = 2.0～6.0 weak rockburst ? ? K u = 6.0～9.0 rockburst of medium extent ? ? K u＞9.0 intensive rockburst ? K u＜2.0 no rockburst

(4)

In general，the greater R，the higher the rockburst tendency ， having the following standards of R＜10 R = 10～18 R＞18 ? ? rockburst of midium extent ? ? intensive rockburst ? no rockburst

Tabulated below (table 2) are the evaluating results of rockburst tendency of several major rock kinds in the mining area obtained from field rock mechanic tests.

Table 2 Evaluation results for rockburst tendency from brittleness index method

Description Rock kinds No. of μ (με) (Overall Evaluation of μ (με) Ku (Brit. rock def. before rockburst (Perm.def.) ind. coef.) sample peak loading) tendency 485-1 JH4 S-3 435-27-4 42.7 85.5 42.7 30.5 809.5 1 647.0 1583.0 944.4 18.95 19.26 37.07 30.96 High High High High

(3)

Tabulated below (table 1) are the evaluating results for the tendency of rockbursts of several kinds of rocks obtained from field experimental studies.

Table 1 Evaluation results from brittleness coefficient method

Description Rock kinds Rt /MPa Migmatitic granite Potassiumized granite Striped gneiss Lamprophre 5.93 6.56 6.02 5.27 Rc /MPa 66.41 86.02 117.77 64.96 R 11.20 13.11 19.56 12.33 Rockburst possibility from evaluation Medium Medium Intensive Medium

Migmatitic i Patassi. granite Striped gneiss Red granite

4.3 Method of stresses

The aforesaid two methods consider the tendency that the rockburst takes place only from the angle of lithological character，i.e.，to ascertain whether there exists internal causes in a rockmass to induce rockbursts. The method of stresses，however，combines the lithological character of a rockmass (including tensile and compressive strengths) to judge the possibility that rockbursts take place. This method introduces two factors of α and β to serve as criterion， α and

4.2 Method of brittleness index

This method makes evaluations on the rockburst possibility through the brittleness index K u of rocks. The brittleness index K u is defined as the ratio of u over u1，the former and the latter being the overall deformation prior to peak loading of rocks and the permanent one respectively as shown in Fig.3.

σmax

β being defined respectively as the ratio of the rocks

uniaxial compressive strength Rt over the major principle geostress σ 1 ，i.e.， α = Rc / σ 1 and as the ratio of the rocks uniaxial tensile strength Rt over

σ1 ， i.e.，β = Rt / σ 1 . Because the index of the uniaxial compressive can be determined easily， value of α the

is generally used for a criterion，having the following

ε

standards： α＞10

Fig.3 Overall deformation u and permanent deformation u1

The greater K u ，the higher the brittleness of the rocks，i.e.，the higher the rockburst tendency. K u

? ? α = 5～10 weak rockburst ? ? (5) α = 2.5～5 rockburst of medium extent ? ? α＜2.5 intensive rockburst ? According to the distribution of the rock strata in the mining area and to both field testing results and 3D no rockburst

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第 10 期 Zhang Guang，et al. Prediction and Control of Rockburst during Deep Excavation of a Gold Mine in China ? 1611 ?

nonlinear FEM computing results，the possibility of rockbursts was analyzed during excavation at each elevation level，and the results are as shown in table 3.

Table 3 Evaluations on rockburst possibility at each elevation level

Elevation / m －435 －585 －735

goal of slowing down or controlling the rockburst can be attained. The above analyzing results have shown that the deep excavation in the mining area likely to induce rockburst. In this case， may employ the methods of we controlling geostress and controlling deformability to prevent the surrounding rocks from burst.

5.1 Method of controlling stresses

σ 1 / MPa

49.1 63.0 58.5

α

1.75 1.37 1.47

Evaluat.on rockburst possib. Intensive rockburst Intensive rockburst Intensive rockburst

This method is the major measure used in the designing stage，which attains the goal of controlling rockbursts through decreasing the stress of the surrounding rockmass. The method includes the following aspects： (1) In the region of high geostress，the tectonic stress is of superiority and mostly has a nearly horizontal orientation. To decrease the stress in the surrounding rockmass ， the longitudinal axis of underground openings should be parallel to the maximum horizontal stress direction as fully as possible. (2) Any sharp or non-smooth corners are not allowable in designing the opening′cross section. It is s suggested that circular or elliptical cross section be selected as fully as possible.

5.2 Method of controlling deformability

4.4 Synthetical evaluation of rockburst

It has been proved in practice that the aforesaid methods are the criterion that is quite useful. The evaluating results about rockburst tendency using those methods should be combined with the engineering situation to attain the final results，i.e.，carry out synthetical evaluation. The methods of brittleness coefficient and brittleness index make evaluations on the rockburst tendency all from the view of the inherent physicomechanical property of rocks. For example，if the evaluation is made according to the former method， we have the conclusion that the rocks of all kinds except gneiss in the mining area present rockburst tendency of medium extent；whereas if according to the latter，then the conclusion becomes tendency of intensive rockbursts. It can be seen therefore that the rockburst problem arising from deep excavation in the mine can never be neglected if the lithological condition is considered. However ， the evaluating results obtained using the stress method have further proved that almost all excavation activities below the elevation level of – 435 m may possibly induce intensive rockburst and the rockburst intensity becomes greater as the excavating depth goes down.

The method of controlling deformability is the measure taken in the process of excavation to control rockbursts concretely. It includes the followings： (1) Advanced borehole method This method cannot only further the decrement in pressures upon the surrounding rocks but also improve the deformation behavior of the rocks. In carrying out the method，boreholes are drilled vertically (to the working face)， going through the stress increasing area to create limit stress areas. If the boreholes are drilled densely enough，the limit stress areas interlock each other to produce a great number of cracks，resulting in rockmass damage to a certain extent. In addition to the advanced boreholes，boreholes should be drilled in the opening sidewalls，which can improve the stress state. (2) Watering-compaction method This method is the one in which water is injected

5 CONTROL OF ROCKBURSTS

Whether or not the rockburst takes place depends two conditions. The first is the stress state of the rockmass surrounding the opening and the second is the rockmass lithological character. Providing those two factors are changed or controlled artificially，the

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into a rockmass to impel the mechanical properties to be changed，decrease the strength and elastic modulus，strengthen capability of plastic deformation and increase the transversal deformation coefficient. It should be noted that this method is effective only if uniform water-injection is guaranteed，or otherwise the rockmass will be locally subjected to higher pressures.

the designing stage， is suggested that rational gallery it axis arrangement and smoothly transformed gallery cross-section be made so as to improve the stress state in the rockmass. The 3D non-linear FEM calculations of large scale performed in this paper give the geostress field distribution of the mining area，which provides the basis for rational arrangements of gallery axis. It is also suggested that during deep excavation， the method of advanced boreholes and the wateringcompaction method be used to attain the goal of controlling rockburst.

6 CONCLUSIONS

(1) By using the stress-relief method，the stress measurement tests have been conducted 48 times in 8 boreholes at different elevation levels in the mining area and the regression analyses have been performed on the in-situ measured data to obtain the expressions for the initial geostresses in the mining area. (2) 3D nonlinear FEM calculation has been carried out. The computing model has 17 167 nodes and 16 766 elements. The computing results have indicated that the major principle stress at the level of –735 m is high up to 25 MPa， which creates conditions for inducing rockbursts. (3) A great number of experiment studies have been conducted in lab on several major rocks of various kinds. According to both testing and computing results，the synthetical evaluations have been made on the rockburst tendency using the methods of brittleness coefficient and brittle index， and the stress method. The evaluating results have shown that intensive rockbursts are likely to occur if the deep excavation is carried out below the elevation of – 435 m in the mining area. (4) The method or measure to prevent the surrounding rocks from burst has been proposed. In

REFERENCES

1 Xu Dongjun， Zhang Guang， Tingjie， al. On the stress state in rock Li et burst[J]. Chinese Journal of Rock Mechanics and Engineering，2000， 19(2)：169～172 2 Pan Yishan，Xu Bingye. The rockburst analysis of circular chamber under consideration of rock damage[J]. Chinese Journal of Rock Mechanics and Engineering，1999，18(2)：152～156 3 Wang Yuanhan，Li Wodong，Lee P K K，et al. Method of fuzzy comprehensive evaluations for rockburst prediction[J]. Chinese Journal of Rock Mechanics and Engineering，1998，17(5)：493～501 4 Zhou Depei， Hong Kairong. The rockburst features of Taipingyi Tunnel and the prevention methods[J]. Chinese Journal of Rock Mechanics and Engineering，1995，14(2)：171～178 5 Mogi K. Effect of the intermediate principal stress on rock failure[J]. J. Geophys. Res.，1967，72(1)：5 117～5 131 6 Mogi K. Fracture and flow of rocks[J]. J. Tectonophysics，1972， 13(4)：541～568 7 Rubin A M，Ahren T J. Dynamic tensile failure induced velocity deficits in rock[J]. Geophys. Res. Lett.，1991，18：219～223 8 Chen E P，Taylor L M. Fracture of brittle rock under dynamic loading condition[R]. SAND-84-2358C，1985 9 Jonson J，Hult J. Fracture mechanics and damage mechanics，a combined approach[J]. Journal de Mecanique Appliqblce， 1997， 1(1)： 11～ 23

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