J. Cent. South Univ. Technol. (2010) 17: 738-743

DOI: 10.1007/s11771-010-0549-6

Stress field evolution law of mining environment reconstructing structure with change of filling height

CHEN Qing-fa(陈庆发)^{1, 2}, ZHOU Ke-ping(周科平)¹, WANG Li-li(王利利)¹

1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;

2. College of Resources and Metallurgy, Guangxi University, Nanning 530004, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: For improving global stability of mining environment reconstructing structure, the stress field evolution law of the structure with the filling height change of low-grade backfill was studied by ADINA finite element analysis code. Three kinds of filling schemes were designed and calculated, in which the filling heights were 2, 4, and 7 m, separately. The results show that there are some rules in the stress field with the increase of the filling height as follows: (1) the maximum value of tension stress of the roof decreases gradually, and stress conditions are improved gradually; (2) the tension stress status in the vertical pillar is transformed into the compressive stress status, and the carrying capacity is improved gradually; however, when the filling height is beyond 2.8 m, the carrying capacity of the vertical pillar grows very slowly, so, there is little significance to continue to fill the low-grade backfill; (3) the bottom pillar suffers the squeezing action from the vertical pillars at first and then the gravity action of the low-grade backfill, and the maximum value of tension stress of the bottom pillar firstly increases and then decreases. Considering the economic factor, security and other factors, the low-grade backfill has the most reasonable height (2.8 m) in the scope of all filling height.

Key words: mining environment reconstructing structure; stress field; filling height; evolution law

1 Introduction

For the soft broken and complex hard mining orebody with high-value, the traditional mining methods mainly have cut and fill stoping one and open stope filling one [1-3]. The former has shortcomings like poor quality of roof-contacted filling, bad security of later pillar recovery and low productivity, while the latter has some defects such as relatively frequent caving incident during stoping and very poor adaptability in the complicated and variable orebody. In recent years, certain progress has been made in improving the defects both at home and abroad. ZHOU et al [4] and LI et al [5] improved the horizontal cut and fill stoping method by optimizing the slope structural parameters and adjusting filling material mixing proportion, respectively. WU et al [6] optimized the sublevel open stope filling method by adjusting orebody structural parameters. However, the improvements and optimization cannot realize fundamental change in the mining method. At the same time, great requirements for safe and efficient exploitation are proposed for complex goafs existing in this kind of orebody, which makes it exigent to find an innovative design of mining method.

In order to improve the stress status in surrounding rock and control the displacement, some fillings like waste rock, tailing and concrete are often used in the engineering [7-9]. Based on key strata theory of ground control, MA et al [10] established structural mechanical model of surrounding rock of gob-side entry retaining in fully-mechanized coal face with top-coal caving and deduced the support resistance of the backfill in the roadway under different geological conditions. LI et al [11] studied the strength characteristics of cemented tailings backfill in deep-seated mined-out area through the experiment. LIU et al [12] studied the damage model of cemented tailings backfill at different cement-to-sand ratios and the reasonable matching between backfill and rock mass. For guiding the safe construction of the mining environment reconstructing structure in cataclastic ore section, stress field evolution law of the structure with the change of filling height was studied in this work.

2 Mining environment structure reconstruc- ting in cataclastic ore section

2.1 Engineering situation

Gaofeng mine is one of the mine bases of Guangxi  Huaxi Group. Orebody No.105 at -79 m is a rare cassiterite-sulfide polymetallic oreshoot. The integrated average grade of major useful metals including tin, zinc, lead and antimony is about 22%, and the average content of silver is 155.45 g/t. However, the average content of sulfur is about 28%, which is also high, so the ore has the possibility of spontaneous combustion. The ore section No.1 in the orebody No.105 belongs to cataclastic structural orebody which is non-cemented and deciduous and the average size of the cataclastic blocks is about  10 cm. Because of serious interference by private mining, many irregular goafs exist in the ore section, as shown in Fig.1.

Fig.1 Orebody and goafs model

2.2 Treatment scheme design of goafs

The elevation level scopes of goafs No.2, No.3 and No.4 in upper part of the ore section are -79--101, -101--103 and -106--110 m, respectively. These goafs should be treated firstly to form a stable upper structure for the safe mining of the lower resources.

The elevation level scopes of goafs No.15 and No.17 are -131--145 and -140--152 m, respectively. Goafs No.15 and No.17 are considered to be filled completely. Goaf No.18 which connected closely with goaf No.17 is considered to be filled to -140 m firstly.

As for those goafs in the main resources between -110 and -140 m, according to the synergism idea of resources mining and goafs treatment, which is proposed by the authors, they would be adjusted as a part of some cut engineering or free blasting spaces in the mining layout for utilization.

2.3 Innovative design of mining method

In view of the characteristics of cataclastic ore section, and in line with the principles of raising production efficiency and working safety, reducing production cost, guaranteeing lower dilution and loss ratio, and collaborative idea of resources mining and goafs treatment, a new mining method (mining environment reconstructing layering and stripping medium-length hole caving mining method) was brought forward on the basis of mining environment reconstructing theory [13-14], as shown in Fig.2. The main resources were divided into three 10 m-thick ore layers. The cross section size of each strip was 10 m×  10 m. Backward mining mode was used in the strip and ore was drawn by remote-controlled left hand drive machine. Two mining cycles were needed in an ore layer. One strip interval one mode was used in whole, then the high strength cement mortar cemented filling was carried out (the allocated proportion of cement to mortar of 1:4) when ore was mined out in the first mining cycle. In the second mining cycle, the most important vertical pillars formed under the condition of the first mining cycle, and one interval one mode was still used; after the resources were mined out, 3 m-thick high strength fillings formed the bottom pillar, then 7 m-thick low-grade fillings were filled in the residual space scopes.

Fig.2 Mining environment reconstructing layering and stripping medium-length hole caving mining method (1—Haulage roadway;  2—Extracted ore drift; 3—Cutting-ventilating patio; 4—Drilling roadway; 5—High strength backfill; 6—Low-grade backfill; 7—Blasthole; 8—Filling roadway; 9—Filling-ventilating connecting roadway; 10—Ore block boundary): (a) The first mining cycle;   (b) The second mining cycle

2.4 Mining environment structure reconstructing

For mining main resources in the scope from -110  to -140 m, according to mining method, the mining environment frame structure would be formed, as shown in Fig.3.

Fig.3 Mining environment reconstructing structure and finite grid of numerical model

3 Numerical model

Nonlinear finite element analysis software of automatic dynamic incremental nonlinear analysis (ADINA) has implicit and explicit time integral algorithms and powerful function in nonlinear solution and fluid-structure interaction analysis. It becomes one of the firstly selected codes in nonlinear analysis computation. In the aspect of calculating rock-soil deformation and the stability, ADINA also has strong superiority such as abundant geotechnical material patterns, many kinds of geological fault and joint fissure processing method, anchor and anti-slide pile bar element algorithm [15].

Fig.3 shows the center part of numerical model of mining environment reconstructing structure. The model size was 400 m×150 m, dividing into 13 468 quadrangle units. The size of central territory unit was 1 m×1 m and the size of surrounding rock unit was 3 m×3 m. The backfill had the same unit type as surrounding rock. The horizontal restrains were imposed on both sides of model. The vertical restrain was imposed on bottom margin and the upper surface was free. Mohr-Coulomb yield criterion was used in the analysis. The physical and mechanical parameters of orebody and rock mass are shown in Table 1.

4 Stress field evolution law of structure with change of filling height

4.1 Stress field analysis without filling

Fig.4 shows the distribution maps of the minimum and maximum principal stress of the key region of the mining environment structure.

From Fig.4(a), the depth of tension stress scope of the roof was 3.15 m; and the maximum value was   1.95 MPa which was larger than material tensile strength 1 MPa. Thus, the situation of material breakage would appear possibly. At the lower-middle part of the vertical pillar there was tension stress area of 1.4 m in depth, 3.5 m², the center of which was approximately 2.8 m low away from the floor and the tension stress was smaller than 0.25 MPa. The bottom pillar mainly suffered from squeezing action from the vertical pillars. Its tension stress area distribution range was broad, presenting double-X distribution. The maximum tension stress was 0.5 MPa and the stability was good. However, it still had a large surplus bearing capacity. From Fig.4(b), the compressive stress had the lower value in the roof and the higher value in the vertical pillar, which showed that the vertical pillar mainly suffered from the compressive stress of the upper surrounding rock. The maximum compressive stress was up to 9.05 MPa, no more than the compressive strength of the high strength backfill, so, the material would not break.

4.2 Stress field analysis of structure with change of filling height

Through analyzing the structure stability, the tension stress of the roof was beyond the tensile strength. In order to improve the whole stability of structure, the low-grade fillings were considered to be filled into the residual space. The action of low-grade backfill included: translating two-dimensional stress on the surface of the vertical pillar to three-dimensional stress to consolidate its carrying capacity; improving the stability of the roof by reducing space area of stope; and fully developing the residual carrying capacity of the bottom pillar. The filling schemes of three kinds of filling height (2, 4 and 7 m) were designed to study the stress evolution law of the structure. The physical and mechanical parameters of low-grade backfill (the allocated proportion of cement to mortar of 1:8) are shown in Table 1.

Table 1 Physical and mechanical parameters of orebody and rockmass

Fig.4 Principal stress distribution maps of key region of structure: (a) Minimum principal stress; (b) Maximum principal stress

4.2.1 Distribution law of minimum principal stress

Fig.5 shows the distribution maps of the minimum principal stress of structure with different filling heights.

From Fig.5(a), the tension stress scope of the roof was not obviously shrinking at 2 m filling height, while the maximum value reduced a lot up to 1.02 MPa, which still exceeded the tension strength. There was no tension stress in the vertical pillar and the carrying capacity was improved. The maximum tension stress appeared in the center of the bottom pillar surface of 0.04 MPa. From Fig.5(b), the maximum tension stress of the roof reduced to 0.54 MPa at 4 m filling height; and the stress status was improved obviously. Meanwhile, the carrying capacity of the vertical pillar was improved further. Carrying capacity of the bottom pillar was significantly improved due to low-grade backfill, which shared squeezing action from the vertical pillars. The maximum tension stress of the bottom pillar was 0.05 MPa. From Fig.5(c), the maximum tension stress of the roof was further reduced to only 0.46 MPa at 7 m filling height. The stability of the roof was in the best status.

4.2.2 Distribution law of maximum principal stress

Fig.6 shows the distribution maps of the maximum principal stress with different filling heights.

From Fig.6, stress decreasing zone was present in the roof and the bottom pillar. Load from the upper surrounding rock was transferred into the lower surrounding rock through the vertical pillars. The maximum compressive stress was not beyond the compressive strength of the material.

Fig.5 Minimum principal stress distribution with different filling heights: (a) 2 m; (b) 4 m; (c) 7 m

4.3 Stress field evolution law of structure

Fig.7 shows the maximum tension stress change law of the roof and the bottom pillar with the increase of filling height.

The maximum tension stress of the roof was reduced gradually. The phenomenon of tension failure was present without filling in the middle part of the roof and tension status was improved effectively after filling. The maximum tension stress of the bottom pillar decreased at first and then increased. The cause of the tension stress decreasing was that the squeezing action from the vertical pillars was apportioned by the low-grade backfill. The tension stress increasing was due to the gravity action on the bottom pillar, which increased with the rise of filling height. The tension stress was not beyond the tension strength.

Fig.6 Maximum principal stress distribution with different filling heights: (a) 2 m; (b) 4 m; (c) 7 m

Fig.7 Change law of maximum tension stress in roof and bottom pillar

If the safety factor of the engineering was defined as 1.2 according to the fitting curve, when the filling height was no more than 2.8 m, the low-grade backfill could not only improve the tension status of the roof, which improved the carrying capacity of the vertical pillar, but also decrease the squeezing action of the vertical pillars on the bottom pillar effectively.

Fig.8 shows the change law of compressive stress in the middle part of the vertical pillar with the filling height. After the filling height was beyond 2.5 m, the carrying capacity of the vertical pillar raised slowly.

Fig.8 Change law of compressive stress in middle part of vertical pillar

Considering the economic factor, security and other factors, 2.8 m was the most reasonable height in the whole scope of filling height. At this height, the reinforcing action of low-grade backfill to the structure was the best.

5 Conclusions

(1) According to mining environment reconstructing layering and stripping medium-length hole caving mining method, the mining environment structure is reconstructed. In the single layer structure, the tensile stress area in the roof is relatively large and the maximum of it reaches 1.195 MPa, which is beyond the tensile strength. At the lower-middle position of the vertical pillar there is a 1.4 m-deep tensile stress area; the maximum tensile stress of the bottom pillar is 0.5 MPa. However, there is still great surplus bearing capacity that is not fully utilized.

(2) With the increase of filling height, the maximum value of tension stress of the roof decreases gradually, and stress conditions are improved gradually. The tension stress status in the vertical pillar transforms into the compressive stress status, and then the carrying capacity is improved gradually. However, the filling height is beyond 2.8 m, the carrying capacity of the vertical pillar grows very slowly, and there is little significance to continue to fill the low-grade backfill. The squeezing action from the vertical pillars is transformed into the gravity action of the low-grade backfill, and the maximum value of tension stress of the bottom pillar firstly increases and then decreases.

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Foundation item: Project(200911MS01) supported by the Scientific Research Fund of Guangxi Provincial Education Department, China; Project (XBZ100126) supported by the Scientific Research Foundation of Guangxi University, China; Project(2009B005) supported by the Teaching Reform Foundation in the New Century Higher Education of Guangxi Province, China

Received date: 2009-09-19; Accepted date: 2009-12-03

Corresponding author: CHEN Qing-fa, PhD; Tel: +86-771-3232200; E-mail: chqf98121@163.com

(Edited by YANG You-ping)