Stress Distribution Around Mechanized Longwall Face at Deep Mining in Quang Ninh Underground Coal Mine

Quang Ninh underground coal mines are currently in the phase of finishing up the mineral reserves located near the surface. Also, in this phase, a number of coal mines have opened and prepared new mine sites for the extraction of the reserves at greater depth. Several mines have mined at -350 m depth and are driving opening excavations at -500 m depth below sea level. The mining at greater depth faces many difficulties, such as a significant increase in support and excavation pressures. The longwall face pressure is mostly manifested in great magnitude that causes support overloaded and jumped and face spall/roof fall. This paper, based on the geological condition of the Seam 11 Ha Lam coal mine, uses the numerical program UDEC for studying the impact of mining depth on stress distribution around the longwall face. The results show that the deeper the mining is, the greater the plastic deformation zone is. The peak front abutment stress moves closer to the coal wall, mainly concentrating on the immediate roof and top coal. The top coal is greatly broken, and its bearing capacity is decreased. Some solutions to the stability of roof strata are proposed, and a proper working resistance of support is determined. Additionally, the paper suggests that the starting depth for deep mining in Quang Ninh underground coal mines should be -350 m below sea level.


Introduction
Deep mining is an inevitable trend for coal mining in the world. According to statistics, there are many countries that perform the coal extraction at -500 m below sea level, such as -514 m in Russia, -928 m in Germany, -610 m in Poland, and -1,024 m in China [1]. These countries have early implemented research on mining at greater depth. Cheng et al [2] used numerical model in combination with theoretical analysis and site observation to interpret the typical stress distribution in roof and floor strata; deformation and failure of coal wall; and typical stress redistribution when mining depth varies. The study found the relationship between face support and roof strata in the condition of competent roof and deep mining. Liu [3] and Bai et al [4], by using the numerical program UDEC 3.0, studied the caving, stress field distribution and roof strata displacement as well as the impact of depth and dip angle on roof rock stability. The results provided scientific evidence and solutions for control of surrounding rock and face support. For studying behaviour of mine pressure and ground control at deep mining, Du [5] and Bai et al [6] used theoretical and practical investigations to predict the mode of failure zone, method for determination of main roof convergence, and relationship between dynamic pressure and mining depth. The results showed that the rock behaviour at deep mining was not clear, the frequency of coal wall spall/roof fall increased as mining depth increased, and the surrounding rock failed significantly. Qi et al [7] analysed ground pressure in deep mining from which fundamental and technical framework for prevention of dynamic impact at different scales in China. At the same time, Li et al [8] found a change in stress distribution at seam floor using numerical model, physical model and site measurement. Recently, Zhang et al [9] studied the stress distribution and failure characteristics of deep inclined seam. They revealed that the vertical taper structure was formed by the roof, interconnection and maingate. Besides, Chen et al [10] investigated deep mining in existence of face fault [10]. Chaoru [11] used regression analysis for studying distribution of in-situ stress in deep underground coal mines. It was found that an increase in mining depth increased the effect of in-situ stress on surrounding rock deformation and failure. However, as the depth increased, the increasing rate of horizontal stress decreases. The ratio of maximum principal horizontal stress to vertical stress was founded in the range of 0.63-2.42. According to Pathegame et al [12], deep mining becomes a common practice in near future, and it is controlled by the understanding of rock mechanics and mining technologies.
It can be seen that in the world, theories and practices for deep mining have been interpreted to some extent through theoretical analysis and site observation. The distribution of stress at great depth, however, is preliminarily studied through mainly numerical modelling. Only several input parameters have been studied; accordingly, it lacks fundamentals for development of stress distribution around longwall face at great depth. For geological conditions in Vietnam, Le et al [13] used numerical method to study coal mining under competent roof at Quang Hanh coal mine, Quang Ninh coal field. The results showed that the face was affected by overburden pressure more seriously. It caused more face instabilities, especially near two roadways. Nguyen et al [14] developed a numerical model for calculation of failure and subsidence caused by mining for Quang Ninh coal field. An other example can be seen in Le et al in which a safe mining height for extraction of coal under Red River delta was determined and some feasible opening solutions were proposed [1,15]. In general, the above studies are preliminary and lack detailed investigation for mine design and ground control at great depth.

Geological condition of longwall face
The mechanised face belongs to Seam 11 Ha Lam coal mine and in depth from -150 m to -300 m below sea level. The rock units in the mine include conglomerate, sandstone, gritstone, claystone, coalclay and coal seams. The rock strata alternate each other, resulting in relatively consistent stratigraphic sequence in local scale. The roof and floor rocks are mainly siltstone, claystone and sometime sandstone layers. The rocks are medium strong. Seam 11 is ranks as thick seam but inconsistent in the range of 0.97-29.75 m and an average dip angle of 5 degrees. The longwall dimension in dip and strike directions are 120 m and 430 m, respectively. The rock characteristics are shown in Tab. 1.

Model development
To study the stress distribution around longwall top coal caving face, this paper uses the numerical program UDEC 3.0 for modelling of longwall face [16].

Rock strata and coal seam properties
The rock behaviour in model follows Mohr-Coulomb theory. This theory can deal with fundamental rock problems such as slope stability and mining excavation. In coal mining, the theory can capture the failure, rupture and fault, and thus its parameters are of importance for calculation results. From the laboratory test, field observation and modelling experience from past users, the properties are chosen as follows: Tab

Front abutment stress distribution in variation of mining depth
Because the top coal fails under roof pressure, the front abutment stress forms destressed zone, concentrated zone, and pre-mining stress zone. Fig. 2 shows the stress distribution around longwall face when the mining depth is 300 m, 350 m, 400 m and 450 m. When the depth increases, the stress contour of front abutment stress varies. When the depth is 300 m, the concentrated front abutment stress zone locates at immediate roof and about 18 m ahead of coal wall with a peak stress magnitude of 20 MPa. When the depth is 350 m, the concentrated stress zone locates at immediate roof, about 15 m ahead of coal wall with a peak stress value of 27 MPa. When the depth is 400 m, the concentrated stress zone locates below immediate roof. The zone is about 13 m ahead of coal wall with a peak lue of 22 MPa.
When the depth is 450 m, the zone locates at top coal section, about 8 m height compared to face floor and it has a peak stress value of 26 MPa. The numerical results indicate that when the mining depth increases, top coal fails significantly and the bearing capacity of top coal and immediate roof decreases. The concentrated stress zone moves closer to coal wall that in turns facilitates the failure of top coal. This however causes difficulty in face pressure control and support.        350 m to 450 m, the peak stress' location follows an increasing trend, from 5.2 m to 11.9 m. At the same time, the magnitude of peak front abutment stress follows a clear increasing trend. From the depth of 300 m to 350 m, the stress' magnitude increases significantly, from 12.3 MPa to 23.9 MPa. From 350 m to 450 m, the magnitude increases slightly. The modelling results clear reveal that when the mining depth increases, the peak front abutment stress increases. The decreasing rate of front abutment stress behind the peak point also increases. This study suggests a boundary for separating the shallow and deep mining portion of -350 m for Quang Ninh coal field.     In general, the roof structure in longwall top coal caving face after failure are also in beam form. The load on face support is smaller when the immediate roof ruptures before the rupture of main roof. This load is greater when immediate roof and main roof rupture at the same time. Thus, it is seen that the load support fluctuates cyclically with different values. In comparison with support capacity, the on-site support are totally capable of supporting the roof.

Distribution of support load along panel width
The distribution of roof load on face support along panel width is shown in Fig. 11. This load distributes unevenly along the dip direction. At about 25 m near maingate and tailgate, the support load is greater than that in the interval between the two locations. Furthermore, the load on front leg pressure is commonly greater than that in rear leg, as shown in Fig. 11.   Fig. 11. Distribution of load on support along panel width. MPa with an average of 8.56 MPa. However, at both face ends, the load is sometimes zero due to the breakage of safety valve as reported by on-site engineers. This also demonstrates the great magnitude of face roof pressure near two roadways.

Conclusions
1) When mining depth increases, the location of concentrated stress zone moves closer to coal wall, mainly distributed in immediate roof and top coal. The bearing capacity of top coal and immediate roof significantly decreases. An increase in mining depth therefore facilitates the failure of top coal while it causes difficulty for support and ground control.
2) The magnitude of peak front abutment stress varies clearly in variation of mining depth. When the depth increases from 300 m to 350 m, the stress increases rapidly with high rate, from 12.3 MPa to 23.9 MPa. The stress afterwards increases slightly when the depth increases to -450 m. Accordingly, this study proposes a boundary for separating shallow and deep mining portion of -350 m for Quang Ninh coal field.
3) When mining depth falls in the range of 300-350 m, the plastic deformation zone extends rapidly with a rate of 0.08 m/m. When the depth falls in the range of 350-400 m, the zone extends slightly with a rate of 0.01 m/m. The deformation zone extends greatly again with a rate of 0.032 m/m when the depth increases from 400 m to 450 m. It is concluded that an increase in mining depth causes an increase in extent of deformation zone as well but with different rate.
4) The working load on face support along panel strike changes periodically with an interval of 15-25 m. The load is also not consistent along panel width. It is commonly greater near T-junctions and smaller in the interval between the two junctions. The study confirms that the on-site face support totally meets the requirement of roof control.

Acknowledgements
The paper was presented during the 6th VIET -POL International Conference on Scientific-Research Cooperation between Vietnam and Poland, 10-14.11.2021, HUMG, Hanoi, Vietnam.