The dynamic disasters are aggravating with the increase of exploitation scale and intensity in Chinese coal mines, to further understand this problem, we studied the mechanisms of mining tremors induced by key strata movement and instability under large scale exploitation. First the mechanisms were categorized into two groups that is main key strata fracture and movement as well as subkey strata instability again under adjacent mining activities. Based on the key strata theory in ground control we revealed three basic mechanisms of key strata destabilization that are rotary and sliding of low subkey strata, shear sliding of the high subkey strata, and the main key strata rupture and cave at limit span, respectively. The microseismic observing systems were applied to monitor the mining tremor events and verify the theoretical analysis in different coal mines. The characteristics of time-space evolution of tremors show that low inferior key strata causing the most, followed by the high inferior key strata and the main key strata least, however the released energy was just opposite.
Recent years, series of advanced mining technologies such as longwall mining with top coal caving, large height mining, and small or none pillar between adjacent work faces have developed considerably in Chinese collieries [
The roof strata rupture and instability related to mining activities are major concerns in coal science, numerous researches have been done in the past to investigate the roof behavior, and great achievements have been obtained [
Thick hard strata that play key roles in the movement of overlying strata are defined as key strata which can be divided into two types: main key strata whose broken and caving will lead to synergistic effect until the surface and inferior key strata that only control local area [
The traditional classification of the overlying strata is based on the controlling rules for the subsidence. The key strata have similar function in ground control rather than instability mechanism and fracture type especially under larger scale exploitation. So we need to make some new definitions in order to predigest the researches based on the instability mechanism. Figure
Schematic structure chart of overburden strata after two work faces exploitation: (1) adjacent gob, (2) district sublevel entry of the longwall face, (3) fracture line of the low subkey strata, (4) fracture line of the high subkey strata, (5) fracture line of the main key strata, (6) face end of the adjacent gob with uncaving top coal, and (7) small pillar between two faces 8-longwall face; (A, B, and C) block of the fractured key strata.
Planar graph
A-A profile graph
The main key strata is characterized by great thickness, high strength, and large caving interval, and in some coal mines the main key stratum keep integrated and stable until there are two or more working faces without big pillars between each other. It is known that most overlying strata of coal seams are sedimentary with obvious layers and weak planes which can be separated easily under shearing stress, so the main key stratum can be regarded as thin plate to analyze the stress distribution under the fixed boundary conditions using Navier’s solution [
Assume that
Curve of relationship between normal stress and the plate length.
In the process of the main key strata fracturing and sliding between breakage rock blocks, most of the accumulated elastic strain energy released to damage the rock body and a small part about 1–10% transforms into seismic wave inducing the mining tremors as shown in Figure
Mining tremors induced by main key strata rupture and waveforms recorded by microseismic system.
The stability of the rock blocks in next goaf especially on the boundary significantly affected the strata behaviors of the working face, and mining tremors occurred frequently around the roadways and in the gob. Figure
Mechanical model of the low subkey rock block.
The minimal horizontal force
Using the force equilibrium in
Substituting (
The condition that no sliding instability occurred between blocks A and B can be expressed as the following equation:
Equation (
Curve of relationship between horizontal force and rotary angle.
The fracture lines of high subkey strata above the two neighboring longwall faces do not coincide with each other so the structure is more stable than the low subkeys strata as the rock mass in the middle of the fracture lines can resist and buffer the mine disturbance to a considerable extent; therefore, together with the rocks on both sides a balanced structure like a “bridge” is formed. Mining tremors occurring in this area have much more energy than low subkey strata because of huger accumulated energy in the structure system as inadequate fracture and larger rock blocks. We can analyze the mechanical equilibrium criterion based on Figure
The mining tremors occur frequently and induced tens of rock bursts that caused great damage to the work face and roadways; hence, the Seismological Observation System called “SOS” for short was introduced to monitor the seismic evens. The “SOS” developed by the Central Mining Institute of Poland is one of the most advanced monitoring equipment in the world. It comprised the seismic data transmission system with maximum distance of 10 km, seismic geophones of frequency band 1–600 Hz, and the AS-1 Seismic Recording System. It can accurately determine the source parameters such as occurring time, coordinates, and energy of the low energy tremors of the order 10–100 J with the horizontal location error less than 20 m, the vertical error 50 m under the optimal configuration of seismological network.
The Huating coal mine in Northwest China is now extracting the third fully mechanized top coal caving longwall face LW250103 in the first district of second lever. LW250103 is adjacent to the 250102 gob in the west with a 5 m width pillar between each other and solid coal seam in the east with 200 m face length. The coal seam is 12 m thick with an average dip angle of 5 degrees, the immediate roof is mudstone of about 7.5 m thick and 30 m fine sandstone constitutes the main roof and also plays the role of subkey strata, and 20 m gritstone with gravel 80 m away from the coal seam is analyzed to be main key strata. Figure
Layout of the mining seismic monitoring system installed in Huating coal mine.
During the exploitation of LW250102 which was adjacent to LW250101 and a 20 m width coal pillar between them, mining induced tremors occurred frequently. The locations of the tremors mainly concentrated within LW250102 and most of the big tremors located in the coal pillar but very few in 250101 gob, which indicated the 20 m wide coal pillar can effectively separate the connection of overlying strata movement between LW250102 and 250101 gob. In order to solve the pillar-burst problem, small pillar that is 5 m wide was adopted between LW250103 and 250101 gobs, but when LW250103 is mining, lots of tremors were located in 250101 gob and induced several rockburst accidents to the gob-side roadway, causing difficulties for normal production.
The time-space evolution process can reflect the rupture and movement of the overlying strata, for example, in the period that is from the beginning of the mining to the first weighting of the main roof, especially when the work face advanced ahead 120 m–150 m from the open-off cut, the frequency and total energy per day increased sharply and extended to 250101 gob, as Figure
Distribution of seismic source close to the gob side of LW250103.
March 1–15 with energy between 103 and 105 J
March 1–15 with energy exceeding 105 J
March 15 to April 1 with energy between 103 and 105 J
March 15 to April 1 with energy exceeding 105 J
The fully mechanized top coal caving LW10302 in number 10 district of Baodian colliery in southeast China is the second work face from the south boundary and is surrounded by gobs also called “isolated island” encountering mining earthquakes severely, so the 20-channel “SOS” was installed in the coal mine and nine stations were arranged around number 10 district in which there are seven in underground and two on the surface in order to monitor the tremors; the locations are given in Figure
Layout of the seismic station around number 10 district in Baodiao coal mine.
The coal seam is 8.7 m thick with a huge thick fine sandstone stratum arranged from 100 m to 150 m overlying about 200 m away from the coal seam. As can be calculated based on the key strata theory the huge thick stratum is the main key stratum with the limit span exceeding 350 m, which indicates after the winning of LW10301 the main key strata can still keep integrated and bears the overburden. So the south zone of LW10302 is subcritical extraction while the north area is full subsidence because of large scale of gobs. According to the theory analysis of the normal stress distribution rules with the work face length and width, we can conclude the stress will increases 64 times when the width doubles, so in most cases will exceed the limit of the main key stratum. Thus, during the extraction of LW10302 the main key strata are most likely to rupture.
A total of 40 big mining earthquakes with energy exceeding 105 J occurred during the LW10302 extraction from July 15, 2008, to April 30, 2009. These big tremors caused surface buildings quake, underground space damage but no real coal bump in the work face and roadways (Figure
Tremors with energy exceeding 105 J in the main key strata during the extraction of LW10302.
About 1–10% of the energy released by the tremor induced by key strata instability in gobs propagates in the form of seismic wave and imposes dynamic load on the coal and surrounding rock. The rock bursts will happen when the total energy that seismic energy
Layout of the deep hole blasting during LW250103 mining.
Plan sketch of the deep hole blasting in the roof
A-A profile of deep hole blasting
Also during the coal wining of LW10302 in Baodian colliery, systematic deep-hole blasting into the roof is carried out thereby reducing the disaster effectively and guaranteeing the safety of miners.
To reveal the mechanisms of mining tremors induced by key strata movement and instability under large scale exploitation, we use elasticity theory and structural analysis to explain the instable process comprehensively; meanwhile the seismological observation system installed in Baodian coal mine supplied in site investigation results for authentication, and the following conclusions can be drawn. Small pillar cannot separate the connection and influence of the gobs adjacent to each other, and the balanced structure formed by the key strata is going to be instable again under the mining disturbance and then induces mining tremors that cause damage to roadways. Mechanisms of the mining tremors can be inducted into three basic types that are rotary and sliding instability of low subkey strata, shear sliding of the high subkey strata, and the main key strata rupture and cave at limit span. The time-space distribution of the seismic sources recorded by the SOS system indicates that the mining tremors show an evolution rule that first small tremors occurred in low subkey strata and then developed to high sub key strata with energy increasing greatly. And instability process of high subkey strata in the gob lags behind that in work face area. Tremors occur in the low subkey strata most frequently with lowest energy and those in the high sub key strata less than in the low subkey strata but with higher energy, tremors caused by main key strata have the least number but with the strongest energy; however, most damage to the work face and roadways is attributed to the shear and sliding instability of high subkey strata, and main key strata tremors impact the least due to being far away from the work face.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work received financial support, provided by the Fundamental Research Funds for the Central Universities (2013QNB30); The National Basic Research Program of China (2010CB226805); The Twelfth Five-Year National Key Technology R&D Program (2012BAK09B01); The Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); and The Independent Foundation of State Key Laboratory of Coal Resources and Safe Mining (SKLCRSM10X05), which are gratefully acknowledged.