， ，，湯泉，劉康

(山東科技大學(xué) 礦山災(zāi)害預(yù)防控制省部共建國(guó)家重點(diǎn)實(shí)驗(yàn)室培育基地，山東 青島 266590)

0 導(dǎo)讀

礦山災(zāi)害嚴(yán)重威脅著中國(guó)煤礦的安全高效開采。實(shí)踐表明，幾乎所有煤礦重大災(zāi)害都與巖層運(yùn)動(dòng)及相關(guān)應(yīng)力轉(zhuǎn)移有關(guān)。研究礦山巖層運(yùn)動(dòng)規(guī)律是揭示并控制煤礦重大災(zāi)害的關(guān)鍵，被國(guó)內(nèi)外學(xué)者重點(diǎn)關(guān)注并加以研究。20世紀(jì)80年代，中國(guó)科學(xué)院院士宋振騏緊密聯(lián)系煤礦現(xiàn)場(chǎng)生產(chǎn)一線，創(chuàng)建了以巖層運(yùn)動(dòng)為中心的“實(shí)用礦山壓力控制理論”，被學(xué)術(shù)界稱為“傳遞巖梁理論”。 “實(shí)用礦山壓力控制理論”體系完整，用于指導(dǎo)開采實(shí)踐取得突出成就。

1) 針對(duì)煤礦采場(chǎng)不斷推進(jìn)的工程特點(diǎn)，以及實(shí)現(xiàn)煤礦安全高效開采必須面對(duì)復(fù)雜多變的地理環(huán)境和煤層地質(zhì)條件，明確了理論指導(dǎo)思想和體系，定義了相關(guān)學(xué)術(shù)名詞和術(shù)語(yǔ)，闡述了相應(yīng)的基本概念。主要成果包括：

① 區(qū)分和定義了“礦山壓力”(促使圍巖向已采空間運(yùn)動(dòng)的力)和“礦山壓力顯現(xiàn)”(在礦山壓力作用下產(chǎn)生的圍巖破壞及支架受力現(xiàn)象)兩個(gè)基本概念，指出“礦山壓力”存在的絕對(duì)性(采動(dòng)即有)和“礦山壓力顯現(xiàn)”的相對(duì)性，以及礦山壓力控制(設(shè)計(jì))的目標(biāo)是把“礦山壓力顯現(xiàn)”控制在安全上可靠、技術(shù)上可能、經(jīng)濟(jì)上合理的范圍。

② 隨著采場(chǎng)不斷推進(jìn)，無論是“礦山壓力”還是“礦山壓力顯現(xiàn)”都在不斷發(fā)展變化之中，變化規(guī)律是由巖層運(yùn)動(dòng)和破壞決定的。提出礦山壓力控制研究必須以“巖層運(yùn)動(dòng)為中心”，把不同采動(dòng)條件下覆巖運(yùn)動(dòng)和支承壓力大小分布范圍及其發(fā)展變化的規(guī)律，包括相關(guān)動(dòng)態(tài)結(jié)構(gòu)模型的建設(shè)和相關(guān)結(jié)構(gòu)參數(shù)的確定放在首要地位。

③ 指出任何開采深度條件下，影響“礦山壓力顯現(xiàn)”的巖層范圍是有限的，即采動(dòng)破壞失去向四周支承傳遞作用力的巖層(直接頂)和保持了傳遞作用力聯(lián)系、運(yùn)動(dòng)時(shí)對(duì)礦壓顯現(xiàn)明顯影響的巖層(老頂)兩個(gè)部分；可知的，即通過巖層動(dòng)態(tài)監(jiān)測(cè)可以確定；可變的，即可通過采動(dòng)條件的變更加以改變，因而可以認(rèn)為是可控的。

④ 對(duì)于可能垮坍的冒落巖層(直接頂)一般只能采取“堅(jiān)決頂住”，即“給定載荷”的控制方案。認(rèn)為對(duì)“老頂”相關(guān)巖層的運(yùn)動(dòng)，可以根據(jù)安全可靠、技術(shù)可能、經(jīng)濟(jì)合理的原則和相應(yīng)的力學(xué)保證條件，分別采取“給定變形”，即將作用力完全轉(zhuǎn)移給四周支承體上，和“限定變形”，即將沉降限定到要求狀態(tài)(位態(tài))兩種方案。對(duì)于老頂相關(guān)巖層的運(yùn)動(dòng)，不分條件地采用強(qiáng)頂硬抗的控制方案是不科學(xué)的、錯(cuò)誤的。

2) 根據(jù)安全高效開采，特別是頂板沖擊地壓、瓦斯煤層突出、透水突水及開采沉陷災(zāi)害有效控制的目標(biāo)，建立、發(fā)展并逐步完善了以“采動(dòng)覆巖運(yùn)動(dòng)和支承壓力分布”為核心的采場(chǎng)結(jié)構(gòu)力學(xué)模型和各類重大事故控制決策結(jié)構(gòu)力學(xué)模型體系，設(shè)計(jì)相關(guān)結(jié)構(gòu)參數(shù)確定的方法，建立了數(shù)學(xué)模型。把煤礦安全高效生產(chǎn)，特別是重大事故的控制決策從統(tǒng)計(jì)經(jīng)驗(yàn)推進(jìn)到針對(duì)具體煤層條件定量研究的發(fā)展階段。

① 確定了采場(chǎng)結(jié)構(gòu)力學(xué)模型的內(nèi)涵和結(jié)構(gòu)模型的結(jié)構(gòu)組成特征。針對(duì)定量控制設(shè)計(jì)需要，把采場(chǎng)推進(jìn)過程中分布在四周煤壁上的支承壓力分為煤壁已進(jìn)入破壞其受力由工作面長(zhǎng)度等采動(dòng)條件確定的“結(jié)構(gòu)拱”內(nèi)巖層運(yùn)動(dòng)決定的“內(nèi)應(yīng)力場(chǎng)”和“結(jié)構(gòu)拱”內(nèi)、外參與運(yùn)動(dòng)的巖層作用力決定的“外應(yīng)力場(chǎng)”兩個(gè)部分。

② 指出采場(chǎng)結(jié)構(gòu)模型(包括結(jié)構(gòu)組成和相關(guān)結(jié)構(gòu)參數(shù))隨采場(chǎng)推進(jìn)處于不斷發(fā)展變化之中，并從控制的差異出發(fā)，界定了“第一次來壓”和“正常推進(jìn)”兩個(gè)發(fā)展階段，并揭示了兩個(gè)發(fā)展階段動(dòng)態(tài)結(jié)構(gòu)模型的結(jié)構(gòu)組成和結(jié)構(gòu)參數(shù)運(yùn)動(dòng)發(fā)展的規(guī)律。

③ 在動(dòng)態(tài)結(jié)構(gòu)模型發(fā)展規(guī)律，特別是支承壓力分布和巖層運(yùn)動(dòng)關(guān)系的基礎(chǔ)上，提出了通過“礦山壓力顯現(xiàn)”推斷支承壓力分布(包括分布范圍及“內(nèi)應(yīng)力場(chǎng)”、“外應(yīng)力場(chǎng)”及應(yīng)力高峰位置)預(yù)測(cè)采場(chǎng)頂板來壓的“井下巖層動(dòng)態(tài)觀測(cè)研究方法”，并成功研究相關(guān)監(jiān)測(cè)儀表測(cè)試系統(tǒng)，較好地解決了通過現(xiàn)場(chǎng)實(shí)測(cè)確定相關(guān)結(jié)構(gòu)參數(shù)問題。

④ 通過大量現(xiàn)場(chǎng)事故案例分析和成功的控制實(shí)踐，揭示重大事故與生產(chǎn)現(xiàn)場(chǎng)“采場(chǎng)結(jié)構(gòu)模型”建設(shè)關(guān)系，在推進(jìn)重大事故控制決策模型體系的建設(shè)研究方面取得了進(jìn)展。

實(shí)用礦山壓力控制理論的提出和發(fā)展，為把采場(chǎng)礦山壓力研究從定性推向定量，使我國(guó)煤炭生產(chǎn)現(xiàn)場(chǎng)頂板控制由主要依靠統(tǒng)計(jì)經(jīng)驗(yàn)決策提高到能夠針對(duì)具體煤層條件的定量發(fā)展階段。

1 Introduction

Coal resource has been the most important energy resource in China for a long time[1-3]. Nowadays, more than 5 000 coal mines and 10 000 coal panels exist in China. With coal resource consumption increasing, different kinds of coal mine disasters have appeared in the past few decades. Rock burst, coal and gas outburst, a large area of the roof collapsing, large roadway deformation etc., have been threatening coal mine safety in China[4-7]. Practices indicate that the occurrence and control of most of the disasters mentioned above are related to ground pressure and strata movement. Therefore, ground pressure and strata control theory is of great importance for revealing the disaster mechanism and control method[7-11]. In the past decades, ground pressure theory has developed rapidly and disasters associated with strata movement have been controlled effectively[12-16].

Transfer rock beam theory (TRBT), which was established and developed by Academician Song Zhenqi of the Chinese Academy of Sciences, is one of the two major schools of thought in this area (the other is voussoir beam theory)[17-20]. TRBT is obtained from the coal ground field and actual production lines, and it plays an extremely important role in China’s mining safety. The coal and gas burst mechanism and the occurrence rule could be revealed well using TRBT[21-25]. Under the guidance of TRBT, the roof loading time, space distribution rule and pillar design can be obtained; the roadway deformation rule of underground pressure can be well understood; and the design and construction of the entry retaining wall under different mining conditions can be completed properly[22,26-28]. Thus, comprehending TRBT is of great importance for preventing and controlling mine disasters associated with strata movement. In this paper, the fundamentals and a summary of TRBT have been introduced, and the theory of the direction of strata movement has been proposed as well.

2 Definitions of “ground pressure” and “ground pressure behaviors”

2.1 Concepts of ground pressure (GP) and ground pressure behaviors (GPB)

These two basic terms are strictly defined in TRBT. In this theory, GP is defined as the force that pushes the surrounding rock towards the mined-out space after mining[12-14]. This force consists of two parts: The stress existing in the rock mass surrounding the mined-out space and the mining-induced force exerted by the moving strata on the surrounding rock boundary (the wall surrounded by the mining space). GPB refers to the phenomena that include the deformation and failure of the surrounding rock and the related abutment pressure, which are derived from ground pressure.

2.2 Differences and the connections between GP and GPB

Based on actual mine pressure control research, TRBT puts forward the absoluteness (i.e. the objective existence of this after mining activities) of GP′s existence and the relativity of GPB. The meaning of this relativity include two parts: (i) Only when the rock mass, that is surrounding the mining space, bears pressure that can greatly deform or even destroy the surrounding rock will the pressure markedly appear. (ii) The value of the stress endured by surrounding rock depends on the inherent structural and mechanical features of the support and the resistance to deformation of the surrounding rock provided by the support. Based on this, TRBT clearly points out that the aim of the research on ground pressure control is to keep ground pressure behaviors from inducing disastrous accidents and make it technically feasible and economically reasonable. The above-mentioned facts have demonstrated the relationship of the dialectical unity between the two key terms, which are different from each other but closely related, which have been created to control ground pressure.

3 The origin of GP and its components

GP， the force that, after mining, causes the movement of the surrounding rock towards the mined space, it includes the stress endured by the surrounding rock and the force caused by strata movement[8,20].

3.1 The stress in the surrounding rock

In coal mines, roadways are always dug and protected and work faces are always advanced in the original stress field or the re-distributed stress field after mining has occurred. Thereby the mining space is formed. The stress surrounding in the rock stored and distributed in the rock mass. The existence of this stress is the root of the surrounding rock’s failure and it related to accidents and disasters. TRBT emphasizes the major differences between the “original gravity field” in the original stress field and the tectonic stress field which is formed by the force of tectonic movement and is comprised of the residual tectonic stress after the generation of a coalfield. It was correctly pointed out that, for a long time in China, great mistakes have been made in decision making, deployment and the safety management of coal mining design, such as the rules of protecting the roadway with a coal pillar with a retaining fault and the classified management of coal mines based on statistical experience. Additionally, major accidents caused by these mistakes such as rock burst and coal and gas burst were all derived from the ignorance of the great difference between the two kinds of original stress fields.

From a practical objective, TRBT emphasizes that the stress field redistributed by mining is an uneven stress field. The stress peak in this stress field may be several times higher than the stress value of the original gravity field. The larger range of the strata without support and suspension after mining, the higher the stress value concentrated at the position of stress peak. At present, conventional longwall mining with coal pillars protecting roadways has been adopted in China’s coal mines. However, if mine workers neglect to calculate the distribution conditions of mining-induced stress field and then incorrectly excavate and protect roadways at the position of stress peak and the advance working face, the surrounding rock will be forced by the stress that is several times higher than the original stress to be pushed towards the mined-out space. This phenomenon is the root of major accidents and disasters including rock burst and coal and gas outburst.

3.2 The moving force of strata

It mainly refers to the force exerted by the movement (including even subsidence or impacting subsidence) of overburden on the mining space on the surrounding rock and support. It is obvious that the magnitude and the action process of the force are determined by the range of strata participating in the movement under corresponding mining conditions. The more strata that participates in the movement, the longer the development process of the acting force would be and the higher the pressure intensity during the movement would be. Therefore, figuring out the range of the moving strata and developing a rule of the strata′s movement under different mining conditions is the key to forecasting and controlling this force.

4 The distribution of abutment pressure

TRBT defines the pressure which is distributed on the coal wall surrounding the mining space during the advancement of the work face as abutment pressure, which is shown in Fig.1.

Fig.1 Distribution of abutment pressure[8,18-19]

In order to satisfy the practical requirements of safe and efficient production and the control of the relevant accidents and disasters, TRBT creatively divided this abutment pressure into two parts: One is the internal stress field, which is determined by the strata′s movement inside the structure′s arch, and the other is the external stress field, which is determined by the strata′s movement outside the structure′s arch. The theory points out that the excavation and protection of roadways should avoid the position of stress peak and should be completed in internal stress field that stabilizes after the movement of the strata fracture inside the fracture arch, which is the key to avoiding serious accidents and disasters such as rock burst or coal and gas outburst and in order to keep the surrounding rock stable for a long time under safe and economical support conditions[27-31].

5 The range of strata influencing GPB and the related concept of the control of the strata′s movement and working conditions of support

To achieve practical targets, TRBT has proposed the relevant notions and ideas.

5.1 The strata range influencing GPB and its two components

TRBT states that the range of moving strata, which is involved in mining and needs supporting control when excavating a roadway and an advance working face in any mining depth, is finite and knowable. The theory breaks down the tradition of classified definition according to the strength and thickness of the strata, and scientifically defines its two components from two aspects of structural features during the development of the related strata and the features that the moving strata transfer force to surrounding rock in mining space and support.

1) Immediate roof: It consists of self-caving strata at the position without support (goaf) beyond the face width during the advancement of mining space. The force acting in this area, generated by the movement of a fracture and the cavity of the strata (roof), should be borne by the support each time during the advancement of the work face, except for a wide hanging arch span caused by hard strata.

2) Main roof: The movement during mining can obviously influence the constitution of transferring the strata beam of the GPB in the stope, as shown in Fig.2. Each transferring strata beam in the range of the main roof is comprised of the strata (including supporting strata), which move simultaneously or almost simultaneously and thus influence the GPB in the stope, and the overlying follow-up strata. This rock beam maintains a connection of the structural mechanics all the time during the full period of the stope advancement, which includes the status of the continuous beam before the first fracture, the formation of a voussoir beam which has a crack throughout it after the fracture movement and the final completion of the subsidence movement. The above-mentioned connection refers to how the rock beam transfers its own weight and its overburden to the stope and the gangue in the goaf.

Fig.2 The range of moving strata influencing GPB in the stope

5.2 Immediate and main roof control and the concept of support working conditions

Only when the immediate roof is hard enough and the hanging arch span of the goaf is very wide can the immediate roof work under the condition of a cantilever, on the premise that the support is solid enough to bear the dynamic pressure impact of the immediate roof, according to the preset deformation when the immediate roof falls onto the gangue.

The theory has denied the traditional proposal that the main roof and its transferring rock beam (voussoir beam) can merely allow support work under the condition of preset deformation. It has added the idea that the support can work on the condition of limited deformation which controls the rock beam movement of the main roof. The theory has also clearly pointed out that it is possible and even necessary under certain conditions to control the main roof in the practical production process and especially the position and state of the rock beam under it, that is, the support should be forced to work under the required working conditions of limited deformation. Therefore, the theory has established an approximate calculation formula of rock beam control for position and state: The position and state formula of the strata beam of main roof to boost the theory of roof control into a developing stage of scientific measurement under specific roof conditions.

6 Structural mechanical model of the stope and its laws of formation and development

6.1 Compositions and functions of mining structural (mechanical) model

In TRBT, the model showing the temporal and spatial distribution of the two dynamic structures which describe the source of ground pressure in the mining process is defined as the mining structural (mechanical) model (abbreviated to mining structural model)[26]. Due to the fact that the structural characteristics of each component of the model are constantly changing as the mining space advances, the model is often called the “dynamic structural mechanical model”. Under the guidance of the “practical ground pressure theory”, the “dynamic observation method of strata formation” and corresponding monitoring methods have been adopted to scientifically determine the structural characteristics and dynamic development of the model under different mining conditions, which has laid the foundation for effective prediction and control of mine pressure and related accidents and disasters. Changing the mining conditions and the structural characteristics of the corresponding components of the control model is also a method for achieving those goals.

The structural model generated by the advancement of the working face has been shown in Fig.3, and its structure consists of two parts.

Fig.3 The model of the stope structure

1) The stope promotes the overlying strata where the movement enters the fracture failure state (the range of the overlying rock failure), including the broken strata in the goaf and the fractured strata (“voussoir beam” or “transferring rock beam”) that transfer the weight of themselves and overlying rocks to the surrounding coal (rock), and the two parts have been shown in Fig.3. The boundary of this range starts from the boundary of the coal seam which has been compressed to failure, and the fracture boundary line of the related strata which has been circled by “fracture arch”. When the strata related to this range are suspended before and after the fracture movement, all of them are weight components of the abutment pressure distributed in the front of the mining boundary. The release of the flexural compressed elastic energy, stored during the occurrence of the strata fracture movement and its acting force during the subsidence movement, is the driving force for the impact damage of the strata surrounding the mining space and the further development of the damaged rock. Obviously, when the subsidence movement of the fractured and damaged strata is fully realized under the corresponding mining conditions, the movement and failure of the rock (roof, floor and side wall) surrounding the mining space will stop and enter a “completely” stable state. At this moment, even if a new roadway is excavated and maintained along the original mining boundary, the obvious ground pressure will not appear.

2) The abutment pressure is distributed in front of the peripheral wall of the stope (“distribution of mining-induced stress field”). For the established mining depth and overlying rock conditions, as long as the mining height, the length of the working face and the other mining conditions adapt to each other, the advancement of the stope to a certain position will result in the compression and failure of the coal wall, and produce the appearance of two stress fields in the distribution of the abutment pressure in front of the coal wall. As was mentioned earlier, at this time if the workers are forced to excavate and maintain a roadway in high-stress areas with coal pillars protecting roadways, without reliable roadway support, rock burst, coal and gas outburst and other major accidents would be unavoidable. On the contrary, if mining deployment of non-coal pillar protected roadways are adopted to achieve excavation and maintenance of mining roadways in which the internal stress field has stabilized after the fracture and subsidence of the strata inside the fracture arch, rock burst and other related disasters could be avoided.

6.2 The rules of the formation and development of dynamic structural model (DSM)

As was mentioned earlier, the “stope structural model” includes the range of fractured and damaged strata, which has been circled by the “fracture arch”, and the abutment pressure of the related strata distributed outside the fracture arch. In order to fully understand and master the characteristics of its structure and composition, and the requirements for the formation of its related structures as mining progresses, the dynamic process laws of its formation and development are fundamental to the control and research on GPB and related disasters and accidents.

6.2.1 Dynamic rules of the formation and development of the strata fracture movement inside the “fracture arch”

With the advancement of the stope, the down-up combined motion of overburden includes: (i) Adjacent strata are forced to subside and bend by gravity to make the shear stress increase beyond their limit, thus forming a separation layer; (ii) According to the requirements of the simultaneous movement of the adjacent strata, the composite simultaneously-moving transferring rock beam (transferring rock plate) continues to subside; (iii) The area of suspension and span of the overburden reaches the fracture, and since the tensile stress exceeds its limit, the overburden will complete the whole process of fracture and subsidence movement, from fracturing at its fixed end to fracturing at the middle position.

Among these processes, when the subsidence value of the fractured strata exceeds the thickness of the supporting layer of transferring rock beam, the rock beam would be unstable, damaged and cave in, it would lose the structural mechanical connection with the surrounding supports and fail to transfer the force acting on them. On the contrary, even if the rock beam droops and touches the floor or the gangue, the structural mechanical characteristics of a transferring rock beam mean it can transfer its own weight to the overlying rocks and it will still be maintained. The mechanical conditions of the formation of the above-mentioned development processes are as follows.

1) The mechanical conditions of the interlayer separation formation.

The formation of the separation layer between adjacent overlying strata results from the shear stress, which is generated in the subsidence and bending process of the interface between the upper and lower strata and the soft layer clamped between them, when it increases beyond the strength limit.

This condition has been shown in Fig.4 and can be calculated from formula (1).

Fig.4 The mode of the interlayer shearing damage

(1)

2) The mechanical conditions for the simultaneous movement of the upper and lower strata that generates the separation layer.

The criteria for judging as to whether the upper stratum moves simultaneously with the lower stratum can be obtained from the “deflection method” and the “interval method”.

a) The criteria of the deflection method are as follows:

If the adjacent strata form a combined “transferring rock beam” and if they move simultaneously, then

(2)

If the adjacent strata form a single “transferring rock beam” and if they move separately, then:

τ=τ0=c+σntanφ.

(3)

WhereECandESare the elastic modulus of the lower and upper strata respectively. They can also be replaced by the tensile or compressive strength of the strata.

b) The criteria of the cracked interval method are as follows:

If the adjacent strata move simultaneously, then

CS≥KCC,K>1，

(4)

If the adjacent strata form a single “transfer rock beam” and move separately, then

CS

(5)

c) The mechanical conditions when the “transferring rock beam” fails to transfer the force and collapses, are as follows:

Under the conditions of a common rock thickness, the condition of instability and collapse of the transferring rock beam is that the thickness of the supporting strata of the transferring rock beam is less than the subsidence value of the rock beam.

(6)

Wherehis the mining depth;mnis the thickness of the collapsed strata in the lower part; andKAis the bulking coefficient of the collapsed strata.

Inside the fracture arch, the bottom-up development process of the relevant transferring rock beams (transferring rock plates) during stope advancement includes two stages; they start from the first fracture movement of the lowest rock beam shown in Fig.5, then they develop to the stage of first loading when the first fracture movement of the top rock beam has finished and ends when all the rock beams have entered a stage of normal advancement of periodic fracture movement.

Fig.5 Development laws of the fracture movement of rock beams in the fracture arch

The process of the formation and development of the fracture movement of each transferring rock beam (rock plate) in the embedded state is as follows: i) The embedded boundary (i.e. the end) cracks and enters a hinged state; ii) The maximum bending moment is transferred to the middle, leading to cracking at this position; iii) The rock beam (plate) achieves connection between the cracks of the embedded boundary and that of the middle with the supports at four sides of the border with the rotary axis under the action of gravity; iv) The rock beam (plate) enters a rapid rotation subsidence process until it reaches the “support” at the bottom.

With the stope advancing, the work face is in a fixed state (both of the two sides are free from mining influence) and it completes the above-mentioned fracture movement. The fracture structure formed in this process and corresponding structural parameters have been shown in Fig.6.

Fig.6 The fracture structure of the work face in the fixed state

The limit decomposition method of rock plates has been adopted to obtain the span of the fracture at both sides according to the virtual work principle, and the results are as follows.

The first fracture:

(7)

And the periodic fractures:

(8)

WhereLis the length of the working face, andC0andCare the first fracture (weighting) interval and the periodic fracture interval for the rock beam respectively, which can be expressed by formulas (9) and (10), according to the mechanical conditions of rock beam fracture.

(9)

(10)

Wheremnis the thickness of the supporting strata of the transferring rock beam (plate), andmcis the thickness of the follow-up strata. If the transferring rock beam (plate) is composed of a single stratum, then:

(11)

(12)

Research has proven that if the differences between every periodical fracture are ignored, then the approximate value of the periodic weighting interval can be selected.

In the above-mentioned formula, [σn] is the tensile strength of the supporting strata of the rock beam.

In Fig.6, the structural parameters of a sub-horizontal coal seam can be approximately calculated as:

The first fracture:

e0=L-2d，

(13)

And the periodic fractures:

e=L-2d.

(14)

As for a sub-horizontal coal seam, under conditions of a certain composition of overburden and length of work face, the transferring rock beam is contained in the fracture arch formed when the stope advances to the length of the work face, and the corresponding structural parameters have been shown in Fig.7.

Fig.7 Structural parameters of the fracture arch and the related cracked rock beam

6.2.2 Dynamic laws of the formation and development of “abutment pressure” distribution on the coal seam and strata outside the “fracture arch”

As for the abutment pressure acting on the front of the coal wall by constant mining and the movement of overburden, its size, distribution characteristics and the laws of its formation and development are determined by the thickness of the mined coal seam (mining height), the length of the work face and other mining conditions for a given mining depth.

The rules revealed by the theory include the direction of advance and the direction parallel to the working face.

1) During the process of mining, along the direction of coal mining, the rules of distribution, development and change of abutment pressure apply at the front of the coal wall.

The development rule of the abutment pressure distribution revealed by the theory has been shown in Fig.8. Its development process includes three stages as follows: (i) The coal wall is not damaged by compression, with the pressure peak in the elastic compression stage along the edge of the coal wall (l1); (ii) Plastic damage occurs to the coal wall, with the pressure peak transferring forward (l2); (iii) The “internal stress field” that occurs is directly related to the movement of the fracture arch towards the strata.

The abutment pressure distribution, which is formed at the third stage of development, includes the “internal stress field” that is directly related to the pressure behavior and the loading from the relevant strata fracture inside the fracture arch and the “external stress field” with the local plastic damage in the elastic compression state. The relevant distribution range and corresponding stress reach a maximum value when the working face advances to the length of it.

Fig.8 The development rule of abutment pressure distribution in the direction of the work face

2) The development rule of abutment pressure distribution on the two sides (along the direction of the working face length) during the mining process.

The abutment pressure distribution that is revealed by the theory, on the two sides when the stope advances to the given position, has been shown in Fig.9.

Fig.9 The law of the abutment pressure distribution on the two sides as the stope advances to the given position

Research has shown that the distribution characteristics of the abutment pressure in the front of the work face and that of the coal wall at two sides are almost the same at any given position that the stope in sub-horizontal coal seam advances to. The abutment pressure value and the law of the development and change of its distribution are also similar. In the mined-out areas, the distribution characteristics and range of the abutment pressure on both sides in the different parts at a given advancement position of the work face will be different. With the stope advancing, the characteristics of the abutment pressure formed at this position, which include the distribution range, especially the range of internal stress field, the value of the pressure borne by the corresponding positions and the behavior characteristics of the pressure, will experience significant changes. The changes will be directly related to the formation of the fracture arch and the development and stabilization process of the fracture movement of the relevant strata inside the fracture arch. The law of its development and change, which includes the compression and damage process of the coal wall before fracture and damage of the lowest rock beam inside the fracture arch and the change of pressure borne by the coal wall during the whole process, from the fracturing of the relevant rock beams inside the fracture arch to subsidence and to the final stabilization, has been shown in Fig.10.

Fig.10 Variation rule of the stress distribution with strata movement

7 Discussion

For a long time TRBT has had a significant impact on coal mine production and the subject of coal mining in China. The achievements that TRBT has created consist of three aspects.

1) A subject theory system with the core of the overlying strata movement and the supporting pressure distribution rule was established, which was obtained according to the engineering characteristics of the continuously advancing working face.

2) The decision theory and model was established, with the goal to achieve safe and efficient production in coal mines, especially the control of major accidents and environmental disasters.

3) Guided by PCPGT, great achievements in the mining method, mining technology, and especially in major disaster control, have been realized. These achievements include roof disaster control, rock burst and gas burst control, overlying three zone division and mining subsidence control, and mining without coal pillars.

8 Conclusion

On the basis of in depth study and research into traditional ground pressure theories, both in China and abroad, the TRBT has been established according to the engineering characteristics and the geological conditions of mining as the stope constantly advances, and it has achieved remarkable progress including:

1) It has made the research goals in this field clearer. The theory has clearly proposed that the research goal of ground pressure control is to keep ground pressure behaviors from inducing disastrous accidents and make it technically feasible and economically reasonable.

2) It has pointed out a guiding thought that is centered on the research on strata movement and it has given priority to research on further revealing the relationship between the ground pressure behaviors and the relevant disasters caused by them and the strata movement and stress distribution characteristics in the relevant stress field. The theory has correctly stated that with the stope constantly advancing, both the ground pressure and ground pressure behaviors continuously develop, and this change has certain laws which are determined by the movement and damage of the relevant strata. It has clearly proposed that the research on ground pressure control must keep the strata movement at its core and put in first place the overburden movement under different mining conditions, as well as the distribution range of abutment pressure value and the law of this range developing and changing with the stope advancing, which includes the establishment of the relevant dynamic structural model and the relevant structural parameters. It has correctly pointed out that the range of strata movement that affects the “ground pressure behaviors” at any mining depth is “definite”, which includes: the strata (immediate roof) which may be destroyed and fail to transfer the force acting on it to the supports around it after mining and the strata (main roof) which can keep transferring its own weight and overburden to the supports around it during the whole movement process and whose movement can greatly influence the ground pressure behaviors in the stope. Moreover, it has clearly pointed out that they are knowable under given mining conditions, that is, they can be determined through theoretical deduction and especially through the monitoring of the conditions of the ground pressure behaviors on the walls surrounding the mining space. They are variable as well, that is, they can be changed by varying the length of the coal work face, the mining procedures and other mining conditions. In this way, the range of the research on ground pressure control has been scientifically defined and the key points of the relevant research have been highlighted.

3) On the basis of the above research, the theory has creatively put forward the “dynamic observation method of underground strata”, which can deduce the distribution of abutment pressure and forecast the loading of the stope roof, by monitoring the abutment pressure behaviors on the walls surrounding the mining space and the variation law of roof movement. It has also integrated the prediction, forecast, control design and control performance of ground pressure distribution and roof activities, thus achieving unity of “theory” and “method”.

The above mentioned research outcomes have laid a solid foundation to boost the traditional research on ground pressure theories to the development stage where scientific and quantitative analysis can be conducted according to concrete coal seam conditions.