Mechanism and control of strong rock pressure in the gully area under compound key strata

In view of the problem strong rock pressure is easy to occur in coal seam mining in western gully terrain. Based on the coal seam mining conditions in the gully area under the compound key strata of Zhujiamao Coal Mine, the methods of similar simulation, theoretical calculation and analysis, numerical simulation and field measurement are adopted. The dynamic structure model of the key strata of the gully is constructed, and the mechanism and prevention and control measures of the strong rock pressure in the gully area of the working face under the compound key strata are analyzed. The research shows that the upper slope section of the gully area first undergoes rotary deformation, and then the chain structure formed by the broken block occurs sliding instability and impacts inferior key strata, resulting in strong rock pressure behavior in the working face. The calculation formula of the first breaking step distance of the main key strata rotation deformation and sliding instability during the movement of the overlying strata is derived. The calculation results show that the initial rotation deformation and sliding instability breaking step distances are 27 and 142 m, respectively, which are in good agreement with the support crushing at 145 m of the first gully. Using 3DEC numerical simulation, the stress of the second gully is reduced by 3.8 MPa after pressure relief, which verifies the position of strong rock pressure calculated by the formula and provides a theoretical basis for subsequent hydraulic fracturing. According to the mechanism, the comprehensive prevention and control measures of strong rock pressure in gully terrain are put forward. The support resistance of the working face is calculated to be 10,011 kN by the support resistance formula, and the ZY11000/14/34D hydraulic support is used. The working resistance of the calculated dangerous area after hydraulic fracturing is basically lower than 30 MPa, and the control effect is good. It has reference significance for coal seam mining through gully terrain.


| INTRODUCTION
With the transfer of the mining center of China's coal resources to the west, the loess gully landform in the western mining area is widespread.The bedrock is thin, the thickness of the overlying sand is large, the surface gully is vertical and horizontal, and the surface gully is fragmented.Especially in the gully, the key strata of bedrock is missing due to erosion, which makes the dynamic load of mine production appear violent. 1,2In the mining process of Yushen mining area in the Jurassic coalfield of northern Shaanxi, the surface bench subsidence caused by gully topography, the roof breaking of the working face, the serious rib spalling and the sharp shrinkage of support column have a great impact on mine safety production. 3,4Therefore, it is urgent to carry out research on the mechanism of dynamic mine pressure in the gully area, provide a reasonable scheme for mining prevention and control technology in the gully area, and ensure safe and green mining during the mine crossing the gully.
Many scholars have carried out effective research on the comparison of the occurrence area and influencing factors of dynamic mine pressure in the gully terrain.Xu 5,6 pointed out that the mine pressure mainly occurs in the uphill section of the gully.When the key strata passes through the uphill section of the gully, it is not easy to form a stable masonry beam structure due to the lack of lateral horizontal squeezing pressure.In view of the fact that the mine pressure of Nanliang Mine mainly occurs at the bottom of the gully, Zhang et al. 7 obtained the calculation formula of the "sliding rotary load" of the overlying strata, and revealed the fracture and instantaneous slip mechanism of the nonuniform load beam.Zhang et al. [8][9][10] pointed out that there were obvious differences in the terrain of nongully areas, the downslope section of the gully and the bottom of the trench, and analyzed the main reasons affecting the dynamic load pressure of the working face from key factors such as trench depth and slope angle.Zhao et al. 11 pointed out that the key strata of the uphill section exists in the form of a cantilever beam under nonuniform load.When the gully slope angle is less than 57°, the instability form is rotary deformation instability.When the gully slope angle is greater than 57°, the instability form is sliding instability.Li 12 quantified the influence of gully occurrence on key strata, deduced the calculation formula of the additional stress of surface gully on key strata, and revealed the fracture characteristic of rock strata affected by gully occurrence.In the aspect of overburden control, Hou et al. 13,14 derived the calculation formula of the first weighting step and the periodic weighting step of the combined key strata based on the analysis of the relevant parameters of the combined key strata.Zhang 15 pointed out that the development of overlying strata movement to the gully after the gully has a great influence on the overlying strata movement, and clarified the basic roof cycle breaking step, strength and gully slope movement characteristics.Wang et al. 16 believed that the initial fracture of the main roof was asymmetric when mining under the gully slope.Under the same geological conditions, the larger the slope angle, the greater the influence of the mining slope on the mine pressure behavior of the working face.Through similar simulation experiments, Liu et al. 17,18 found that the large-scale pressure of the knife column mining of shallow buried coal seam was actually caused by the domino effect, and the collapse started from the direct top and gradually expanded to the loose strata.Li et al. 19 studied the geological and mining conditions of shallow coal seams in northern Shaanxi and obtained the overburden structure of thin bedrock coal seams in shallow and deep strata in northern Shaanxi by numerical simulation and mining practice.Fan et al. 20 analyzed the dynamic evolution characteristics of overburden strata movement and fracture expansion and distribution in horizontal and vertical directions in shallow coal seams.With the advancement of working face, overburden strata will collapse synchronously with the surface.Cao et al. 21,22 obtained that when the overlying coal seam is directly hard rock, the abutment pressure concentration coefficient and the energy released by the system are the largest.The fracture of hard rock will pose a great threat to the safety production of the working face, which is easy to cause support failure accident or rock burst.Wang et al. 23 studied the relationship between "supportsurrounding rock" during the mine pressure appearance of the working face, established the mechanical model of the first breaking and periodic breaking of the basic roof of the stope under the gully slope, and obtained the support resistance to control the sliding instability of the roof structure.Li et al. 24 studied the influence of gully topography on the stress field distribution and mine pressure behavior of shallow coal seam mining.Zhu et al. 25 studied the mechanism of mining pressure behavior in shallow buried thick coal seam mining in gully area, which provided a useful reference for the characteristic of overburden movement.Li et al. 26 studied the temporal and spatial distribution characteristics, formation mechanisms, and related preventive measures of overburden failure in shallow coal seam mining in the gully area.Zhang et al. 27 studied the occurrence mechanism and influencing factors of dynamic load mine pressure in shallow buried coal seam mining face in gully area, and put forward reasonable prevention and control measures for mine pressure appearance in gully area.
Many scholars have carried out in-depth research on terrain mining through gullies under a single key strata, but the problem of dynamic rock pressure in terrain mining through gullies under the composite key strata is more complicated.The research results of many scholars have laid a firm foundation for the smooth passage of mine mining through the gully area, but most of the research results are aimed at the mechanism of mine pressure appearance in a certain area of gully terrain under a single key strata, while the root cause of dynamic load mine pressure appearance caused by the whole mining process under the composite key strata is relatively less studied.At the same time, the comprehensive prevention and control measures proposed in combination with the mechanism have not been applied to the field practice.In view of this, based on the background of Zhujiamao Coal Mine, the paper study the dynamic load mechanism and field application of shallow buried coal seam crossing gully terrain.By adopting a similar simulation experimental method and combining it with on-site mining pressure monitoring, this study aims to analyze the causes of dynamic mining pressure before and after the gully area, verify the results through numerical simulation analysis, and understand the disaster-causing mechanism in the gully area under the influence of composite key strata.Subsequently, the study aims to optimize the prevention and control measures for dynamic mining pressure in the gully area.The research results provide scientific guidance for safe and efficient mining in the gully area of Zhujiamao coal mine, ensuring smooth mining operations across the gully.Additionally, the findings have certain reference significance for the safe exploitation of mineral resources in the gully area.

| Geological overview and mining conditions
Zhujiamao Coal Mine is located in the northeastern part of the Yuheng Mining Area (Southern District) of the Jurassic Coalfield in Northern Shaanxi Province.and on the southeast edge of the Maowusu Desert bordering the Loess Plateau, the surface is basically covered by Quaternary loose sediments, and bedrock is sporadically exposed in larger gullies.The overall minefield structure is a nearly westward, gently inclined monoclinic strata.Most of the middle and southern parts of the mine field are loess ridge area, and the northern part is the river gully area.The No. 1305-2 working face discussed in this paper is located in the west limb of the first panel of Zhujiamao Coal Mine, with a strike of 3905 m.Zhujiamao Coal Mine can only mine the No. 3 coal seam of the Jurassic Yanan Formation.The buried depth of coal seam is 183.6 m, the average dip angle of coal seam is 0.8°, and the thickness of coal seam is 3.5 m.The long wall fully mechanized mining is adopted, and the roof is managed by all caving method.The corresponding ground position is mainly loess ridges, with large undulating terrain, fragmented and ravines.There are two ravines passing through the middle and rear parts of the working face, and the flow direction is generally from south to north, and the bedrock of the key strata at the gully is eroded, and the missing thickness is 15-30 m, which belongs to the typical coal seam mining in the gully area.The ZY9000/17/35 hydraulic support is selected, and the rated working resistance is 9000 kN.
The characteristic of mine pressure in the working face is affected by the fracture of overlying strata and the stability of fractured rock mass.The key strata thickness, fracture, and instability form have an obvious influence on the strong rock pressure in the working face.Therefore, the coal seam and roof strata of the working face are sampled and analyzed.According to the key strata theory of strata control 28 and the 805 borehole histogram of the No. 1305-2 working face of Zhujiamao Coal Mine, the position of the key strata of the overlying strata of the working face is judged.There are two key strata in the overlying strata of the No. 3 coal seam, which belongs to the structure of composite key strata.The rock strata that play a decisive role in the whole or part of the rock mass activity when there are multilayer hard rock strata in the overlying strata of the stope are called the key strata.As shown in Table 1, the inferior key strata is medium sandstone with a thickness of 15.1 m, and the main key strata is medium sandstone with a thickness of 11.5 m.The main key strata is missing at the valley.

| The phenomenon of dynamic load mine pressure in gully terrain
The No. 1305-2 working face experienced two consecutive support failure events in the uphill section of the first gully, resulting in a total production interruption of 16 days.After the advancing distance of the No. 1305-2 working face was about 2400 m, severe pressure occurred at the first gully, which caused the roof of 46-122# support in the working face to be disconnected and cut at the rib, among which 65-95# support was all crushed to death, causing damage to the field equipment and affecting the production for 10 days.Subsequently, the second severe pressure occurred, and the resistance of most of the 0-110# supports suddenly increased by about 15 MPa in a short time.Most of the 45-95# supports in the middle of the working face were crushed to death, and the hydraulic pipeline was seriously damaged, accompanied by a huge sound, affecting production for 6 days.Compared with the conventional periodic weighting of the working face, the mining through the gully terrain had a sudden and huge energy.
Mining under the gully has led to obvious cracks on the surface.At the same time, with the step-sinking phenomenon.This has caused damage to the ecological environment of the surface.For the underground working face, the roadway side drum and roof subsidence have been caused.Rib and roof cutting are more serious.It causes damage to hydraulic supports, pipelines, and other equipment, resulting in significant economic losses.It has a serious impact on the safety production of the working face and the surface environment.Figure 1 shows the surface and underground damage during the gully terrain.The purpose of this similar simulation experiment was to explore the failure characteristics of the overlying strata under the gully terrain.Combined with the analysis of the working resistance of the simulated support, the characteristic of mine pressure in different regions was clarified, and the mechanism of dynamic mine pressure was studied, which provided a theoretical basis for the prevention and control of dynamic load on the working face.According to the field coal rock histogram and the physical and mechanical parameters of coal rock, the field engineering research object was reduced to an experimental model.The physical material model design of the gully area of Zhujiamao Coal Mine is shown in Figure 2. The length, width, and height of the model design are 300, 20, and 175 cm, respectively.The geometric similarity ratio C l is 1:150, the time similarity ratio C t is 1:12, the velocity similarity ratio C v is 1:12, and the displacement similarity ratio C s is 1:150.According to the design and related requirements of the model construction, combined with the actual engineering background, the main material used in the physical similarity simulation experiment was river sand, and the auxiliary materials were white powder, gypsum, mica, and water.When the coal seam was mixed, fly ash was added, and the rock strata of different lithologies were simulated according to the determined ratio material, and the mica powder was laid between the strata to simulate the rock bedding.Due to the lack of key strata in the gully, especially the thin bedrock cover in the upper slope section, it was possible to break and fall, which led to severe mine pressure.Therefore, it was necessary to accurately monitor the pressure in the process of working face advancing.In the process of model mining, the simulated support sensor was used to monitor the support resistance, and the working resistance values of the support at different advancing positions were analyzed.To analyze the movement characteristics of overburden rock, the displacement monitoring points were arranged above the key strata of coal seam and on the surface, so as to obtain the characteristic of mine pressure in different areas, revealed the coupling disastercausing characteristics of "stress field-displacement field" and the characteristic of mining-induced stress in the mining of working face, and clarified the location of dynamic mine pressure.

| Movement and failure characteristics of overlying strata in different areas
Different from the mining of conventional coal seams, the lack of critical strata in the gully area made the characteristics of roof caving and breaking rings often different.Therefore, the movement characteristics of overlying strata and the characteristics of pressure in the process of rock breaking were analyzed.
Figure 3 shows the failure characteristics of overburden rock under different advancing distances.By comparing the failure characteristics of overburden rock in different areas, the particularity of overburden rock failure in the gully area was explored, so as to clarify the causes of mine pressure.Before the working face advanced to 240 m, the integrity of the key strata was good because of the nongully area.The hard rock strata above the goaf were broken into neatly arranged rock blocks in the fracture zone, and the rock blocks were hinged by horizontal thrust, which was easier to form a masonry beam structure.When the working face was mined to 264 m after moving the support, at the 2/3 position of the upper slope section, there were end face roof breaking and main key strata sliding.In this process, the roof of the working face also appeared serious caving and rib spalling.
The variation curve of the working resistance of the support with the advance of the working face is shown in Figure 4. Through the monitoring and analysis of the support resistance of the working face in the experiment, it was shown that in the nongully area, the support pressure was about 25 MPa, and there was a regular periodic weighting below the rated working resistance.In the downhill section of the gully, the support resistance had a certain fluctuation, but no severe pressure occurred, and the support resistance was low, indicating that the possibility of dynamic rock pressure was small.When the working face advanced to the 2/3 position of the uphill section of the gully, the support resistance increased to 35 MPa suddenly,.After reaching the maximum value, it returned to the normal value.The uphill section of the first gully was about 210 m long, corresponding to the dynamic load mine pressure at about 140 m on the mine field, which was more consistent with the support failure event at 145 m on the mine field.
To summarize, the formation of a masonry beam structure is facilitated by the collapse of key strata in the non-gully section of mining.The mine pressure within stops follows a relatively regular pattern and occurs in a controllable cycle.Upon entering the gully area, however, due to the absence of the main key strata, a stable structure cannot be formed due to limitations imposed by lateral horizontal forces; instead, it exists as a cantilever beam.As a result of variations in bedrock thickness, the overlying strata assume a "nonuniform load beam" structure that is supported by the key strata.In uphill sections, rotational deformation or sliding instability may occur.Compression takes place on the upper slope of gullies.When strong ore pressure arises, rock mass instability primarily manifests as sliding instability with more severe damage.Face support must provide appropriate support force to prevent sliding and destabilization of the main key strata structure.Considering both failure patterns observed in overlying rock and support resistance during operation reveals that masonry beam structures are prone to form in downslope sections of working faces while strong ore pressure tends to occur on upper slopes of gullies.However, there is currently no theoretical basis or calculation formula available for determining specific locations where this occurs, nor is there clarity regarding mechanisms driving strong ore pressure development on terrain characterized by gullies.

| Dynamic structure of gully key strata
Based on the characteristics of mine pressure and the evolution characteristics of the overburden structure mastered by similar simulation experiments, the dynamic structure model of key strata in the gully was established, and the mechanism of mine pressure appearance in the gully terrain was studied.It can be seen from Figure 5 that the coal seam mining caused the stress disturbance of the original rock.In the downhill section of the mining gully area, the subkey strata collapsed and fell to form a masonry beam structure.The critical blocks bit each other.When the main key strata rotated and sank, it can be subjected to the lateral restriction of the rear broken block structure.There was a certain lateral horizontal pressure limit, which was conducive to the stability of the block structure and was easier to form periodic collapse.Therefore, when crossed the downhill section of gully terrain, the mine pressure of the working face was generally normal, and the phenomenon of dynamic load mine pressure was not easy to occur.
In the uphill section of the mining gully area, the subkey strata sank above the goaf, and the overburden failure continued to pass upward.When passing through the uphill section of the gully area, the left side lacked the limitation of lateral horizontal force, and the slope angle of the gully was small.The main key strata existed in the cantilever beam structure would undergo rotational instability at the initial stage due to the influence of gravity.After instability, the broken blocks were hinged to each other in the form of E 1 , E 2 , and E 3 shown in Figure 5.When the working face was continuously mined, the chain structure formed by the hinged broken blocks would slide and lose stability due to the lack of horizontal friction force, resulting in a strong downward impact force.The critical block B 2 , which played the role of bearing load, would not be able to continue to maintain a stable structure after being strongly impacted, other words, the impact load was formed on the critical block B 2 in an instant.The load was then transmitted to the support, resulting in the impact load of the support to initiate the mine pressure, which was also the root cause of the dynamic mine pressure in the uphill section of the gully area.
The characteristics of mine pressure behavior in the gully area were as follows: it occurred mostly in the upper slope of the gully, and the characteristic of mine pressure behavior was mainly induced by the sliding instability of the main key strata breaking block.In the actual mining process, taking pressure relief measures in advance to prevent energy accumulation will become an effective prevention and control mechanism for safe and efficient mining in the gully area.Therefore, it was the first problem to be solved to clarify its sliding instability position.

| Stress analysis of gully key strata structure
In the previous section, the structure diagram of the key strata of the gully is constructed, which shows the law of mine pressure with the main key strata breaking block sliding instability as the main inducing factor.However, the exact location of the pressure is not clear.Therefore, the mechanical model is constructed according to the key strata structure diagram of the gully, and the mechanical characteristics are analyzed.To clear the pressure position, we must first construct a reasonable mechanical model for mechanical analysis.The construction of the mechanical model is based on the key strata structure diagram of the gully to study the mechanical characteristics of the continuous gully.
In the uphill section of the gully, the overlying strata of the key strata gradually became thicker along the slope angle.On the contrary, in the downhill section, the overlying strata of the key strata gradually became thinner along the slope angle.The effect of gravity on the main key strata will be increasing or decreasing.Therefore, the key strata of the overlying strata was considered to be subjected to nonuniform load.Due to the bearing effect of the lower subkey strata, when the main key strata passed through the gully area, the key strata of the overburden rock can be regarded as a cantilever beam structure before the initial fracture.According to the structural change characteristics of gully overburden rock in Zhujiamao Coal Mine, its mechanical bearing characteristics had certain rules.The mechanical analysis model of the key strata of overlying strata in continuous gully mining was shown in Figure 6, where a was the uphill section of the first gully, b was the downhill section of the second gully, and ab section constituted the basic unit of the mechanical model of continuous gully.
The distance of the uphill section of the first gully is about 210 m.Combined with the breaking characteristics of the cantilever beam and the on-site pressure situation, the first breaking will occur in section a. First, section a is taken as the research object.The normal stress of any point in the beam is: The shear force at this point is: q x is the load of overlying strata at any point, if the beam is taken as the unit width.
The section moment of the beam: Then the normal stress of any point: According to its nonuniform load, the load of the overlying strata at any point is q xρg θ = tan x .According to the calculation of material mechanics, the shear force F S and bending moment M S at the fracture are: According to the stress condition, the maximum tensile stress is: The maximum shear stress occurs on the neutral axis of the rectangular section beam, that is, y = 0. Therefore, the maximum shear stress τ max is: S max 2 (7)   According to the maximum tensile stress and the maximum shear stress failure criterion of the material, the conditions of no tensile failure and shear failure in the roof strata of the working face can be established as follows: The rotation deformation and sliding instability of the key strata of the overlying strata in the uphill section of the working face can be expressed as follows: In the formula, a is the uphill section of the valley; M is the bending moment of the section where the point is located; y is the distance from the point to the neutral axis of the cross-section; Jz is the section moment of the symmetric neutral axis; ρg is the average rock mass weight of the overlying strata of the key strata, taking 0.018 MN/m 3 ; h is the thickness of the main key strata, taking 11.5 m; θ is the slope angle of the upper slope section of the gully, taking 16°; In addition, the allowable tensile strength σ ( ) t = 0.8 MPa and the allowable shear strength τ ( ) = 6.8 MPa of sandstone in the main key strata are measured.Substituting the above data into Equations ( 10) and ( 11), X 1 = 27 m and X 2 = 142 m are obtained.By analyzing the similar simulation, mechanical model, calculated value and field actual situation, it can be concluded that the main key strata has rotary deformation in the early stage of mining through the gully uphill section, and the initial fracture distance is 27 m.Then, due to the superposition effect of nonuniform load, the stress gradually increases, and the fracture distance will become shorter, which is consistent with the current periodic weighting step of the No. 1305-2 working face is about 20 m.At the same time, it can be concluded that the rotary deformation has less impact on the masonry beam structure formed by the lower subkey strata, and the manifestation is that there is a regular pressure phenomenon in the working face.When the chain structure formed by the upper broken block cannot maintain the stable structure due to the lack of friction, the initial shear occurs at 142 m, and the huge impact force generated by the sliding instability impacts the stable structure formed by the lower subkey strata.The manifestation is that the working face will produce severe pressure, which is consistent with the first support failure phenomenon at about 145 m from the bottom of the gully in the uphill section.
Similarly, as the upward mining continues along the uphill section, the instantaneous load at any point gradually increases, and the load on the rock strata per unit length increases, and the distance of sliding instability will become shorter.This is also the second pressure.The root cause of the support failure is only after the first 45 m.Section b is the downhill section of the second gully.As the overlying strata become thinner, the load gradually decreases, and there is a phenomenon of stress transfer.Section ab will concentrate on sliding instability in section a, and the energy will be concentrated and released after the uphill section slides.Section b will not be prone to sliding instability.Therefore, dynamic mine pressure is less likely to occur in the downhill section of the gully area.This is consistent with the absence of strong compression in the first downhill section of the area.

| STRESS-STRAIN CHARACTERISTICS OF GULLY BEFORE AND AFTER PRESSURE RELIEF
According to the strata and lithology data exposed by the drilling of the No. 1305-2 working face, combined with the rock mechanics parameters, the 3DEC discrete element numerical simulation analysis software is used to establish the 3DEC numerical simulation model of the working face crossing the ditch.The dimensions of the model are 430 × 30 × 200 m (length × width × height), as shown in Figure 7.
In the process of coal seam mining, the original stress state of the surrounding rock changes, and the phenomenon of support failure is caused by sliding instability, which has a great impact on the safety production Therefore, it is necessary to simulate the pressure position when the No. 1305-2 working face passes through the second gully.At the same time, the displacement change and stress distribution before and after pressure relief are compared, which provides a theoretical basis for the subsequent hydraulic fracturing.The distribution characteristics of displacement before and after pressure relief in working face mining are shown in Figure 8A,B.The maximum vertical displacement when mines to the downhill section, slope bottom and uphill section is 3.275 m.It shows that the mining in the gully area is very easy to cause the roof breaking of the overburdened rock, and in serious cases, it can cause the surface step subsidence.At the same time, the displacement change after pressure relief has a significant mitigation effect.The periodic weighting step distance before pressure relief is 16 m, and the periodic weighting step distance after pressure relief is 11 m, which has a significant pressure relief effect.The characteristics of stress distribution before and after pressure relief in working face mining are shown in Figure 8C,D.The stress state shows an obvious trend of increasing stress with the increase of buried depth.The vertical stress in the process of uphill mining is obviously larger than that in other stages.The vertical stress reaches the maximum of 18.4 MPa at about 144 m in the uphill section, and the maximum vertical stress becomes 14.6 MPa after pressure relief, indicating that hydraulic fracturing and other measures can play a good pressure relief effect on the mining of gully terrain.
The simulation results of the pressure position of the No. 1305-2 working face passing through the second gully can provide the basis for the subsequent theoretical calculation of the pressure position.At the same time, the good effect after pressure relief provides support for hydraulic fracturing on the working face.

| Comprehensive prevention and control measures of dynamic mine pressure in gully terrain
According to the above analysis, the mining dynamic rock pressure in the gully area has certain characteristics, so there are many ways to prevent and control the dynamic rock pressure in the gully area.First, the time and place of weighting should be determined by means of real-time stress detection, prediction of mine pressure appearance and other technical means, and protective measures should be taken in advance.For example, the pressure position is predicted by the calculation method mentioned above, the pressure relief measures are made in advance, and the control effect is monitored by the borehole peeper.Second, field management should be used to prevent and reduce the mine pressure of the working face.For example, through support management, a reasonable support model should be selected according to the support resistance, and the quality monitoring of the support condition should be done by the analysis of the periodic weighting characteristic of the monitoring, so as to ensure good engineering quality in the dangerous area of dynamic load mine pressure.Through roof management, hydraulic fracturing, blasting and other means are used to relieve pressure, and critical monitoring and prevention are carried out for the hidden danger location.The comprehensive prevention and control measures of dynamic load mine pressure in gully terrain are summarized as shown in Figure 9.

| Engineering practice
The selection of support equipment is closely related to the safety production of the working face.In the nongully section, the selected ZY9000/17/35 hydraulic support can meet the normal support demand.According to the mechanism of mine pressure appearance in the gully terrain introduced above, due to the sliding instability of the uphill section in the gully area, the pressure is transmitted to the support.Whether the rated support resistance of the support can meet the normal needs of the safety production will become an important consideration standard.The relationship between the support and the surrounding rock during the mining stage of the upper slope is shown in Figure 10.
The maximum support resistance P m of the working face is the sum of the pressure transmitted by the gravity of the immediate roof rock column and the sliding instability of the main key strata.The calculation formula is as follows: In the formula, γ 0 is the average bulk density of the immediate roof strata above the support; l h is the length of support top control; b is the width of the stent; h 0 is the thickness of immediate roof strata; R D is the pressure transmitted by the sliding instability of the basic roof per unit width.
Take the bending moment at the fracture of the main key strata as zero, that is, Combining ( 12) and ( 13), the calculation formula of support resistance can be obtained as: According to the geological occurrence and production technical conditions of the No. 1305-2 working face in Zhujiamao Coal Mine, l h = 5.0 m, b = 1.75 m, h 0 = 36.5 m, the bulk density of rock strata is 25 kN/ m 3 ; ρg is the average rock mass weight of overlying strata of key strata 0.018 MN/m 3 ; x 0 is the ultimate fracture span of rock mass when sliding instability, 15 m; θ is the slope angle of upper slope section of gully, 16°.
Substituting the above parameters into Formula (13), the support resistance of the No. 1305-2 working face is calculated to be 10,011 kN.Therefore, the ZY9000/17/35 hydraulic support selected for the No. 1305-2 working face cannot meet the requirements.The ZY11000/14/34D hydraulic support can be used to realize safe mining, and then it is applied in the working field.
The implementation of comprehensive prevention and control measures for dynamic rock pressure in gully terrain should first clarify the position of weighting.The position of weighting calculated according to the above formula of the initial breaking step of sliding instability is basically consistent with the minefield.Therefore, it is applied to the actual mining of the second gully, and the position of the No. 1305-2 working face when passing through the second surface gully is calculated.θ 2 is 15°, the rest parameters remain unchanged, and it is calculated that the sliding instability position is located at 147 m from the bottom of the gully.The comparison of pressure in each area of No. 1305-2 working face under different methods is shown in Table 2.
To ensure that similar support failure events do not occur in the upper slope section of the working face, hydraulic fracturing is then carried out in the strong rock pressure danger zone within the calculated influence range of the second gully of the No. 1305-2 working face.Hydraulic fracturing technology has a very obvious effect on the control of hard roof.After hydraulic fracturing, the periodic weighting step is decressed from 18 to 12 m.The timely caving roof strata can more effectively support the higher strata and relieve the dynamic load pressure of the roof.
By comparing the working resistance of the support, the strong rock pressure before and after the prevention and control is analyzed.The working resistance of the support before and after hydraulic fracturing is shown in Figure 11.
When the prevention and control measures were not taken in the first gully, the maximum working resistance of the support exceeded 45 MPa, which was much larger than the rated working resistance, and the support was crushed at 145 and 190 m from the bottom of the gully in the uphill section, which seriously threatened the safety production of the mine.The predicted pressure position of the second gully was calculated, and hydraulic fracturing was carried out in the dangerous area.During the No. 1305-2 working face passing through the second gully, the working resistance of the support was basically lower than 30 MPa, the periodic weighting was relatively stable, and there was no support failure accident, which ensured the safe mining of the working face.

| CONCLUSIONS
(1) Through field observation and physical similarity simulation methods.In the nongully area of mining, the pressure law of the working face shows a periodic change of about 25 MPa below the rated working resistance.In the gully area, due to the lack of main key strata, the lack of lateral horizontal force limits cannot form a stable structure.When the working face advances to the 2/3 position of the uphill section of the gully, the support resistance suddenly increases to about 35 MPa.Destructive phenomena such as support crushing, coal wall spalling and surface step subsidence are caused by sliding instability.
(2) The dynamic structure of the key strata in the gully is constructed, and the law of strong rock pressure is obtained, which is mainly induced by the sliding instability and occurs in the upper slope of the gully.Thus, the mechanical analysis model of key strata in continuous gully terrain is constructed.The calculation formula of the initial fracture distance of the main key strata rotation deformation and sliding instability is derived.It is calculated that the initial breaking distance of the main key strata in the uphill section of the gully area is 27 m, and the sliding instability breaking distance is 142 m.This is in good agreement with the first support crushing phenomenon at about 145 m in the uphill section.It is explained that as the overlying strata become thinner, the load gradually decreases, and there is a stress transfer phenomenon.This is the fundamental reason why strong rock pressure is not easy to occur in the downhill section.(3) The displacement change and stress distribution characteristics of the second gully of No. 1305-2 working face before and after pressure relief were simulated by 3DEC discrete element numerical simulation.The maximum vertical displacement of the uphill section of the gully is 3.275 m, and the vertical stress reaches a maximum of 18.4 MPa at about 144 m of the uphill section.After pressure relief, the periodic weighting step is changed from 16 to 11 m, and the vertical stress is reduced by 3.8 MPa after pressure relief.The pressure relief effect is good, which provides a theoretical basis for the subsequent hydraulic fracturing of the working face.(4) According to the characteristics of strong rock pressure in the mining of the gully area, the comprehensive prevention and control measures of the dynamic load rock pressure in the gully area are put forward.The support resistance of the working face is calculated to be 10011kN by the support resistance formula.Replace ZY11000/14/34D hydraulic support.Hydraulic fracturing was carried out in the dangerous area of strong mine pressure within the influence range of 147 m in the second gully of the No. 1305-2 working face.The working resistance of the field support is basically lower than 30 MPa, and there is no crushing accident.It ensures the safe mining of the working face and provides reference experience for the prevention and control of strong mine pressure in mining through gully terrain.

T A B L E 1
Identification and parameters of key strata on the No. 1305-2 working face.

F I G U R E 1
Phenomenon of surface and underground failure when strong rock pressure occurred.(A) Surface step subsidence, (B) surface tensile cracks, (C) roadway side drum, and (D) roof convergence.F I G U R E 2 Physical material models and equipment of gully terrain.
Failure characteristics of overlying strata under different regions.(A) Mining nonvalley area and (B) mining valley area.

F I G U R E 5
Dynamic structure diagram of gully key strata.F I G U R E 6 Mechanical analysis model of key strata in continuous gully terrain.

F I G U R E 7
Before the excavation of the second gully model.F I G U R E 8 Comparison of stress-strain characteristics before and after pressure relief.Displacement comparison diagram (A) before and (B) after pressure relief.Stress comparison diagram (C) before and (D) after pressure relief.

F I G U R E 9
Comprehensive prevention and control measures of dynamic load mine pressure in gully terrain.

F-
I G U R E 10 The relationship between support and surrounding rock during mining in uphill stage.T A B L E 2 Identification and parameters of key strata on the No. 1305-2 working face.No severe pressure occurred.144 On-site pressure position (m) 145 No severe pressure occurred.No severe pressure occurred after prevention and control.