Failure Mechanism And Control Strategy Of Deep Large-section Coal Roadway With Composite Roof In Zhaozhuang Coal Mine

Coal-bearing strata have typical layered features,and structural planes such as engineering rock mass bedding,fissures,and weak interlayers are developed.Among them,the layered composite roof roadway accounts for a large proportion.As a kind of complex difficult roadway,the stability control of surrounding rock of composite roof roadway has always been the focus and difficulty in the field of roadway support.With the increase of mining depth,the engineering response of the rock mass will change fundamentally compared to the shallow part.For the deep large-section composite roof coal roadway,the stability control problem will be even worse.Based on the deep multi-section composite roof coal roadway in Zhaozhuang Mine,this paper comprehensively uses field investigation,theoretical analysis,numerical simulation and field engineering test to study the deformation and failure mechanism of deep large-section composite roof coal roadway.The stress state of surrounding rock of roadway with different sections is analyzed,and the shape of roadway section is optimized.Starting from controlling the load effect of surrounding rock,the superimposed coupling support technology of large and small structures is put forward,which is based on strong bolt and high prestressing anchor cable,and with "closed surrounding rock,strengthened small structure and mobilized large structure" as the core..The main conclusions are as follows:(1)The geomechanical tests of surrounding rock of roadway were carried out,and the distribution law of original rock stress field,clay mineral content of surrounding rock and surrounding rock mechanics parameters were obtained,and the stability of surrounding rock of roadway was preliminarily classified.Horizontal tectonic stress was dominant in the original rock stress field,the maximum horizontal princial stress azimuth was N350W,and the lateral pressure coefficient was 1.17.The clay mineral content of the mudstone in the roof of the roadway is more than 50%,and it is easy to weather and crack when it meets water;the strength of the coal body is less than 8MPa,which is relatively soft.Using fuzzy clustering analysis method,the stability classification of 15 coal roadways in Zhaozhuang mining area was carried out,and the clustering center of surrounding rock stability classification was obtained,and the classification index template of surrounding rock stability of coal roadway was established.(2)The deformation and failure characteristics of large-section composite roof coal roadway are summarized and analyzed,and the evolution law of composite roof separation layer is clarified,and the deformation and instability mechanism of large-section composite roof coal roadway is revealed.The roof sinks severely and the hidden danger of roof falling is great.The coal wall is extremely easy to slice,and the extrusion deformation is significant.The damage of the supporting structure is serious,and the high rate of roadway repair is the typical failure characteristic of the large section composite roof coal roadway.The internal structure of the composite roof is variable,showing discontinuous and jumping damage.The evolution process of the composite roof of the large-section coal roadway is as follows:roof deflection-interlayer shear-non-coordinated deformation-separation expansion.The width of the roadway,the lateral pressure coefficient and the layer thickness have a significant influence on the deformation of the composite roof.The weak interlayer can easily lead to the destruction of the interlayer and the layer of the composite roof.With the number of weak interlayers increases,the height of the roof and the risk increase.The existence of the weak interlayer is the key factor for the discontinuous and jumping damage of the composite roof.The degree and extent of coal gang damage increase,and the stability of composite roof is reduced.In order to ensure the stability of roadway,the principle of "top-assisted control" must be adhered to.The wet environment in the underground aggravates the weathering and fragmentation of the roof.After the roadway is excavated,the concrete layer must be sprayed in time to reduce the strength degradation of the engineering rock mass.The composite roof of large section coal roadway produces a deflection layer under the action of vertical load and horizontal load.With the evolution of the separation layer,a potential falling block is formed above the roof;during the process of flexural deformation of the potential falling block,the support structure fails and the support strength decreases.When the sliding resistance of the potential falling block is insufficient to overcome the sliding shear force,the composite roof will be unstable.The engineering geological conditions are complex,the surrounding rock strength is low,the roof structure is variable,and the separation from the layer is significant;the coal gang is soft and broken,and it is difficult to provide effective support for the roof;the roof mudstone is easily weathered and fractured,and the anchor cable prestress loss is serious;Poor pertinence of support scheme and low bearing capacity of surrounding rock are the key factors causing deformation and instability of large section composite roof coal roadway.(3)The force model of the circular arched roof of the roadway is constructed.The mechanical properties of the roof under the influence of different factors are studied,and the cross-section shape of the composite roof coal roadway is optimized.From the point of view of structural mechanics,the force model of circular arched roof in composite roadway is constructed,and the changes of bending moment,shearing force and axial force at different positions of circular arched roof are obtained under the influence of different span ratio and tunnel width.The numerical analysis is used to analyze the stress state,plastic zone characteristics and displacement distribution of surrounding rock under the influence of 11 sections.With the increase of the span ratio of the roadway,the stress state of the surrounding rock gradually becomes better,which is beneficial to the control of the surrounding rock.When the span ratio reaches 0.3 and continues to increase,the increase in the roadway stress state is no longer obvious.At the same time,considering the difficulty of construction,the large-section composite roof coal roadway adopts a straight wall arc-shaped section with a sagittal span ratio of 0.3.(4)Based on the regulation of surrounding rock load effect,the superimposed coupling support technology based on strong anchor bolt and high prestressed anchor cable is proposed,which is based on "closed surrounding rock,strengthened small structure and mobilized large structure".The main problems in the composite roof coal roadway support are analyzed:the various weak structural planes in the engineering rock mass are insufficiently considered,and the mechanism of deformation and instability of the composite roof can not be correctly recognized;the geotechnical test of roadway surrounding rock is not emphasized and the roadway support method is single resulting in insufficient regional support and local support waste;insufficient understanding of the synergistic mechanism of anchor and anchor cable can not achieve coordinated support of anchor and anchor cable;lack of timely monitoring of mine road pressure data,The rationality of design of the protection program cannot be effectively evaluated in roadway support.On this basis,the idea of surrounding rock control in composite roof coal roadway is proposed.The spacing of bolts and anchors increases,the degree of superposition of supporting stress fields decreases,and the structural effect of group pressure arches transforms into individual effects;dense anchor bolt support is conducive to the formation of double-layer bearing arch structures in surrounding rocks;When the cable spacing is too small,although the outer layer bearing arch structure with higher rigidity can be formed,the anchor bearing cable has a limited supporting range.The pre-tightening force of the anchor cable increases,and the superposition degree of the support stress field increases,which is beneficial to the formation of a double-layered bearing arch structure with greater rigidity,which increases the hierarchical shape of the support and is beneficial to improve the reliability of the support system.As the length of the anchor cable increases,the range of support and reinforcement of the surrounding rock increases gradually,but the effective supporting stress is reduced.For the structurally complex composite roof,the long and short combination of the anchor cable can be considered on the basis of the bolt support.The level of support forms a three-layer pressure-bearing arch structure,making the best of the self-supporting ability of the surrounding rock.The pre-tightening force is the key factor affecting the control effect of the anchor cable on the composite roof.It should be ensured that the design pre-tightening force can form an effective compressive stress zone in the surrounding rock,so that the weak interlayer is in the clamped state,avoiding its degradation and extending along the layer.Significantly reduce the sensitivity of composite roof deformation and damage to engineering disturbances.According to the difference of deformation and damage degree of the roof in different depths of large section coal roadway,it is divided into unsteady layer,metastable layer and stable layer.In order to ensure the stability of the surrounding rock,it is necessary to focus on controlling the unsteady layer in the shallow part and the metastable layer in the middle part,and mobilizing the deep stable layer to carry.The shallow unsteady layer and the metastable layer in the middle are considered as small structures of the surrounding rock,and the deep stable layer is regarded as the large structure of the surrounding rock.Based on the regulation of surrounding rock load effect,the superimposed coupling support technology of large and small structures based on "strongly enclosed rock,strengthened small structure and mobilized large structure" based on strong anchor and high prestressed anchor cable is proposed.(5)Based on the superposition and coupling support technology of large and small structures,the typical test roadway is selected,the specific support scheme and key technical parameters are proposed,and the on-site engineering test is carried out,and good support effect is obtained.The large and small structure superposition coupling support technology relies on the support system of "long and short combination,strong and weak combination,dense and dense combination" to form multi-level support.Select a typical test roadway,select the strong anchor bolt and the high prestressed anchor cable to support the roof according to the specific engineering geological conditions,form a continuous prestressed load-bearing structure,eliminate or reduce the influence of the weak structural surface in the composite roof;and choose reasonable The watch guard member,at the same time strengthen the coal control,and timely spray the layer to seal the surrounding rock.The on-site monitoring results show that after the new support scheme,the surrounding rock deformation of the roadway is small,the long-term stability is high,and the support effect is good.