<p>巷道掘进过程中的空顶区处于暂时未支护状态，这是其成为顶板事故频发区和重灾 区的主要原因，而顶板岩层表面裂隙由表及里的扩展是造成岩层断裂和岩体破裂以致冒 顶的根本原因。本文以含表面裂隙的巷道空顶区顶板岩层为研究对象，通过构建岩体 （层）表面裂隙的几何模型，采用理论分析、室内实验和数值仿真相结合的方法，分析 了岩体表面裂隙扩展模式转变的几何要素及其量化指标，揭示了岩体表面裂隙分别在单 向压缩载荷条件下和三点弯曲载荷条件下的扩展机理、演化过程及强度变化规律。在此 基础上，通过构建巷道顶板岩层表面裂隙的力学模型，分析了岩层表面裂隙的扩展穿层 机制及岩层分离特征，揭示了表面裂隙对巷道空顶区顶板单一岩层和复合岩层的断裂模 式及断裂结构的影响机制。</p> <p>（1）分析了裂隙深厚比和倾角对岩体表面裂隙扩展模式的影响规律。岩体表面裂 隙的扩展模式转变发生于岩体厚度的中心面，可用贯通率进行表征。岩体表面裂隙在扩 展的最危险状态下可用二维裂隙来表征。巷道顶板岩层表面裂隙的贯通率可忽略岩体尺 度、裂隙倾角和偏置系数的影响。</p> <p>（2）外载形式、裂隙位置与几何特征是影响岩体表面裂隙的应力强度因子、扩展 方向和起裂载荷的主要因素。单向压缩载荷条件下，初裂角与裂隙倾角和贯通率均负相 关，而与摩擦系数和侧压系数均正相关；起裂载荷与抗剪强度之比与贯通率负相关，与 摩擦系数正相关，而随侧压的增大呈先增大后减小的变化规律。三点弯曲载荷条件下， 初裂角与裂隙倾角和偏置系数均正相关，与贯通率的关系受裂隙倾角的影响而呈相反的 变化规律；起裂载荷与Ⅰ型断裂韧度之比与裂隙倾角和偏置系数均正相关，而与贯通率 的关系受裂隙倾角和偏置系数的影响显著。裂尖椭圆形模型和等效模型法可用于岩体表 面裂隙扩展路径的量化表征。</p> <p>（3）表面裂隙岩体的断裂模式和峰值载荷主要受外载形式、裂隙位置与几何特征 影响。单轴压缩载荷条件下，岩体表面裂隙尖端可能产生剪裂纹或张裂纹；裂尖和裂隙 中部可能产生片裂。表面裂隙岩体的峰值强度随裂隙倾角的增大呈先减小后增大的变化 规律。三点弯曲载荷条件下，岩体表面裂隙的扩展及断裂随裂隙倾角、贯通率和偏置系 数的不同可能与表面裂隙无直接关系，且不产生片裂。表面裂隙岩体的峰值载荷与裂隙的位置密切相关。理论预测的岩体表面裂隙扩展路径与试验结果大致相符。</p> <p>（4）表面裂隙引起巷道顶板岩层减厚和弱化，由其形成的顶板结构称之为&ldquo;减厚 梁&rdquo;。重力载荷作用下，&ldquo;减厚梁&rdquo;可能产生沿裂尖断裂、沿岩梁中部断裂和复合型断 裂，且其断裂模式由岩梁中部的最大拉应力和裂尖的应力强度因子决定。水平应力作用 下，&ldquo;减厚梁&rdquo;可能产生屈曲失稳和沿裂尖压剪断裂，临界厚跨比可用于其判定。复合 载荷作用下&ldquo;减厚梁&rdquo;可能产生屈曲破坏、弯曲断裂和压剪断裂。&ldquo;减厚梁&rdquo;屈曲破坏 的首要位置由最大拉应力的位置决定，而裂尖的应力强度因子则控制&ldquo;减厚梁&rdquo;的弯曲 断裂和压剪断裂。</p> <p>（5）受表面裂隙的影响巷道复合顶板形成&ldquo;减厚复合梁&rdquo;的力学结构。&ldquo;减厚复合 梁&rdquo;的裂隙扩展穿层受层理粘结强度的影响会产生跨层不偏折或跨层偏折。&ldquo;减厚复合 梁&rdquo;破断后形成的正态拱或叠合正态拱有利于巷道顶板的稳定。巷道顶板岩层断裂不可 避免时，促使表面裂隙朝着有利于巷道顶板岩层能形成自稳结构的方向扩展成为巷道顶 板支护的新要求。</p> <p>基于对岩体表面裂隙扩展机理、巷道空顶区顶板岩层破断机制等方面研究，揭示了 巷道空顶区顶板岩层的破断模式及其力学条件，提出了巷道空顶区顶板岩层破断后的自 稳形式和顶板支护的新要求，对实现巷道顶板精准有效的防控具有重要理论意义和工程 应用价值。</p>

<p>The empty roof area in the process of roadway excavation is in a temporary unsupported state, which is the main reason why it has become a frequent and severely affected area for roof accidents. The propagation of the surface cracks of the roof stratum from the outside to the inside is the fundamental reason for the fracture of the stratum and rock mass and the roof fall. In this paper, the rock stratum of roadway empty roof area with surface cracks is taken as the research object. By constructing the geometric model of rock mass (rock stratum) surface cracks, combining theoretical analysis, laboratory experiment and numerical simulation, the geometric elements and quantitative indexes of the propagation transformation mode of surface crack rock mass are analyzed, the crack propagation mechanism, evolution process, and strength variation law of surface crack rock mass respectively under uniaxial compression load conditions and three-point bending load conditions are revealed. On this basis, by constructing a mechanical model of surface crack in the roof strata of the roadway, the mechanism of crack propagation through bedding of surface crack and the characteristics of rock stratum separation were analyzed, the influence mechanism of surface crack on the fracture mode and fracture structure of single strata and composite stratum in the roof of the empty area of the roadway was revealed.</p> <p>(1) The influence law of crack depth to thickness ratio and inclination angle on propagation mode of rock mass surface crack is analyzed. The transformation of propagation mode of rock mass surface crack occurs in the central plane of rock mass thickness, which can be characterized by penetration ratio. The surface crack of rock mass can be characterized by two-dimensional crack in the most dangerous state of propagation. The influence of rock mass size, crack inclination angle and offset coefficient can be ignored in the penetration ratio of surface crack rock in the roadway roof.</p> <p>(2) The external loading form, crack location and geometric features are the main factors affecting the stress intensity factor, propagation direction, and initiation load of surface crack&nbsp;rock mass. Under uniaxial compression load conditions, the initial fracture angle is negatively correlated with the crack inclination angle and penetration ratio, but positively correlated with the friction coefficient and lateral pressure coefficient. The ratio of initiation load to shear strength is negatively correlated with the penetration ratio, positively correlated with the friction coefficient, and shows a pattern of first increasing and then decreasing with the increase of lateral pressure. Under three-point bending load conditions, the initial fracture angle is positively correlated with the crack inclination angle and offset coefficient, while the relationship with the penetration ratio shows an opposite change pattern due to the influence of the crack inclination angle. The ratio of initiation load to type I fracture toughness is positively correlated with the crack inclination angle and offset coefficient, while the relationship with the penetration ratio is significantly influenced by the crack inclination angle and offset coefficient. The elliptical crack tip model and equivalent model method can be used to quantitatively characterize the propagation path of surface crack rock mass.</p> <p>(3) The fracture mode and peak load of surface crack rock mass are mainly affected by the external loading form, crack location and geometric features. Under uniaxial compression load conditions, shear cracks or tensile cracks may occur at the crack tip on the rock surface. Spallation may occur at the crack tip and the middle of the crack.The peak strength of surface crack rock mass decreases first and then increases with the increase of crack inclination angle. Under the condition of three-point bending load, the propagation and fracture of rock surface crack may not be directly related to the surface crack with the difference of crack inclination, penetration ratio and offset coefficient, and there is no spallation phenomenon. The peak load of surface crack rock mass is closely related to the location of the crack. The theoretical prediction of the propagation path of rock surface crack is roughly consistent with the experimental results.</p> <p>(4) The surface crack lead to the thickening and weakening of roadway roof stratum, and the roof structure formed by them is called &ldquo;thickened beam&rdquo;. Under the action of gravity load, the &ldquo;thickened beam&rdquo; may produce fracture along the crack tip, along the middle of the rock beam, and composite fracture, and its fracture mode is determined by the maximum tensile stress in the middle of the rock beam and the stress intensity factor at the crack tip. Under the action of horizontal stress, the &ldquo;thickened beam&rdquo; may produce buckling instability and compression shear fracture along the crack tip, and the critical thickness span ratio can be used to determine it. The &ldquo;thickened beams&rdquo; may undergo buckling failure, bending fracture, and compression shear fracture under combined loads. The primary location of buckling failure of &ldquo;thickened beam&rdquo; is determined by the location of the maximum tensile stress,while the stress intensity factor at the crack tip controls the bending fracture and compression shear fracture of &ldquo;thickened beam&rdquo;.</p> <p>(5) Under the influence of surface crack, the roadway composite roof forms the mechanical structure of &ldquo;thickened composite beam&rdquo;. The crack propagation through bedding of the &ldquo;thickened composite beam&rdquo; will be affected by the bonding strength of bedding, resulting in cross bedding non deflection or cross bedding deflection. The normal arch or superimposed normal arch formed after the fracture of &ldquo;thickened composite beam&rdquo; is conducive to the stability of roadway roof. When the fracture of roadway roof stratum is inevitable, it is a new requirement of roadway roof support to promote the propagation of surface cracks in the direction of conducive to the formation of self stable structure of roadway roof stratum.</p> <p>Based on the study of the mechanism of rock surface crack propagation and the mechanism of roof rock fracture in the empty area of the roadway, the fracture mode and mechanical conditions of the roof rock in the empty area of the roadway were revealed. The new requirements of the roof support and the self stabilization formation of the roof after fracture in the empty area of the roadway were proposed, which has important theoretical significance and engineering application value for achieving accurate and effective prevention and control of the roadway roof.</p>