水力压裂是煤矿井下卸压的重要技术手段，更好地理解裂缝扩展机理对煤层安全开采具有重要意义。为深入探究水力裂缝扩展规律，针对实验室常用的类岩石试件，利用颗粒离散元数值模拟软件MatDEM，建立了二维水力压裂数值模型，开展了不同注液压力增量的水力压裂试验。试验研究了注液压力增量对水力裂缝扩展的影响，揭示了模型起裂机理，从细观尺度分析了裂隙生成及裂缝扩展规律，并对水力裂缝扩展特性进行了讨论。结果表明：① 注液压力增量对模型起裂压力与起裂时间变化趋势的影响相反。起裂压力随注液压力增量变大，其增长趋势不断变缓，并逐渐趋近于5.6 MPa。起裂时间随注液压力增量变大不断减小，其减小趋势同样逐渐减缓。② 累计裂隙数目随时间呈指数型增长，将水力压裂过程分为4个阶段(Ⅰ～Ⅳ)：无裂隙阶段、裂隙缓慢增长阶段、裂隙稳定增长阶段和裂隙急速增长阶段，分别对应模型起裂前、模型起裂后裂缝形成前、主裂缝扩展以及次级裂缝扩展过程。随注液压力增量变大，第Ⅰ、Ⅱ和Ⅲ阶段时长不断减小，第Ⅳ阶段时长呈波动式增长。各阶段裂隙数目关系为第Ⅳ阶段最多，第Ⅲ阶段次之、第Ⅱ阶段最少。③ 随注液压力增量变大，次级裂缝数目由8条增至16条，裂缝增长速度在第Ⅲ阶段以前逐渐减慢，进入第Ⅳ阶段后不断增快。注液压力增量由0.03 MPa增大至0.7 MPa，裂缝最终长度增大1.79倍。④ 模型内部能量随注液压力增量的变大而增加，且能量输入速度逐渐变快。模型起裂后，高压水在裂缝尖端形成应力集中，促使裂缝继续扩展延伸。较高注液压力增量下，裂缝扩展速度变快，颗粒位移量由压裂孔至模型外侧逐渐减小。注液压力增量的变大会使次级裂缝形成位置向压裂孔靠近，对主裂缝的形成及扩展起抑制作用，而对次级裂缝的形成及扩展起促进作用，裂隙类型均为张拉裂隙。

Hydraulic fracturing is an important technical means to relieve the pressure of coal seam roof. Better understanding of fracture propagation mechanism is of great significance to the safe mining of coal seam. In order to further explore the law of hydraulic fracture propagation, aiming at rock-like specimens commonly used in the laboratory, MatDEM, a particle discrete element numerical simulation software, was used to establish a two-dimensional discrete element numerical model of hydraulic fracturing, and various hydraulic fracturing tests with different injection pressure increments were carried out. The effect of injection pressure increment on the propagation of hydraulic fractures was studied, and the mechanism of model initiation was revealed. The law of fracture generation and propagation was analyzed from mesoscale, and the propagation characteristics of hydraulic fractures were discussed. The results show that ① the effect of injection pressure increment on the model initiation pressure and initiation time presents an opposite trend. With the increase of injection pressure, the increase trend of initiation pressure is slow and gradually approaches to 5.6 MPa. The initiation time decreases with the increase of injection pressure, and the decreasing trend slows down gradually. ② The cumulative number of fractures increases exponentially with time. The hydraulic fracturing process can be divided into four stages (Ⅰ−Ⅳ): no fracture stage, slow fracture growth stage, steady fracture growth stage and rapid fracture growth stage, which correspond to the pre-crack initiation, pre-crack formation, primary fracture propagation and secondary fracture propagation processes respectively. As the injection pressure increment increases, the durations of stage Ⅰ, Ⅱ and Ⅲ decrease, while the duration of stage Ⅳ increases in a fluctuating manner. The number of cracks in each stage is the highest in stage Ⅳ, followed by stage Ⅲ and stage Ⅱ. ③ As the injection pressure increment increases, the number of secondary fractures increases from 8 to 16, and the growth rate of fractures gradually slows down before the stage Ⅲ, and increases rapidly after the stage Ⅳ. When the injection pressure increment increases from 0.03 MPa to 0.7 MPa, the final fracture length increases by 1.79 times. ④ The internal energy of the model increases with the increase of the injection pressure increment, and the energy input speed gradually becomes faster. After the model initiation, high-pressure water forms stress concentration at the crack tip, which promotes the crack to continue to extend. At higher injection pressure increment, the fracture propagation speed becomes faster, and the particle displacement decreases gradually from the pressure hole to the outside of the model. The increment of injection pressure makes the secondary fracture forming position close to the pressure hole, which inhibits the formation and expansion of the primary fracture and promotes the formation and expansion of the secondary fracture. All fracture types are tensile fractures.