Tree-type drilling is a new technique for drilling radial tree-type boreholes in coal seams in underground mines using water jets to break the coal. The aim is to drain gas from the coal seams in larger quantities and from larger areas than can be done by conventional gas drainage using single boreholes. The self-propelled drill bit is the most important component for this technique. The bit generates a self-propelling force to move forward, break coal, and form a borehole. This paper investigates the relationships between the physical parameters of the forward nozzles in the bit and the diameter and shape of the borehole drilled. The effect of different physical parameters on the rock-breaking efficiency is studied by conducting drilling experiments. The results show that the size, orientation and number of the nozzles significantly affect the rock-breaking efficiency of the bit. To obtain a better rock-breaking efficiency under the experimental conditions used for this paper, the axial angle for forward nozzles should be 15°, the radial angle 90°, and nozzles should be arranged 2.1 mm from the center of a 12 mm drill bit. The experimental results provide a reference for the design of multi-nozzle bits for many applications such as radial jet drilling (RJD) and bent pipe cleaning.

High-pressure water jets under nozzle moving conditions are widely utilized for rock breakage in geotechnical engineering. The nozzle motion state significantly influences the jet flow field and flow characteristics, thus changing its rock-breaking performance. In this paper, a single-jet nozzle model with different traverse speeds was established to investigate the jet flow field features and deflection characteristics and analyze the effects of nozzle traverse speed and jet pressure through numerical simulations. The jet flow fields under nozzle fixed and moving conditions were compared in terms of the distributions of vorticity, velocity, pressure and turbulence kinetic energy, and deflection characterization. Jet flow patterns photographed by an ultra-high-speed camera indicated the deflection of the jet flow field under the nozzle moving condition. The simulation results show that the flow field structure of the moving jet contains the non-deflection zone and deflection zone. The jet time-averaged velocity and vorticity are distributed symmetrically in the non-deflection zone, which is similar to the flow patterns under the nozzle fixed condition. As the jet prolongs further, the vorticity value increases and the vorticity distribution is more concentrated and closer to the nozzle outlet in the moving direction owing to more intense turbulence, resulting in the non-self-similarity of jet velocity dimensionless distribution on cross-sections and jet flow deflection to the opposite direction. The faster the traverse speed and the lower the jet pressure, the greater the jet flow deflection degree. The jet centerline velocities decay exponentially with the standoff distance, and the attenuation degree increases as the traverse speed increases. As the standoff distance increases, the jet deflection distance increases in a quadratic parabola and the deflection angle and impact angle both increase linearly, and decrease exponentially with increasing jet pressure. There is a positive relationship between the jet deflection characterization and the traverse speed.

Water-jet drilling (WJD) is an effective method for stimulating, exploring, and developing oil and gas resources. The self-rotatory water-jet bit (SRWJB) is a type of potentially efficient bit applied in WJD. In this study, the three-dimension drilling model of the SRWJB is established, and the numerical simulation of this model is conducted based on the sliding mesh model combined with RNG turbulence model. The numerical simulation method was validated using experimental results. To improve the drilling performance, the effects of the standoff distance, bit rotation speed, and nozzle layout parameters on the SRWJB drilling capability were investigated. The results show that there exists an optimal standoff distance for the single water jet under the condition of maximum removal volume, that is 15 mm in this experiment. A high rotation speed will make the streamline bend and decrease the jet convergence, which weakens the jet rock-breaking capacity. To achieve a high rock-breaking performance, the design considerations of the SRWJB are as follows: (1) There is an optimal rock-breaking rotation speed for SRWJB by comprehensively considering the jet deflection, moving speed and impact times, such as 200 rpm for the 12 MPa; and the optimal rotation speed increases with the increase of jet pressure. (2) The nozzle arrangement angles should be near 10° to weaken the water cushioning effect. (3) There is an optimal combination of distribution radii for the middle nozzles and lateral nozzles for effectively drilling a large and round borehole. This work will provide a reference for the optimal design of an SRWJB and its application in drilling.

Hydraulic fracturing can improve the permeability of soft coal seams. However, the uncertainty of coal gas enrichment zone results in the unreasonable borehole layout and low gas extraction efficiency. Here this paper presented a dynamic model for investigating the gas migration mechanisms and enrichment law by combining the theoretical analysis and field experiment results. The water-driven gas effect led to gas migration in soft coal seams during fracturing; the hydraulic fractures closed easily after fracturing, forming coalbed gas concentrations in some regions. The results show that the water content of the coal seam surrounding the fracturing borehole increased, but decreased parabolically to the original level with distance from the hole. Free gas was redistributed, sequentially forming a gas reduction zone, gas enrichment zone, and normal zone. The gas enrichment zone was located near the fracturing influence boundaries. The gas drainage concentration in the drilling fields around the fracturing hole followed a parabolic curve, peaking at the gas enrichment zone. Compared with the non-fractured area, gas drainage concentration in the fracturing zone was relatively stable and maintained a high concentration of 62–75%, increasing by 18–37%. The average drainage rate of a single borehole was high, an increase of 1.8–4.2 times. The extracted gas volume over six months was 3.6 times that of the non-fracturing region. The study results bear an important significance on providing guidelines for the layout of gas drainage boreholes in soft coal seams.

The evolution law of internal force and deformation of an anti-slide pile affects the slope stability and prevention design in a significant way. Based on the similarity theory, a test system for the bearing characteristics of a cantilever anti-slide pile was constructed, and the physical model test for the bearing characteristics of a cantilever anti-slide pile under trapezoidal thrust load was carried out. The distribution laws of internal force and deformation of a cantilever anti-slide pile were revealed, and the optimized calculation method for internal force of a cantilever anti-slide pile was proposed by taking the elastoplastic characteristics of steel bars and concrete into consideration. Furthermore, a numerical model was employed to conduct a parametric analysis of a cantilever anti-slide pile. The results show that the whole process of stress and deformation of a cantilever anti-slide pile can be classified as the uncracked stage, the cracks emerging and developing stage, and the steel bars yielding–failing stage. In the uncracked stage, the bending moment of the cantilever anti-slide pile calculated by the traditional method is smaller than that calculated by the optimized calculation method established in this paper. The traditional calculation method is no longer applicable in the stage of cracks emerging and developing. The lateral displacement and bending moment of the cantilever anti-slide pile are negatively and positively correlated with the strength of the pile material, respectively, and the influence of the deterioration of steel bars’ strength on the ultimate bearing performance of the anti-slide pile is more obvious than that of the deterioration of concrete strength. The bearing capacity of the anti-slide pile could not be significantly improved by increasing the length of the anchored section when the strength of the rock stratum embedded in anchored section was large enough. As the thrust load behind the pile increased, the difference of the bearing performances of the cantilever anti-slide pile under the uniform load and trapezoidal load increased gradually. The research results can provide guidance for the evaluation of the service performance of the cantilever anti-slide pile and the slope stability.