One of the key points in numerical simulation of coal-rock cutting by conical picks is to select a proper coal-rock constitutive model. In order to find a reasonable coal-rock constitutive model, a uniaxial compression test was conducted to obtain the constitutive model. The several stages for linear elastic deformation and creep, plastic yielding, hardening, and finally brittle cracking of the constitutive units were studied, and the coal-rock constitutive model was established. As a result, the coal-rock cutting by one conical pick or two conical picks was simulated and the results were compared with coal-rock cutting experiment on a Coal-rock Cutting Machine. According to the simulation and experimental results, it is believed that the numerical simulation can reveal coal-rock crushing process. And the total error rate of coal-rock cutting by one conical pick between the simulation and experiment is 8.5%. The maximum deviation of coal-rock cutting by two conical picks between the simulation and experiment is 9.8%. All simulation values are within a reasonable range. The comparison indicates that the coal-rock constitutive model should better be defined considering the coal-rock crushing process by conical picks.

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Establishment of Coal-rock Constitutive Models for Numerical Simulation of Coal-rock Cutting by Conical Picks

Received: 21 June 2020 Accepted: 17 September 2020 During pipe jacking in unstable coal rock formation, it is difficult to ensure the stability of the workface and the structural safety of the equipment. To solve the problem, this paper establishes a numerical model for the rock-breaking and slag discharge of pipe jacking machine (PJM) with different tunneling parameters, and numerically analyzes the variations in the flow rate and speed of slags, and the stress and deformation of cutterhead, at different cutterhead speeds (1.0-4.0r/min) and jacking speeds (0.5-4.0mm/s). Based on the simulation results, the authors discussed how different tunneling parameters affect the tunneling safety. The simulation results highlight the importance of the matching between cutterhead speed and jacking speed to tunneling safety. As the jacking speed increased, the load of the cutterhead increased significantly, which may cause the cutter to wear. At the jacking speed of 4.0mm/s, the equivalent stress and deformation of cutterhead peaked at 397.43MPa, and 10.73mm, respectively. Excessive jacking speed may result in accidents, such as deformation and structural fracture of cutterhead, posing a serious threat to the structural safety of cutterhead. As the cutterhead speed increased from 1.0r/min to 4.0r/min, the mean axial speed of slags dropped by 68%, and the slag flow rate declined by 76%, due to the rapid discharge of slags from the cabin. In this case, the slag volume in the cabin cannot provide sufficient support to the workface rock, and the risk of collapse soars during the tunneling in unstable formation. In addition, an excessive cutterhead speed increased tunneling energy consumption and aggravated the wear of the cutterhead. The research results promote the setting of control parameters for the safe pipe jacking in unstable coal rock formation.

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Discrete- and Finite-Element Analysis on the Tunneling Safety of Pipe Jacking Machine in Coal Rock Formation

This paper explores the safety of surface structures over a deep coal seam (12#) in a coalmine of northern China’s Hebei Province. If 12# coal seam is mined by block caving, the surface might collapse and the surface structure could be damaged. First, FLAC3D simulation was conducted to simulate the influence of block caving in 12# coal seam and the shallower 9# coal seam on roof movement. The results show that the mining of 9# coal seam does not affect surface safety; but the mining of 12# coal seam would cause the surface deformation to exceed the requirements of the current regulation, posing a threat to the safety of surface structures. Hence, the excavation method for the coal seam was replaced with cut-and-fill mining. Further simulation on this coal seam indicates that the safety of surface structures could be guaranteed, when the equivalent mining thickness (EMT) was 0.2m. Under EMT=0.2m, the fill strength that best protects the surface structures and controls the surface deformation was found to be 0.5 times of coal strength. Finally, the EMT and fill strength for the cut-and-fill mining of 12# coal seam were optimized as 0.2m and 0.5 times of coal strength, respectively, laying a scientific basis for fill preparation.

Numerical simulation of roof movement and fill strength of coal deep mining

Received: November 11, 2019 Revised: November 27, 2019 Accepted: November 27, 2019 For the economical construction of a tunnel by TBM, the selection of TBM optimized with the various project conditions is important, and also necessary to predict the performances of selected TBM in advance. This study was conducted to comprehensively evaluate the performance of the EPB shield TBM screw conveyor by the discrete element method. The sticky particles were used for the excavated material models, and screw conveyor with 11 different inclined angles were simulated to evaluate the performance depending on the different inclined angles. The four different rotational speed conditions of the screw were used, and torque, required power, extra energy for muck discharge, and the muck discharge rate were selected as four performance indicators. As a result, the optimized inclined angle was selected, and selected angle accords with the fact that EPB shield TBM screw conveyor is generally installed and adjusted at the inclined angle between 20.0° and 30.0° in the field.

Evaluation of Screw Conveyor Model Performance depending on the Inclined Angle by Discrete Element Method

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Geotechnical data analysis to select a feasible method for development of a long axis, large diameter vertical ventilation shaft

Received: 20 May 2019 Accepted: 26 October 2019 This paper aims to design a suitable method to analyze the relationship between the rockbreaking performance and tunneling performance of the cutters of tunnel boring machine (TBM). Considering the cutting discontinuity in broken coal rock formation, the authors put forward a simulation method for the rock-breaking performance of TBM cutters. The material properties of broken coal rock with low mechanical strength and poor uniformity were described by a linear elastic constitutive model and a Drucker-Prager plastic model. The variations in the cutting force with cutting depths (5, 10 and 15mm) and rake angles (5, 10 and 15°) were observed through the simulations, aiming to disclose the mechanical nature of the contact, extrusion and peeling of the coal rock under the action of the cutter. The results show that a high cutting depth and a small rake angle are favorable for tunneling efficiency, but unfavorable for tunneling safety. Overall, the optimal cutting parameters were determined as the cutting depth of 10mm and rake angle of 15°. Our simulations were proved valid through comparison with the results of Evans’ cutting resistance model for wedge cutters. The research results shed new light on the research into the TBM cutting of unstable coal rock.

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Rock-Breaking Performance of Cutters of Tunnel Boring Machine in Broken Coal Rock Formation

A rapid construction method of pipe jacking for underground rescue tunnel with large section is disclosed. The present invention realizes rock breaking through the cutting head of a roadheader cutting unit, and pushes pipe jacking forward through a pushing thruster. During the process, a protecting bush fixedly connected with a stretchable part is adopted for circumferential support; and since the cutting head and the protecting bush occupy the rock breaking space, and the pushing thruster pushes pipe jacking forward to push the coal rock in the annular space to the circumferential direction, the effect of moving forward without slag discharge can be achieved. The present invention can establish a rescue tunnel quickly and effectively, which saves rescue time and greatly reduces the threat to the life safety of trapped people. The present invention can establish a fast and safe underground rescue tunnel with large section for the rescue of people.

Rapid construction method of pipe jacking for underground rescue tunnel with large section

Received: 19 May 2020 Accepted: 1 September 2020 In unstable coal rock formations, the rescue channels should be constructed through safe and efficient tunneling. The rock breaking performance of the tunneling equipment directly hinges on the cutter-head layout. Focusing on the conditions of unstable coal rock formation, this paper adopts the extended Drucker-Prager (D-P) plastic model to define the properties of bedrock materials of the coal rock with low mechanical strength and poor homogeneity. Then, a finite-element model was established on ABAQUS for the coal rock cut by two disc cutters, and used to simulate the breaking of the coal rock and the peeling of slags from the bedrock. On this basis, the authors analyzed the influence of cutter spacing (30, 35, 40, and 45mm) over cutting force, rock breaking amount, and specific energy under two cutting methods: simultaneous cutting and sequential cutting. Finally, a cutter deployment strategy was designed for safe and efficient tunneling in unstable coal rock formations. The results show that: Under simultaneous cutting, as the cutter spacing increased from 30 to 35mm, the rock breaking amount increased, while the specific energy declined; as the cutter spacing further rose from 35 to 45mm, the rock breaking amount dropped, while the specific energy increased. Under the coal rock conditions in our research, the optimal cutter deployment strategy is: simultaneous cutting with cutter spacing of 35mm. The research results provide theoretical support for the cutter-head design of rescue equipment for collapsed coalmines.

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Shuai Lv

Papers

Rock-Breaking Performance of Cutters of Tunnel Boring Machine in Broken Coal Rock Formation

Discrete- and Finite-Element Analysis on the Tunneling Safety of Pipe Jacking Machine in Coal Rock Formation

Rapid construction method of pipe jacking for underground rescue tunnel with large section

Rock Breaking Performance of Two Disc Cutters of Tunnel Boring Machine for Safe Tunneling in Unstable Coal Rock Stratum