Mechanism And Control Techniques Of Mining-activities-induced Floor Heave In The Entry Employing Yield Pillar

In order to deal with gas challenges in longwall panels cutting highly gassy coal seam, multi-entry gateroad system is widely used in various coalfields including Changzhi, Jincheng, Yangquan, etc. Developing an entry employing yield pillar(EEYP) between adjacent panels can greatly improve the recovery rate and meanwhile, ensure the entry stability. However, EEYP generally has severe issue of floor heave due to the strong side abutment pressures induced by the retreat of both the adjacent panels. To address such problem, multiple methodologies including mechanical analysis, numerical modeling, rock testing, and case study were used in this work. The key questions needing to be answered include: a) what are the main factors significantly influence the stress distribution in the entry floor; b) how the strong floor heave gradually accumulates; and c) what control strategies can be used for effective control of floor heave. Focused on such concerns, the key findings in this work can be summarized as:(1) The change of the magnitudes and directions of the three principal stresses in EEYP floor during mining activities were obtained, followed by the mechanical discussion on the influence of three interesting factors on the stress distribution in EEYP floor. Note that such three interesting factors refer to increment of the principal stress close to vertical direction(factor A), increment of the principal stress with larger magnitude in or close to horizontal direction(factor B), and the angles between entry axis and the orientation of the principal stress with larger magnitude in or close to horizontal direction(factor C). It was found that the stress re-distribution or concerntation in EEYP floor during entry development were more influenced by factors B and C. However, during the retreat of the second panel, factor A is more notable in term of influencing the stress distribution in entry floor, comparing to factors B and C.(2) Based on the proposed mechanical model in three dimension, the change of the yield zone in EEYP floor during mining activities was analyzed. During entry development, the range of the yield zone in entry corner is larger than that in the mid-line of entry floor. After the retreat of the second panel, the change of the yield zone in mid-line of entry floor is ignorable; while the yield zone in the entry corner is significantly developed.(3) It is interesting to find that the ratio of the dissipated energy in strain energy(to be simple, it would be called dissipated energy ratio in this work) has a strong correlation to the failure state of the coal measure rocks. Generally, the more seriously the rocks fail, the higher is its dissipated energy ratio. Thus, the dissipated energy ratio would be a helpful indicator for the process analysis of EEYP floor heave. The EEYP calculation equations were drived and a subroutine was developed in FLAC3 D, a finite difference method. It was further used to discuss the process of the EEYP floor heave during each mining activities. It has been proved that the high strain energy started to concerntrate at the right corner(or yield pillar side) of EEYP and propogate to the other side of EEYP during entry development. In addition, during the retreat of the second panel, the yield zone and high strain energy concertration area were generally propogate to the left side the EEYP, while further failure would be found for the immediate-floor of EEYP.(4) The influence of in-situ and mining-induced stresses, surrounding rock strengths, and coal-rock interface strengths on floor heave in EEYP was elaborated. It is true that strong surrounding rocks could bear higher strain energy thus reduce floor heave in EEYP. Meanwhile, coal-rock interface strengths could also significantly influence the floor heave of EEYP. A stronger coal-rock interface could reduce the level of energy concerntation in EEYP floor thus the floor heave conditions. To this point of view, reinforcing the coal-rock interface would effectively reduce the magnitude of floor heave in EEYP.(5) The previous discussion has greatly improved our understanding on floor heave mechanism in EEYP. Thus, a combined strategy including floor slotting, floor bolting, and pubmpable cribs was proposed for floor heave control. It is recommended that, during entry development, floor slotting should be employed to reduce the horizontal stress concerntation in the immediate floor. Meanwhile, floor bolting should be used to reinforce the coal-rock interface. The idealized dimensions for slotting and angles for floor bolting were discussed and proposed. In addition, installation of the pubmpable cribs could greatly increase the residual strengths of and the allowed amount of the strain energy stored in the failed roof and floor rocks, thus reduce floor heave in EEYP.A case study has indicated that the proposed control strategies could effectively reduce the amount of the floor heave in S1202 gas entry(EEYP) in Yu Wu coal mine.