Re-discussion on reasonable position and support technology of entry driven under the gob edge of previous split-level longwall panel

Passive support of steel sets has long been used in the entries driven under arc-shaped gob edge(GDAGE)of previous negative pillar longwall panels(NPLP). It is of high labor intensity, high cost, and slow drivage, which limits its promotion and upgrade. Therefore, on the basis of a large number of previous studies, against the practice of the NPLP at the Tangshan Mine of the Kailuan Group, using field measurement, theoretical analysis, and numerical simulation, the reasonable position and support technology for the GDAGE are re-studied and re-discussed. The result shows that:(1) Periodic loading in the NPLP shows the asymmetric and zoning distribution characteristics such as the largest loading in the middle section, the second largest loading in the upper section and the smallest loading in the lower section. Caved rocks in gob is asymmetric with compaction in the lower section, full in the middle section and suspension in the upper section. The lower compaction mitigates the abutment pressure and provides a favorable stress environment for the GDAGE.(2) It is revealed that the connection law between the failure zone of surrounding rock and the failure zone of arc floor with the different offset distances of the working face with the negative pillar. When the roof coal of the gate road is thin, some cracks are easy to occur in the roof and the gradual connection of cracks with that in the arc floor leads to a roof instability.(3) The best position of the GDAGE is directly below the arc floor of the previous negative pillar longwall panel. It can not only retain sufficient thickness of top coal for bolt and cable support, but also avoid the left stress concentration and dramatic change area of upper section gob.(4) The principle of positive support based on the theory of self-stabilizing invisible arch is given. The arch of GDAGE is derived and it presents asymmetric characteristics. The maximum arch height is located 0.1 m to the left of the central axis of the roadway. The research can provide a theoretical support and scientific basis for the surrounding rock control of the GDAGE.