Yubari New Coal Mine has been suffering roadway maintenance problems since the beginning of the mining operation started in 1970. Main roadways driven in thick layers of Horokabetsu shale are situated at a depth of about some 1000m from the surface and at 30m below the working coal seam having mean thickness of 3m. Behavior of the roadways are characterized by the continuing creep-like progress of the road- Way deformation with considerable amount of closure rate which depends on several factors and a great deal of manpower have been obliged constantly to be put into the repair workings. In one case, whole length of the roadway experienced seven times of repairment during 40 months.
It is apparent that the present supporting design of the 125xl5mm three piece steel arches installed at O.5m intervals in conjunction with wooden lagging is not adequate to effectively maintain the road- Way of 5.5x3.6m section.
To overcome these miserable roadway conditions, both field measurements and laboratory studies have been conducted and effort has been made to clarify the mechanism of the excessive roadway deformation. long with these investigations, effective counter-measures and/or supports have been Searched and tested in the field.
A typical example of the vertical closure of the roadway opening is presented in Fig. 1. The measurement started. one week after the completion of the roadway. Mining activities in the vicinity of the measuring section had been absent during the measurement. It is noted that the convergence rate gradually decreased in a few weeks and thereafter kept the constant rate over a period of 200 days. Measurements on the rock movements surrounding the opening, as shown in the exemplified result in Fig. 2,reveals that the floor uplift mainly contributes to the vertical closure, because roof lowering is considerably small.
Small amounts of tilt which have been monitored on the rock mass below the sidewall might indicate that the floor uplift is induced neither by the shear failure occurring presumably at the edges of the opening nor by the radial movement around it but by the swelling of the relaxed zone under the floor(Kinoshita et.al.1979).
Vertical closure is generally recognized to be superior in magnitude to side closure.
With an aid of the borehole jack of Goodman type (Goodman et.al. 1968),the relaxed zone developed around an opening was measured at several sections. It is noticeable that this zone extends to a fairly amount of depths below the floor as depicted in Fig,3. Measurement using multi-strain meters revealed also that the rock mass in this relaxed zone showed higher rate of expansion than those in other deeper zone. In addition, observations by borehole viewer direclty confirmed that the density of fissures is remarkable in the relaxed zone.
(Figure in full paper)
Fig. 4 is a typical pattern of water contents of rock mass around the roadway; In sidewalls(and in floor)they are rather low value, whereas in floor higher values.