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- 15 - <br />1 <br /> <br />1 <br />1 <br />1 <br />1 <br /> <br /> <br /> <br />1 <br /> <br />inclined between 1 and 5 degress; 3 degrees inclination is <br />indicated on Plate 13. <br />The diameter of the holes is usually 4 to 4 z inches; the <br />slotted casing has a diameter of 1 ~ inches. According to our <br />recent experience ,. the costs of the horizontal drains may range <br />between $ 8 and $ 10 per-linear foot of installed drain plus <br />mobilization and demobilization costs of the rig. <br />Maximum spacing of drains as indicated in the drawing is <br />on the order of 30 feet; it should vary according to the per- <br />meability of the soils to be dewatered. <br />An alternate system of dewatering may consist of trenches <br />which are excavated parallel with the slope gradient or per- <br />pendicular to it. The trenches would be filled with pervious <br />material such as clean gravel-sand mixtures; a perforated <br />plastic pipe located close to the bottom of the trench provides <br />a means of collecting and removing water from the trench. <br />This drainage system might be used to dewater the cut <br />slopes above entries 2, 3, and 4; the trenches could be in- <br />stalled after the excavation of the cuts. <br />Although our studies have not dealt with the tunnels which <br />will be driven from the proposed bench, several important fac- <br />tors should be mentioned. As it is shown in individual profiles <br />along the entries, the first tunnel parts will be driven in <br />soils of the slides before intact rock (coal seam) is reached. <br />The overburden of the tunnels will be low; ground water will <br />probably be encountered in some tunnels as the newly installed <br />piezometers seem to indicate. Such tunnelling conditions are <br />difficult and special driving methods will be require8_ This <br />problem could be partly or totally eliminated if the tunnel <br />o~wnroao wraswna r+c <br />