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subsidence monitoring points over two adjacent longwall areas. This <br /> network included three lines of points perpendicular to panel center- <br /> lines and a partial line of points over each panel centerline. <br /> Figure 5 illustrates the general layout of the monitoring net and its <br /> relationship to the underground workings. <br /> Each monitoring point was fabricated of 1- and 2-ft lengths of <br /> EW drill rod (1 .375-in diameter). A bottom 1-ft section had'a harden- <br /> ed steel drive point welded on. A 0.25-in thick steel plate, drilled <br /> with holes to mate with a Wild trivet, was welded to a drill rod - <br /> coupling. This was installed on top of the drill rod subsidence point <br /> once installation was completed. Details of subsidence monitoring point <br /> construction are illustrated on Figure 6. <br /> The installation sequence for monitoring points in soil areas gen- <br /> erally involved the following: <br /> 1 . A drive head was attached to the top coupling <br /> 2. The point was initially driven by two individulas with a <br /> fence post driver. Plumb of the monument was frequently <br /> checked with a "pill box" level . As needed, l-ft or 2-ft <br /> sections of drill rod were added . <br /> 3. The fence post driver was used until approximately 10 to 20 <br /> blows were required to drive the point 1 inch <br /> 4. A Pionjar gasoline powered hammer-drill was then used to <br /> i <br /> 1 <br /> further drive the point. This hammer-drill was used until <br /> refusal . Refusal was judged as 10 to 20 blows per inch of <br /> drivage <br /> 5. The drive head was removed and the top trivet plate was <br /> installed <br /> 18 <br />