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• 11 <br />performed. Also, opal samples were collected and labarateQy tests <br />were made on so-called shattering strength. The field tests on <br />compressive strength of opal irr3icate on the minimum valu.: of 3, 500 <br />psi, while the shattering strerx~i test gives a maximum value of <br />7,000 psi (Table 2). <br />Field tests on roof rocks were again performed by use of the <br />inbound hammer. A canpressive strength of 7, 420 ~i has been deter- <br />mined on the roof Hocks at the distance of about 200 feet from the <br />portal at the 6,400 feet level (Table 2). <br />2.2.2 Pillar stress <br />• The design of mine pillars have long been studied by number of <br />researchers. For the purposes of this report, the Salamrn and <br />Munro's statistical technique of pillars stress deternunation was <br />used. <br />Salomon and Munro, 1967, calculated an average pillar stress ire <br />South African opal mines by taking into consideration the depth of <br />the seam, h, the density of the overburden, d, and he extraction <br />ratio, e, as follows: <br /> <br />• In our case, d = 156 pcf., h = x,100 feet, and e = 0.4~ <br />