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material inside the forward portion of the casing. For casing <br />diameters eight (8) inches or greater, the minimum space <br />between the leading end of the auger, or cutting head as <br />allowed below, and the leading end of the casing shall be no <br />less than one -half the casing diameter. However, the <br />setback shall be increased if necessary to prevent undue <br />flow back of the spoil material. No setback is required for <br />casing diameters less than eight (8) inches. <br />Cutting heads may be used only where the subsurface <br />investigation report or other reliable information indicates <br />the likelihood of encountering a very hard soil strata, rock or <br />other obstructions such as tree stumps; and it is <br />1 determined prior to beginning work that the area of <br />difficulty may be passed by the use of an appropriate <br />cutting head. The cutting head shall remain inside the <br />casing as outlined above except during the passage of such <br />obstructions. <br />On large diameter jacked crossings where clean out of the <br />bore is accomplished by special digging machine or by hand, <br />the distance between the leading end of the casing and the <br />' actual cleanout operation shall be no less than that <br />necessary to ensure that voids will not form around the <br />1 outside of the casing. The use of tunnel liners will be <br />allowed only where the installation method and soil <br />conditions ensure that voids will not be formed around the <br />' outside of the liner during installation. <br />5) Auger size and spacing - The leading auger section used in <br />' conjunction with a cutting head must be full -sized having an <br />outside diameter not less than the inside diameter of the casing <br />less the amount needed to provide the minimum working clearance <br />necessary. In no case shall the auger diameter be less than 1/2 <br />inch smaller than the inside casing diameter unless some other <br />1 positive means of restricting the movement of the cutting head as <br />previously required is assured. Less than full -sized augers that are <br />large enough to remove spoil satisfactorily will be allowed when <br />the auger is not used in conjunction with a cutting head and is to <br />remain within the casing at all times, except as follows: <br />1 07/03 16200 -13 03- 1438.A <br />