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t <br />n <br />I <br />the back of the cutting head and the end of the casing shall <br />be limited to the clearance necessary to allow the cutting <br />head to function without coming in contact with the end of <br />the casing. In areas where sand pockets may be <br />encountered the cutting head must be construction so that <br />it can be retracted into the casing, to within the limits <br />specified in this section for the particular material <br />encountered. <br />b) Hard pan, clay, hard sand -clay and stable cohesive soils - <br />As in rock, the cutting head should normally precede the <br />casing but the type cutting head used must allow no more <br />than two (2) inches between the back of the head and the <br />leading end of the casing. Cutting heads with cylindrical, <br />pointed chippers designed for use in solid rock shall not be <br />used. <br />c) Loose unstable soil - The distance between the leading end <br />of the first auger section and the leading end of the casing <br />shall be as necessary to maintain a solid plug of spoil <br />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 determined <br />prior to beginning work that the area of difficulty may be <br />passed by the use of an appropriate cutting head. The <br />cutting head shall remain inside the casing as outlined <br />above except during the passage of such 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 />09/03 16200 -12 03- 1438.A -R <br />