. Shield and compressed air tunneling . y^ (Fig. 93).. Fig. 93.—Greathead type of grouting machine driven by compressed air engine. 85. Nipple and Valve.—It has been found more convenient notto screw the grout pipe nozzle directly into the grout hole but tointerpose a nipple and valve between the segment and the groutpipe so that, when the space is filled with grout, the valve can beclosed until the grout has set. Otherwise it is necessary toremove the grout pipe and to put in the plug while the grout isstill liquid. This results in loss of grout in all holes above thehorizontal axis.
. Shield and compressed air tunneling . y^ (Fig. 93).. Fig. 93.—Greathead type of grouting machine driven by compressed air engine. 85. Nipple and Valve.—It has been found more convenient notto screw the grout pipe nozzle directly into the grout hole but tointerpose a nipple and valve between the segment and the groutpipe so that, when the space is filled with grout, the valve can beclosed until the grout has set. Otherwise it is necessary toremove the grout pipe and to put in the plug while the grout isstill liquid. This results in loss of grout in all holes above thehorizontal axis. 86. Mounting of Grouting Pan.—For portability the machineis mounted on skids, or l)etter still, on flat cars which can run onthe tunnel tracks. 87. Other Types of Grouting Machines.—Grouting machinesof several different patterns have been made. Some have thecylinder vertical instead of horizontal. In others the grout ismixed and agitated by jets of compressed air instead of bypaddles. These machines give good service and there is prob-ably little to
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