introduCtion Excavation should not precede the jacking operation more than necessary. Material is trimmed so the tunnel bore slightly exceeds the outside diameter of the pipe. Such a procedure results in minimal disturbance to the adjacent soils. The lead pipe is generally contained within a shield projecting from the mining machine or equipped with a cutter or shoe to protect the pipe when excavating by hand. Jacked pipe tends to set or freeze when forward movement is interrupted, resulting in significantly increased frictional resistance, therefore, continuous operation is desirable. Occasionally a lubricant, such as bentonite slurry, is pumped into the space between the tunnel bore and the outside of the pipe to reduce frictional resistance. After the jacked pipe have reached their final position, grout is frequently pumped into this same space to insure continuous bearing with the surrounding soil. When increased resistance develops due to soil conditions or length of run, intermediate jacking stations may be inserted at periodic intervals. The intermediate jacking station pushes only the several lengths ahead while bearing on the pipe behind. The use of intermediate jacking stations reduces axial loads on the pipe and required jacking capacity. The number and capacity of the jacks primarily depends upon the size and length of the pipe to be jacked and the type of soil encountered. Abutments for the jacks must be strong enough and large enough to distribute the maximum capacity of the jacks to the soil behind the backstops. Pits are excavated on each side. The jacks will bear against the back of the left pit so a steel or wood abutment is added for reinforcement. A simple track is added to guide the concrete pipe sections. The jacks are positioned in place on supports. ConCrete pipe A section of concrete pipe is lowered into the pit. The jacks are operated pushing the pipe section forward. stormwater treatment teChnologies The jack rams are retracted and a “spacer” is added between the jacks and pipe. The jacks are operated and the pipe is pushed forward again. box seCtions It may be necessary to repeat the above steps four or five times until the pipe is pushed forward enough to allow room for the next pipe section. It is extremely important, therefore, that the stroke of the jacks be as long as possible to reduce the number of spacers required and thereby reduce the amount of time and cost. The ideal situation would be to have the jack stroke longer than the pipe to completely eliminate the need for spacers. The next section of pipe is lowered into the pit and the above steps repeated. The entire process above is repeated until the operation is complete. manhole and CatCh basin material LAFARGE PIPE eMANUAL PAGE 43