Maximum Allowable Tensile Load
After the bursting head breaks the old pipe and creates a cavity in the ground, the winch pulls the new pipe through this cavity. For the pipe to be pulled, the pulling force has to exceed the friction between the outside surface of the pipe and the surrounding soils. When the coefficient of friction between soil and the pipe is high and the outside surface area of the pipe is large, high pulling forces are needed to overcome this high friction resistance. The high pulling force generates high tensile stresses on the replacement pipe. If the allowable tensile strength of the pipe is less than the anticipated tensile stresses on the pipe, actions to reduce friction must be adopted to avoid excessive strains in the pipe. Examples of these actions are increasing the diameter of the bursting head by approximately an inch to create approximately half an inch of overcut around the pipe, and injecting bentonite and/or polymer lubrication into the annular space behind the bursting head to reduce the frictional forces. If these actions are not sufficient to rectify the problem, a shorter bursting run and relocation of the insertion or pulling shafts must be considered. Pull force calculations should be conducted before bursting operation starts to avoid over stressing the pipe. It is much easier and less costly to incorporate the above-mentioned corrective actions before bursting than during bursting.
However, estimating the pulling force to break the old pipe and overcome friction resistance between the new PE and the surrounding soil is very difficult and currently there is no generally accepted procedure. Many site and project factors interact to make developing an accurate and reliable model problematic including: the strength of the old pipe, the type of backfill material, the type of native material, degree of upsize, bursting system, the amount of overcut, the presence of sags along the line, etc. Comparisons between the actual pulling forces and the calculated forces using the Terzaghi's Silo Theory that is used in calculating the jacking force in pipe jacking operations is presented below.