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.
Atalah et al (1998) instrumented two HDPE pipes with strain gauges and measured the strain in the pipe due to the pipe bursting process. The friction resistance between the pipe and the soil was calculated using Terzaghi's Silo Theory. The figure below presents the maximum stresses recorded in the pipe in comparison to the calculated pipe stresses. The calculated stress was determined on the basis that the soil collapsed around the pipe and exerted a normal pressure on the pipe related to its depth below the ground surface, similar to the frictional drag developed when jacking pipe. The assumptions for ground pressure and frictional resistance followed the typical assumptions for pipe jacking calculation presented in (Atalah 1994) and (Atalah 1996).
Actual Stress vs. Calculated Stress for TTC Test Site #1 and 3
In TTC test site 1 there was substantially less frictional drag on the pipe than would be expected from fully collapsed soil around the pipe. This indicates that the hole remained at least partially open during, and possibly after, the bursting process because the nominal overcut was approximately 0.7 inch and the hammering action of the head compressed the surrounding soil. In comparison, for TTC test site 3, the measured data correlated well with the predicted stresses that were calculated based upon collapsed soil acting around the replacement pipe over its full length. It is not clear why the friction on the pipe in test site 3 is significantly greater than that on the pipe in test site 1. The following conclusions are based on these pipe stress measurements:
- The calculated stresses generally fall within the range of stresses measured
- Methods to retard the collapse of soil around the replacement pipe will lower stresses in the replacement pipe
- None of the measured stresses exceeded approximately two-thirds of the yield stress of the HDPE pipe
- The level of stress in the replacement pipe was less for the pipe with larger upsizing percentage, possibly indicating a more stable expanded hole due to greater soil compression.
- The magnitude of the stress cycling in the replacement pipe during installation is small compared with the mean stress level.
There are two techniques to reduce the pulling force through the pipe: oversize cut and lubrication of the outside surface of the pipe. Oversize cut at the face reduces the pulling force if the soil is highly stable. In unstable soil, oversize cut must be made nevertheless to allow lubricating the outside surface of the pipe, but it should be minimized. Lubrication around the perimeter of the pipe and along the full length of the drive significantly reduces the friction resistance.