Dealing With The Unexpected - What To Do When Things Don't Go As Planned

15th In A Series From The IPBA
By Matt Timberlake, President, Ted Berry Trenchless Technologies Team LLC | August 2012, Vol. 67 No. 8

What if the burst head becomes stuck? In many cases a stuck head is the result of an obstacle (either mechanical or geotechnical) that cannot be overcome. Or, it could be because the pipe bursting equipment used was not of sufficient size to complete the burst. In most cases a stuck head will result in the need for a "rescue pit." However, once encountering the geotechnical obstacle, it can be exposed in a timely matter (usually within one to 4 hours) and then a relief pit can be dug which simply relives the head and allows it to progress forward again. If the pipe bursting operation cannot be resumed, then the bursting operations will need to be terminated and re-launched at that point.

What if an adjacent utility is damaged during a burst? Although breaking a utility is possible, a properly executed burst with nearby utilities that have been pre-marked or exposed prior to pipe bursting should be rare. In the event that a utility, such as a water main, is broken, it is often best to continue with the installation as repairs to the broken utility are made -- if it can be easily confirmed that there will be no harm to the employees, public or any additional damage to property. Once the burst head has passed the obstacle, it can generally continue freely to the desired end point. It is necessary to keep in mind that if a water main is broken, water may want to travel through the pipe being burst or through the annulus created by the burst head and migrate to the receiving or insertion pit. A response for a situation like this should be part of the contingency planning and discussed prior to the start of a burst with the project team.

Strain and heave

What if excessive pipe strain is put on the pipe during an installation? In most cases the pipe strain is minimal and although there is some elongation (see Poisson effect) on an HDPE pipe as it is installed, that strain will be well below the safe pulling allowance of the pipe. As drag increases -- the force that is being transferred to the pipe -- the pipe may experience "jumping" or "hopping" at the insertion pit. If these forces are encroaching on the safe pulling limit, then the burst is often halted to prevent damage to the newly installed pipe. Again the burst plan should take into effect the potential drag the pipe will encounter coupled with the existing soil conditions and spacing of the burst pits established based on that information, but making real time decisions in the field may become a possibility.

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