August 2012, Vol. 67 No. 8
Features
Dealing With The Unexpected - What To Do When Things Don't Go As Planned
Pipe bursting is a reliable and proven method of replacement of deteriorated or under capacity utility lines. The IPBA has developed a recurring series of technical articles and this article will be written with the understanding that the reader has an advanced knowledge of the pipe bursting process.
As with any underground construction there are times when things do not go as planned and pipe bursting is no exception to that rule. Managing changes that are either anticipated or unexpected can often mean the difference between a project being considered successful in the eyes of the owner or public, and profitable for the contractor performing the work.
As a contractor that has field experience with pipe bursting of both water and sewer as well as static and pneumatic processes of pipes from two – 24-inches, I have seen firsthand what real time decision making can do towards the success and the profit and loss of a project. In a pre-job risk assessment it is important to identify what could possibly go wrong and what are the field crews going to do about it. These situations can typically be put into two categories: items within our control and items outside of our control. An experienced project team should be able to adjust in real time to surprises or problems that arise in the field, thus greatly increasing the chances of a successful project.
Items within our control would include employee skills, layout of the project plan, selection of process (static or pneumatic), size and type of bursting machine, execution of the project work plan, equipment reliability and the sequence of the work.
Out of control
Items often considered out of the contractors control include geotechnical conditions, pipe repairs made of unknown materials, narrow trench geometry, etc. A contractor is often able to influence these situations even if they cannot be completely managed or anticipated. For example, pre-existing pipe repairs in a sewer line, although not always clearly indicated prior to the start of work (a water main replacement with mechanical repairs comes to mind) can be reasonably anticipated to exist. Something like this is often referred to as “known unknowns.” By having a project team with the tools and ability to recognize and adapt to such problems dramatically reduces the potential for failure.
Let’s review a list of some of the potential situations that could occur on a pipe bursting project and some of the potential remedies.
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.
What if the ground heaves during the burst? Ground heave is an anticipated calculation done prior to a burst which takes into account the inner diameter (ID) of the existing pipe and the outside diameter (OD) of the expander which is sized +/- 20 percent larger than the OD of the new pipe being installed. Potential impacts of ground heave should be determined prior to the start of a burst. However, in some cases ground heave will subside within 24 hours of an installation as the ground “relaxes” back into contact with the new pipe. Under pavement, severe cases of heave can be ground down after the installation and surface restoration of the pavement complete. In any event, it is important to have a clear level of expectation of a project and all stakeholders in agreement of the decision making process in the field to modify or alter an existing plan.
Although this is a very basic look at some of the potential problems and potential remedies, it is important to keep in mind that the most important thing to have on site is an experienced project team and solid contingency plans for unanticipated situations. The success rate of pipe bursting throughout the world is very high. By continuing efforts to increase the knowledge base of those associated with pipe bursting in North America, the IPBA strives to influence future success and growth of pipe bursting and its practical applications.
FOR MORE INFORMATION:
IPBA (NASSCO), (410) 486-3500, www.nassco.org
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