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Managing Burst Forces
14th In A Series
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 is written with the understanding that the reader has an advanced knowledge of pipe bursting and risk management associated with a pipe bursting project.
In the July 2011 article titled :Geotechnical Conditions And How They Relate To Pipe Bursting, we discussed the three forces that must be overcome to ensure a successful pipe bursting project. Those forces for both static and pneumatic pipe bursting are:
• The force required to "split" or "burst" the existing pipe, plus;
• The force required to expand the existing soil to accommodate the new pipe, plus;
• The "drag" or friction that is being caused by the soil relaxing back on the outside of the newly installed pipe as it is pulled in place; therefore
• Burst + Expand + Drag = Total force required.
This article will focus on managing the "drag" or friction on the pipe and some of the lessons learned by IPBA members. In many pipe bursting designs the length of the pipe that is to be installed is determined by the anticipated combination of the three burst forces. By limiting the amount of drag that is placed on the pipe, the installation lengths can increase. This increases productivity and reduces impact and cost. Although there is no "rule," it is generally accepted that lubrication is used on pipe bursting projects where the new pipe size is larger than +/- 15-inches and for lengths where the anticipated pipe drag may approach safe pulling limits.
Lubrication mixtures can be used in two ways to reduce or manage the effects of drag. By lubricating the pipe as it is installed the friction is reduced, and by stabilizing the hole it is kept in the expansion state for a longer period of time prior to relaxing back onto the pipe. A common mistake made is to use a basic bentonite mix for all installations regardless of pipe size, type, geotechnical conditions or groundwater levels. Lubrication design should be just that – a design as to what it is going to do and how the ground will react to it.