July 2011, Vol. 66 No. 7
Features
Geotechnical Conditions And How They Relate To Pipe Bursting
Pipe bursting is a method of pipe replacement that involves three main forces that must be overcome to accomplish installing a new pipe. A basic understanding of these three forces is required for anyone involved in a pipe bursting project from conception, to design, through final construction. A more detailed level of understanding of the effects of varying ground conditions (geotechnical data) is essential to the success of a project by the senior team members including engineer, owner, contractor and field crews.
The first force, from which pipe bursting derived its name, is the force required to fracture or “burst” an existing pipe. Most types of pipes from two-inches to more than 48-inches can be split or burst and that is accomplished by pulling a hardened steel head through an existing pipe that is configured to focus energy on the pipe wall until it fails from the inside out.
The second and third force will be the focus of this article. The second force is “expansion” which is the force required to expand the existing ground to allow insertion of the new pipe. Typically expansion of the existing hole (existing pipe inner diameter in inches) by 20-25 percent will be required to install the new pipe. Pipe bursting is the only pipeline rehabilitation method that allows the newly installed pipe to have the same or larger ID than the existing pipe.
This force will change dependent on existing ground conditions. Typical geotechnical reports for construction and, more specifically, trenchless construction, will not be of much value to a pipe bursting project unless they are given for the area inside the original trench. Because the pipe bursting process is replacing a pipe that was originally laid in a trench, the required information for properly estimating the force required to expand the soil to the required diameter needs to be supplied from the original trench design. Often times it is not practical to perform standard soil borings in such close proximity to an active pipe like a sewer, water or gas main. However, any borings must provide data that is comparative to those soils found inside the trench.
Soil reaction
Virgin soils may in some areas have been used as backfill in the original trench, but in most cases “fill” was hauled in which replaced the original soils during construction. Often times a “test pit” is specified in the bid but it is not given prior to the bid opening. It is simply a way to pass soil condition responsibility off to the general contractor. A test pit prior to the bid being released is a more practical alternative. This will give the project team and the bidders’ real world information in which to base their burst plan. If any special materials were used for the original launching of the pipe or shoring of the original trench, they are essential to the preplanning stages.
Narrow ledge trenches were often dug to install pipes and they may be only slightly larger than the OD of the existing pipe. A pipe bursting expander head may not physically fit through that narrow trench. These locations should be determined prior to considering the project as they are not conducive to pipe bursting.
The groundwater table is a very important consideration in any pipeline construction plan and pipe bursting is no exception. Although pipe bursting can in many cases be completed successfully with little or no dewatering over the entire length of the project, there may be very specific dewatering needs at the insertion and receiving pits. This should be carefully considered and part of the construction plan.
Certain soils are very favorable to pipe bursting and others are more challenging but can be overcome with properly preplanning the project using actual conditions. It is critical to understand soil dynamics and how varying types and densities can affect the expansion and insertion process.
Drag
The third force is referred to as “drag” and is the force of friction that is being exerted from the soil returning into contact with the new pipe as it is being installed. As the soil is expanded to allow installation of the new pipe, it is only in its expanded state for a short amount of time. Shortly after the expander head passes through the soil, the “relaxation” period starts which is putting the original soil back in contact with the new pipe. This contact will be the final state of the pipe in the ground as the 20-25 percent void is gone approximately four to 24-hours after the burst is complete, depending upon soil conditions.
Each of the three forces can be managed and influenced through proper planning and understanding of the process. For example, the burst force can be directly influenced and reduced through varying burst head and tooling configuration. A vitrified clay pipe sewer line burst head is different from a cast iron burst head. An understanding of the tooling that is used during the burst is important and should be a part of the project plan and submittal. This force can be controlled through the bursting system which can be either static or pneumatic. Expansion forces can be influenced through expander head design and is most often a factor when choosing the actual dynamic tonnages needed to complete a burst. Drag is a very important consideration and must be managed carefully in the field. Typically drag increases as the length of the newly installed pipe is continually inserted into the ground.
Lubrication mixtures can often be used to stabilize the hole and lubricate the new pipe as it is inserted. A true understanding of downhole fluids or lubrication is needed as soils react to fluids in a number of ways dependent on the mixture. A common mistake is to utilize the wrong fluids and actually increase drag in certain ground conditions. A HDD mud school is often a great way to learn the 101 level of the benefits of utilizing lubrication on a burst.
Through continued education involving utility system owners, engineers, manufacturers, contractors and field crews, pipe bursting can continue to become a method of choice for rehabilitating failing underground infrastructure in your community.
Coming in August: Breaking down barriers – How to help sell pipe bursting to utility owners in your community.
FOR MORE INFORMATION:
IPBA (NASSCO), (410) 486-3500, www.nassco.org
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