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Geotechnical Conditions And How They Relate To Pipe Bursting
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.
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.