PIPE CRACKING / PIPE BURSTING

What is Pipe Cracking / Pipe Bursting

Pipe cracking / Pipe bursting is a method for trenchless replacement of worn out and undersized gas, water and sewer pipes. With pipe bursting an existing pipe is replaced size-for-size or up-sized with a new pipe HDPE (High Density Polyethylene) in the same location. Pipe bursting by means of static pull, fractures a pipe and displaces the fragments outwards while a new pipe is drawn in to replace the old pipe.

The Process

Solid rods are fed into the existing pipe at the receive pit by the hydraulic unit (cracking machine). The hydraulic unit (cracking machine) is capable of 60 tons of pull back force. The rods are 1 meter in length and are threaded together through the existing pipe to the launch pit. There, they are attached to the front end of the bursting head, and the new HDPE pipe is connected to its rear end.

The bursting process is done in consecutive sequences (back and forth), in each sequence during the bursting, the hydraulic unit (cracking machine) in the reception pit pulls the rods for the length of the individual rods, and the rods are separated from the rest of rod assembly as they reach the reception pit.

The force for breaking of the existing pipe comes only from pulling the bursting head forward. The tensile force applied to the bursting head is significant. The cone-shaped bursting head transfers this horizontal pulling force into a radial force, which breaks the old pipe and provides a space for the new pipe. 

The diameter of the bursting head is larger than the inside diameter of the existing pipe to cause the fracturing and slightly larger than the outside diameter of the new pipe, to reduce friction on the new pipe and to provide space for maneuvering the pipe. The hydraulic unit (cracking machine) together with the shape of the bursting head keeps the head following the existing pipe. Pipe bursting operation creates outward ground displacements adjacent to the pipe alignment. The ground displacements tend to be localized.

Ground Conditions

The most favorable ground conditions for pipe bursting are soils that can be moderately compacted (reducing the lateral extent of outward ground movements), in which the expanded hole behind the bursting head does not cave in before the replacement pipe is installed (lowering the drag and the tensile stresses in the pipe during installation).

Less favorable ground conditions involve densely compacted soils and backfills and soils below the water table. Each of these soil conditions tends to increase the force required for the bursting operation and to increase the zone of influence of the ground movements. Special soils such as highly expansive soils or collapsible soils will also cause problems.

For most soil conditions, it is simply necessary to provide the required power to effect the burst, displace the soil and pull the replacement pipe in over the length of the burst and to consider the potential effect of the ground displacements and vibrations on adjacent utilities and structures. Longer bursts can be accomplished more easily in favorable ground conditions.

Conclusion

Pipe bursting has limitations. Difficulty can arise in expansive soils, close proximity of other services, point repairs that reinforce the existing pipe with ductile material, a collapsed pipe at a certain point along the pipe, etc.

The bursting operation can cause ground heave or settlement above or at some distance from the pipe alignment. The most critical conditions for ground displacement are when the pipe to be replaced is shallow and ground displacements are primarily directed upward, significant upsizing percentages for large diameter pipes are used, and deteriorated existing utilities are in close proximity to the pipe being replaced.