Page 291 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Pipe Replacement Methods 255

over a reasonably large area. The thrust block, shoring, and soil
behind the shoring must be able to withstand the stresses from the
pulling system. The passive earth pressure of the soil must exceed
the stresses generated by the pulling system with an acceptable factor
of safety.

6.9.3 Matching System Components to Reduce
Risk of Failure
One of the most critical activities prior to initiating the bursting project
operation is to ensure that the system has sufficient power to burst
the existing pipe segment between the insertion shafts to the pulling
shaft. Additionally, the bursting system must overcome the friction
between the soil and the outside surface of the new pipe with a rea-
sonable margin of safety to account for unforeseen repair sleeves,
clamps, and the like. In general, the bursting system components
should be appropriately matched to the project; for example, the
winch capacity should be compatible with the bursting head size and
the conditions of the job.
The contractor should adhere to the sizing guidelines stated in
the operations manual issued by the bursting system manufacturer to
match the system with the needs of the job. The manufacturer should
be consulted if there is any doubt regarding the adequacy of the
system for that specific run under those particular conditions (soil,
depth, type of pipe, etc.). Lubrication of the outside surface of the
new pipe with polymer or bentonite (depending on the type of soil)
can reduce the coefficient of friction between the pipe and the soil,
and consequently, reduce the required pulling force.
6.9.4 Nearby Utilities
The contractor must perform its due diligence to identify, locate, and
verify the nearby underground utilities prior to digging the shafts
and initiating the bursting operation. This includes contacting the
local one-call center and request representatives of the utilities mark
their existing lines on the ground surface. Then the contractor should
then verify the exact location and depth of these utilities via careful
hand digging and/or vacuum excavation. Manual excavation may
be required within a few inches from the existing utilities to avoid
damage. Vacuum excavation is an excellent tool to expose utilities
with minimum surface excavation and minimum risk to the existing
utility.
Underground utilities that are in good conditions are unlikely to
be damaged by vibrations at distances of greater than 2½ ft from the
bursting head for small sizes (less than 12 in. in diameter), in soft
soil, which is a typical application for pneumatic pipe-bursting
operations (Atalah, 1998). The safe distance for large-diameter
bursting (up to 24 in.) is about 7½ ft (see Sec. 6.8.7). These guidelines