Page 281 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Pipe Replacement Methods 245

In very soft or loose soils, significant ground movement may take
place causing significant sags in the new pipeline and damage to
nearby structures. In severe situations, the soil particles migrate to
the interior of the existing pipe, converting the bursting operation
into a piercing operation.
Although there may be some plausible advantages in performing
pipe bursting below the ground watertable—for example, possible
reduced friction between the pipe and soil, as well as reduced effec-
tive soil pressure on the pipe due to the buoyant force of the soil
above—this situation presents significant problems. The groundwa-
ter flows toward the insertion shaft, complicating the operation. Thus,
in general, dewatering should be performed.

6.8.5 Maximum Allowable Operating Pressure
For pressure applications such as water, gas, and force mains, the
maximum allowable pressure should be determined based on the
maximum surge pressure that pipe will be subject to and the maxi-
mum operating pressure for the pipe. The HDPE pipe should be
designed to withstand the maximum allowable operating pressure
(MAOP) and surge pressures according to conventional design pro-
cedures, such as provided in the Plastics Pipe Institute’s Handbook of
Polyethylene Pipe. DR 17 ∗ is typically used for bursting pressure or
gravity pipe unless a higher pressure rating is required. In short,
relatively shallow bursting runs, where high tensile and vertical
forces are not expected, DR 21 may be used (see Sec. 6.10).

6.8.6 Risk Assessment Plan
It is important to pay close attention to the project surroundings
(surface and subsurface conditions) to identify unfavorable condi-
tions and possible risks. The risk conditions require extra attention in
order to ensure the safety of the construction personnel and public, as
well as surrounding facilities and infrastructure. Most underground
and pipeline construction projects entail some risks due to unknown
subsurface conditions. The risks associated with pipe bursting include
damage to nearby facilities, failure to complete the project using the
selected technology, and time and/or budget overrun. The risk of
damage to nearby utilities, buried structures, and pavement increases
under adverse soil conditions, improper construction techniques,
design errors, inaccurate location of utilities, and the like. There are
also risks associated with flow bypass, dewatering, shoring, and the
like, if inappropriate procedures are employed. The additional risks

∗The dimension ratio (DR) of the pipe is the outer diameter divided by the wall
thickness.