Page 282 - Trenchless Technology Piping Installation and Inspection

246 Cha pte r S i x

that may halt or impede the bursting operation and/or create prob-
lems include:
• Settlement at insertion/pulling pits if the density of the back-
fill exceeds that of native soil.
• Bursting through curves.
• Concrete encasement or steel point repair of existing pipe.
• Excessive bursting lengths.
• Damage to the new (replacement) pipe from sharp edges or
fragments of existing pipe being burst/split.
• Damage to laterals from bursting of main line.
• The presence of rock under the existing pipe may create a
“bump” in the replacement pipe.
• Collapsed replacement pipe (see Sec. 6.10.4).
Projects within the Class C category (challenging to extremely
challenging as indicated in Table 6.1) must be carefully examined in
terms of required forces and ground displacements. In addition, due
to the enlargement of the borehole and accompanying compaction of
surrounding soil, the depth of the existing pipe affects the extent of
ground displacement above the pipe. If the pipe is shallow, damage
to the pavement may take place. Saw cutting the pavement prior to
bursting might be advisable. If the existing pipe is below the ground-
water table (GWT), the difficulties increase. Insertion and pulling
shafts will be larger and more complex as the depth increases.
If there are unacceptable sags in the existing sewer line, these sags
must be corrected before bursting. The sags can be corrected by local
excavation, surface grouting, or grouting from within the pipe. Some
reduction of sag magnitude may be expected (without corrective
measures) from the bursting operation, but the extent to which the
problem is corrected depends on the relative stiffness of the soil below
the sagging section.
If there is erosion of the soil around the pipe, the bursting head
and the following new pipe will tend to deviate toward the void or
lower density region. Similarly, if there is a hard soil layer or rock
close to the pipe, the bursting head will tend to displace toward the
softer soil. In shallow conditions, the ground will deviate mostly
upward toward the ground surface. If the conditions change substan-
tially along the length of the burst, this may cause some change in the
grade and/or alignment of the pipe. When the grade is critical, these
possibilities should be considered.
As mentioned in Chap. 2 trenchless operations in general and pipe-
bursting projects specifically can be performed successfully and safely
if site and project conditions are known before bursting and appropri-
ate measures are taken to address these conditions. There are well-
known solutions to all of the above mentioned risks and problems, and