Page 378 - Trenchless Technology Piping Installation and Inspection

328 Cha pte r Ei g h t

Geotechnical Investigation
A geotechnical investigation involves identifying and classifying soil
conditions along the center line of the installation. It is important to
know if PTMT is a feasible method for the given conditions. For a
simple trenchless project or an open-cut project, a geotechnical review
may be sufficient. In a preliminary geotechnical review, the data is
gathered from the surrounding constructions such as buildings,
bridges, another pipeline project, and the like. This review would
give an approximate estimation of the conditions to be encountered
at the project site. For PTMT projects, however, it is important to
know soil stratification at the project site. Boreholes must be made at
various locations along the centerline of the pipeline to obtain the soil
samples. These soil samples must be tested in laboratory to know the
actual soil conditions. Also, the groundwater levels along the center-
line of the pipeline must be measured. It must be verified that there
are no large stones or cobbles along the centerline of the pipeline.
PTMT is not a suitable method if there are a large number of boulders
along the alignment. A thorough geotechnical investigation of the
project site will lead to a good understanding of the subsurface con-
ditions at the site.

Alignment Consideration
The next step is identifying a feasible pipeline alignment. This can be
a difficult task, especially if the project is located in an urban area. For
a trenchless method like PTMT, the alignment most of the time can be
made straight between two points. The number and alignment of the
utility lines already laid may play an important role in deciding the
alignment. If there are too many utility lines, the new pipeline can
be laid deeper. Similarly, the existence of the structures around the
project alignment must be considered. The PTMT installation must
be at a safe distance from any of the laid foundations. The distance
between the shafts must be considered, which is normally not more
than 400 ft. Also the space required for machinery around the jacking
pits must be considered in choosing shaft locations.

Overcut Allowance
The overcut of the enlargement casing must be limited to no more
than 1 in. of the diameter of the product pipe to be installed.

Lubrication Fluids
Lubricant must be selected by the contractor to achieve successful
installation of the pipe system. Any additives used must meet NSF
standard 060.
Figure 8.16 suggests a checklist for inspection of pilot-tube micro-
tunneling operations.