Underground Construction - March 2019 - 22

First Look
CASE STUDIES

Locating a telecom duct-GIS record (yellow), Contractor postulation
(white) and true map from duct probe (red).

Verification of GIS Data for Existing HDD jobs
A telecom operator was informed that a canal it crossed with an
HDD was about to be dredged to deepen it. To ensure the existing
HDD conduit was installed deep enough, the operator requested
to map the pipe segment between manholes A and B using pipe
mapping tool. The operator knew that the data in the GIS platform
(original installation was in 1998) was incorrect (yellow line)
because the pipe entered manhole B from the south. The telecom
operator presumed the HDD bore had been drilled in a long curve
(white dotted line).
After completing the mapping, it turned out that the true as-built
(red line) was in a significantly different location than presumed
or drawn. Following this measurement, the operator decided to
obtain true as-built maps of all HDDs that were recorded in the GIS
platform as a straight line between the entry and the exit point. On
the upside, the existing drilling turned out to be deep enough.

Duct probe results.
C

Inclination assessment of a gravity sewer
In post installation of a new gravity sewer, the client required verification of exact inclination of each segment.
When the pipe was installed, one GPS reading was taken on each
joint using a conventional GPS surveying method. However, the
client wanted to verify that the pipe had not shifted vertically.
The pipeline mapping system logs at 100Hz (100 samples per
second) and the probe was pulled through the pipe at about 3-feetper-second, giving a point frequency of about 0.5 inches.
Contrary to the conventional method, the result showed several
locations where the pipe inclination was positive rather than the
specified -0.5 degrees.

Pipe bundle torque assessment.

Assessment of Pipe Bundle Torque
A power utility had a 3,300-foot long HDD cable installed under the
False Creek in Vancouver, Canada. Six, 10-inch HDPE conduits of
cables were pulled in but could not be aligned with the HDD exit point
in Laurel Street, so they were pulled through a series of 90-degree
bends (yellow dotted line).
To get through the 90-degree bends, the bundle of ducts had to
twist 180 degrees. The client was concerned that as the bundle was
pulled into the borehole, the torque effect would continue with the
power cables. In that case, a higher drag factor had to be accounted
for, thus requiring a stronger and more-expensive power cable.
Reduct mapped four of the six pipes in one afternoon. The resulting
profiles were projected in a 3D viewer which showed that the pipes
remained almost perfectly aligned and did not torque. As a result, the
less-expensive power cable would serve the needs.

MARCH 2019 | UConOnline.com

Impact loss of HDD steering signal. Preferred (yellow) and Actual pipe
location from Duct probe.

HDD Steering Affected by
Electromagnetic Interference
An HDD contractor used a walk-over beacon system to steer a
1,700-foot-long HDD installation in Michigan. The design specified
that the drilling followed the northern sidewalk. The figure below
highlights (white circle) where the beacon signal was lost, probably
due to electromagnetic interference.
But the rig master steered "blind" until the signal was picked up
again-about 7 feet away from the planned profile (yellow dotted
line). The utility owner insisted on the post-installation measurement
to avoid future damage to its infrastructure. It also revealed a sharp
change in depth at the same spot. *

CMY

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Underground Construction - March 2019

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