Underground Construction - December 2019 - 22

Sliplining with an HDD Twist
Due to site restrictions the drill-rig
anchorage point had to be located over
175 feet from the tunnel inlet. In order to
control the unsupported drill stem pipe
length over a live stream, a drill stem
bridge and guide system also had to be
designed and constructed.

Inserting drill pipe
One of the most complex challenges was
successfully inserting more than 7,800
linear feet of 6 5⁄8-inch steel drill stem
weighing over 297,000 pounds into,
through and safely out of the 7,632-foot
tunnel without causing damage or losing
control of the drill stem string.
Global provided a creative solution
with a 120,000-pound pull industrial
winch, modular sheave roller, rope/cable
and re-spooler system. First the winch
was placed just beyond the tunnel outlet,
then at the inlet a buoy was floated
downstream with a 1⁄4-inch rope attached.
The rope was captured at the exit portal
vault, threaded through the modular
sheave roller system to keep the rope and
cable from cutting into the tunnel roof
and connected to the winch.
Rope/cable was progressively upsized
to larger diameters and strength until a
1½-inch steel cable was pulled through the
tunnel. The cable was connected to the drill
stem string using a 58-ton swivel and 16-

inch diameter safety pig to prevent coiling/
recoiling and floor and wall damage.
Global used its American Augers DD440T (440,000 pound thrust/pullback
capacity HDD drill) to connect the drill
stem string together and pushed as the
cable tension on the winch was carefully
monitored while the string was pulled
directly through the tunnel. After all of
the drill stem was inserted, the swivel
and pig were removed and replaced by a
550-ton swivel which would connect to
the reinforced pulling head assembly and
HDPE pipe string. The assembly was sealed
to prevent contamination from entering
the HDPE pipe string as it was pulled thru
the live wastewater flow.
Global fused a 42-inch Polycam DR11/
steel pulling head to a 42x48 DR11
concentric reducer to a 48-inch DR13.5
pup, not to exceed the retrieval flume
length of 17 feet, 6 inches. Customized
design of internal reinforcement included
a 30-inch DR11, 1-inch thick steel pipe
inserts, optimal symmetrical pattern of
11⁄4-inch bolt/nut connections, DR13.5
wall wedges and watertight seal disc.
They were fabricated in-house to obtain
maximum pull force strength.
Most pipe fusing was conducted inside
a tent that enclosed a McElroy 1648
fusion machine provided by ISCO. Fusion
parameters were predetermined by PPI,
ISCO, WL Plastics and Global Underground
engineers and technicians.
The wall thickness on the pipe was 3.55
inches and weighed 218 pounds per foot. It
was butt fused according to ASTM F-3183.
The average facing time was 15 minutes.
The pipe ends were held against the heat
plate (400-450 degrees F) at an interfacial pressure of 65 psi for a 25-minute
heat soak period. After the heater plate
was removed, the pipe ends were forced
together at an interfacial pressure of 1,000
psi with 100 psi drag pressure for one hour
to complete the fuse.
Quality of fuses was verified using a
McElroy DataLogger 6 with a link that
downloaded the joint information to the
McElroy Vault. The fusion manager would
immediately analyze the joint.

Careful monitoring
During pipe pullback with the HDD drill
unit, rig pressures were monitored con22

DECEMBER 2019 | UConOnline.com
http://hollypipe.com http://www.UConOnline.com
Underground Construction - December 2019

Table of Contents for the Digital Edition of Underground Construction - December 2019
