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ment: “We tend to use Christmas tree heads for softer materials,
but in hard rock, they wear very hard on the ABM, particularly
on the gear box and transmission. Roller cone heads cut
rock a little better, but there is still the problem with maintaining
the ABM. With the Robbins Motorized SBU in particular, all
of the rock cutting is done at the front — the ABM is simply
pushing the casing and turning a small invert auger.”
Raising the Stakes in Kentucky
In 2010, Capitol completed a record-setting bore at the
Ripple Creek Sanitary Sewer in Cold Springs. The 440-ft
long crossing was excavated in just 24 days using a 24-in.
diameter SBU-A with 4-in. hex auger and a 48-in. ABM with
60,000 lbs of thrust.
The crew ran into some difficulty when excavating in
unpredictably mixed layers of shale and limestone. Mud
seams ran throughout the rock and at one point the SBU-A
was excavating in a full face of dirt. “We fought the head drifting
for most of the bore because of the layers in the rock. We
made adjustments to the steering fins to help control this
and on several occasions we had to pull the casing back and
forth, reaming the hole out to allow the casing and cutters to
clear the spoils from the bottom and left hand side to avoid
further drifting,” said Kyle Lucas. The crew continued to pull
the auger string and check the grade throughout the bore.
The machine broke through on Nov. 24, 2010, 8 in. off
line and exactly on grade. “I believe we could have pushed
another 100 ft or so in this material. The jacking pressures
were lower than expected, and our rotational pressure was
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Capitol has several ongoing potential record-breakers,
including a 435-ft long crossing using a 48-in.
diameter Robbins Motorized SBU in Pennsylvania.
next to nothing. Our foreman, Steven Barker, and his threeperson
crew should be congratulated for guiding the
machine to its successful finish,” continued Kyle Lucus.
Despite the length, the 24-in. SBU-A required no cutter
changes during the bore. “We generally don’t need to change
the cutters until after the project. Typically after completion of
the bore we take the cutters out, check them for wear and tear,
www.trenchlessonline.com February 2011 TRENCHLESS TECHNOLOGY 41