Author: Professor, Dr. Dietrich Stein - TrenchlessOnline

INTERNATIONAL
hydraulic shoes allow for continuous control of line and
grade, while a laser target system provides instantaneous
measurement of boring accuracy.
SBU Technology & Microtunneling
Many of the crossings on the pipeline are using microtunneling
machines (MTBMs) — 11 crossings utilized MTBMs for
excavation during Phase II of the project, mostly in soft soil
to mixed ground conditions with expected water ingress.
MTBMs are superior in jobs below the water table or in soft
ground; however, they can be highly inefficient in hard rock
conditions.The machines use tungsten carbide drag bits on
the cutting head,which tend to break off in rock above 75 to
100 MPa UCS. The broken bits require expensive replacements,
therefore slowing progress and increasing man-hours.
In dry or near dry conditions, when an auger is used in
conjunction with the MTBM for spoils removal, the microtunneling
method is still more expensive than an SBU
machine with ABM.An MTBM using an auger in this fashion
requires that all rock chipped from the face be further
crushed for removal, resulting in slower excavation rates.
SBU systems, in contrast, use interchangeable single disc
cutters to size the rock chips directly at the face.
While MTBMs are superior in some conditions, they are
much more expensive for hard rock excavation than either
an SBU-A/ABM combination or an SBU-M/ABM setup.
Purchasing a typical MTBM will often cost five to 10 times
more than using an SBU-A, and two to three times as much
as purchasing an SBU-M of the same size.
Advantages of SBU Technology
Williams based his decision to use Robbins SBUs on past
jobsite experience. B&W originally purchased two Robbins
SBU-As in 2002 for a similar natural gas pipeline through siltstone.This
previous project involved six hard rock crossings
along a 70-km pipeline extending from St. Fergus to
Aberdeen in Scotland, United Kingdom.
B&W Tunnelling sees its use of SBUs as a competitive
advantage in the United Kingdom, where the product is not
well known compared to the United States. “These
machines can definitely compete with microtunneling
machines given that they can use concrete pipe and sacrificial
casings. SBUs also do not require slurry — microtunneling
machines are dependent on slurry, which can often be
messy and inefficient,” explained Williams.
Excavating the Crossings
As of summer 2007, SBU excavation has gone extremely
well.Average rates of SBU-M excavation are 1.5 to 2.0 m per
hour and none of the machines have required rebuilding
between crossings.
B&W began excavating the crossings using five Robbins
machines — three 1.2-m diameter SBU-As and two 1.2-m diameter
SBU-Ms.The machines utilize 6.5-,9.5- or 11.5-in.diameter
disc cutters (depending on the rock hardness) to bore
through geology between 70 and 200 MPa UCS. Many of the
crossings are in ground consisting of hard rock interbedded
with clay and gravel, necessitating careful cutterhead design.
Circle 22 on reader service card. Circle 52 on reader service card.
54 TRENCHLESS TECHNOLOGY September 2007 www.trenchlessonline.com