Singapore - Trenchless International

PROjeCts
October 2010 - Trenchless International
36
The Robbins Double Shield Rockhead is steered continuously from an in-shield operator’s console.
The Robbins Rockhead is excavating the first
900 m with a mixed ground cutterhead for
soft shale and limestone, before switching to
a hard rock cutterhead.
the seven tunnels, with their varying
grades, result in a change of
54 vertical metres over the course of the
project. As a result of the elevation change,
the tunnels travel through different strata
consisting of softer shale and limestone for
the first 900 m, changing to harder, drier
shale and limestone for the later tunnels.
to effectively excavate in the conditions,
Midwest Mole needed a solution to excavate
mixed ground as well as hard rock up to
165 MPa UCs.
“the Rockhead was the best suited to the
project conditions based on our previous
experience with similar Robbins machines.
two-pass tunnelling with a Double shield
Rockhead was also more cost effective
than microtunnelling when we prepared our
estimates,” said Mr Abernathy.
the original contract allowed for either
microtunnelling or two-pass tunnelling,
which allows for installation of a primary
liner before placing the carrier pipe.
the Double shield Rockhead was
designed with a mixed ground cutterhead
that can be changed out for a hard rock
cutterhead later on. the mixed ground cutterhead
features 6.5 inch single disc cutters
and carbide bits, combined with large
openings in the cutterhead to ingest mixed
ground and to allow for cutter changes.
the hard rock cutterhead is dressed
with 11.5 inch diameter single disc cutters
and abrasion-resistant muck scrapers, as
well as a cutterhead opening for cutter
changes. Crews will switch out the cutterhead
between crossings in one of the
launch/receiving shafts.
how it works
the Robbins Double shield Rockhead
(sBU-RHDs) is a tunnelling machine for use
on longer utility installations, usually over
180 m, in unstable ground. It is also typically
used for line and grade-critical installations
such as gravity sewers, as the machine can
be continuously steered from an in-shield
operator’s console. Line and grade are
monitored continuously using a laser targeting
system.
the machine, available in diameters from
1.3 to 2 m, consists of a circular cutterhead
optimised for either mixed ground or hard
rock. Hard rock disc cutters are capable of
excavating ground ranging from 25 to over
175 MPa UCs.
As the cutterhead rotates, disc cutters
penetrate the rock face and create a ‘crush
zone’ through which fractures propagate.
Material between adjacent crush zones is
then chipped from the rock face.
Muck scrapers scoop the muck into
openings on the cutterhead called muck
buckets, which transfer the material to a
machine belt conveyor. Muck removal from
the site is by either a belt conveyor or
muck cars.
crossing excavation
Excavation of the first tunnel began on
19 May 2010, from an 11 m deep shaft.
the Robbins Rockhead completed its first
474 m long drive in three months, holing
through on 18 August into a shaft site.
“the mixed ground cutterhead is ideal for
the ground. After 474 m there is almost no
perceptible wear of the teeth or disc cutters.
the machine also drives very smoothly and
has plenty of power for these conditions,”
said Mr Abernathy.
Crews reported advance rates averaging
150 mm per minute in low strength
shale using the mixed ground cutterhead.
Advance rates topped out at 32 m in
24 hours, or 21 1.5 m long ring beam and
board sets in two ten-hour shifts. the swift
advance comes despite some unforeseen
difficulties, including a section directly below
the creek bed with significant groundwater,
and gripper slippage in the soft, wet ground
during machine pushes.
the machine was launched from its
receiving shaft to bore a second 575 m
long tunnel on 30 August, following some
maintenance and modifications. Changes
included modifying the hydraulic system
for increased gripper and roll correction
cylinder pressure. the increases allowed
the machine to grip and roll correct in the
extremely soft, often wet rock.
As the machine bores, a primary liner of
ring beams and lagging is set every 1.5 m.
After each ring is built, a muck train consisting
of a battery-operated locomotive and
three muck cars removes spoils from the
tunnel. the muck will be used as back-fill
around some of the shaft sites after project
completion, and potentially as fill on private
property in the area.
Work on the launch/receiving shafts is
ongoing – four shafts are now complete
using a combination of drill and blast techniques
and manual excavation. some open
cut pipe jacking operations are also connecting
existing, peripheral lines to the new
sewer line.
Midwest Mole recently utilised a
760 mm diameter Robbins sBU-A for a
46 m long crossing of the creek, which will
connect a 460 mm diameter PVC sewer to
the main line.
the work to date has been successful,
in large part due to teamwork between
Midwest Mole, Robbins field service, and
the project owner.
“If there is one thing we would like to
emphasise, it would be how willing the contractor
has been to work with us throughout
the project. In addition to proposing the
tunnelling method, they planned value
engineering that resulted in substantial
cost savings,” said Mr Bloom.
“When the project was bid, there were
originally eleven shafts, but Midwest Mole
proposed to eliminate three shafts. they
obtained easement rights to alter the project
alignment, and shortened the project schedule
with their new plan.”
All seven of the tunnels and the eight
manhole shafts are expected to be completed
by May 2012.
Primary Program (Jan. 25-27)
I Introduction to Sewer & Water Rehab Technologies
II Sewer Construction, Rehabilitation & New Technology
III Cured-in-Place Pipe
IV Pipe Bursting: View from the Field
V WaterWorks Conference: Facing Reality
VI Storm Water
VII Trenchless Technical Symposium
VIII HDD: Rigonomics & Operations
IX Alternative Energy Construction
X Damage Prevention & Safety Conference
XI Underground Utilities Construction
CEUs and PDHs available!
AKKERMAN
Reduced
Registration
Rates In
Effect!