The Parramatta Rail Link will be a new, mostly underground, 28-kilometre
(17-mile) -long railway linking Parramatta and Chatswood via Epping
(Figure 1). It will add four new stations to the Sydney Rail (CityRail)
network and enable the upgrading of seven existing stations, including
a new A$100 million transport interchange at Parramatta. The project
includes twin-bored heavy rail tunnels running 12.5 km (7.5 miles)
from Epping to Chatswood. The rail link represents a major investment
in public transport by the New South Wales Government.
PB has teamed with consultant Gutteridge Haskins and Davey (GHD)
to provide the design services for the civil and rail systems contract,
which covers the Epping to Chatswood section. We played a major
role in the pretender design for the winning group. Subsequent to
the design/build contract being awarded to the Thiess Hochtief Joint
Venture in July 2002 for approximately A$850 million, our team has
been engaged to provide continued services. Our scope covers design
of the rail alignment, tunnel and cavern excavation and support,
and station structures; technical overview of ventilation design;
and modelling of all temporary access works.
Figure 1: Alignment of Parramatta Rail Link. |
Overview of the Parramatta Rail Link
The main component of the Parramatta Rail Link works will be 17-kilometre
(11-mile) -long twin tunnels from Carlingford in Sydney’s
northwest to Chatswood, on Sydney’s north shore. The tunnels
will:
- Measure 7 m (23 feet) in diameter and vary in depth from about
15 m (50 feet) to about 60 m (200 feet)
- Run mostly parallel to each other
- Pass under commercial and residential properties, universities
and other institutions, and the Lane Cove National Park
- Have cross passages located at regular intervals (approximately
240 m, or 790 feet) for emergency and maintenance access between
the rail tunnels.
These tunnels, along with another 3 km (2 miles) of twin tunnels
proposed to connect Rosehill and Parramatta in western Sydney, account
for about 70 percent of the entire Parramatta Rail Link route. The
remainder of the project includes:
- A cut-and-cover tunnel under the Lane Cove River
- Duplication and upgrading of the Carlingford Line
- Dive structures at locations where the underground tunnels
meet the surface tracks
- Integration works to connect the new rail link with the existing
Main North, Main West and North Shore lines, including rebuilding/
upgrading several road and rail bridges and associated infrastructure.
Innovative Design
We developed the following strategies during the tender design to
solve problems presented by restrictive water inflow criteria and
the need to build caverns in shale.
Geology and Hydrogeology on The
Civil and Rail Systems Contract
The Epping to Chatswood section of the project is located
in the upper sedimentary formations of the Sydney Basin, with
a stratigraphic sequence that consists of Hawkesbury Sandstone,
Mittagong Formation and Ashfield Shale. The Ashfield Shale
forms a “capping layer” on elevated portions of
the route, including Chatswood, Delhi Road, Macquarie Park
and Epping. The residual soil overlying the bedrock is generally
less than 2 m (7 feet) thick over sandstone, but up to 7 m
(23 feet) thick over shale.
The Lane Cove River Valley where the cut-and-cover tunnel
is to be constructed under the river is a palaeovalley comprising
alluvial and estuarine sediments to depths of 17 m (56 feet)
below ground level overlying fractured sandstone. High horizontal
stresses have led to valley bulging and localised high permeability
zones.
The tunnel will be constructed mostly within the Hawkesbury
Sandstone groundwater system. The deeper water in the sandstone
is semi-confined — that is, the aquifer is confined
by a low-permeability layer that permits water to flow through
it slowly.
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Tunnel Alignment. The tunnel alignment at the
new Macquarie Park Station was lowered to maximise the extent of
mined station cavern in the more competent Hawkesbury Sandstone.
This step allowed the designers to achieve settlement criteria,
reduced ground support in the overlying shale and allowed the contractor
to maximise production rates in the station caverns.
Ground Support. Based on numerical analysis and
local experience with large span openings in Hawkesbury Sandstone,
we also established support criteria for the vertical alignment
design to select requirements for ground support. Doing so allowed
the use of an observational approach to ensure the degree of support
was optimised by matching the ground support to the in-situ conditions.
Groundwater Flow Analyses. One of the major project
issues was groundwater control to limit groundwater level drawdown
and surface settlement, and reduce discharge quantities of tunnel
inflow to treatment facilities and the environment.
The tunnels will be driven by a tunnel boring machine (TBM). The
economics of tunnel excavation using such equipment is predicated
on rapid and uninterrupted machine advance. Large uncontrolled inflows
and/or the stoppage or delay of tunnel drive for probe drilling
and grouting are disadvantageous to TBM progress and performance.
About 80 percent of all inflows were derived from just ten locations
between Chatswood and Epping, where long-term infiltration rates
were estimated to exceed 1 L/sec/km.
Probe Holes. We proposed that forward observation holes
up to 30 m (100 feet) long be driven beneath the alluvial valleys
and areas of high predicted inflow to quantify the groundwater inflows
and provide a staging area for pre-excavation grouting of these
areas. This forward heading will provide the ability to assess steady
state groundwater inflows prior to full width excavation, drainage
placement and lining.
Probe hole grouting in the zones identified will allow a more reliable
indication of the rock quality and anticipated water inflows than
is possible based on the borehole data. Probe holes may be drilled
up to 30 m (100 feet) in front of the heading and the tunnel advanced
to within 5 m (16.5 feet) of the end of the probe hole before the
probe hole is extended. Thus, probe drilling may be performed during
the maintenance or off shift.
Water inflows will be monitored and should flows exceed the desired
level, cement fan grouting performed from the tunnel heading may
be undertaken. Probe drilling/fan grouting in this manner will allow
the shortening of the grouted zones, as grouting would only be performed
as dictated by measured inflow. As the water pressure testing results
apply only to the zone around the boreholes tested, it is not yet
possible to verify the true extent of the leaky rock zones. Probe
drilling will allow individual zones to be tested and will generally
reduce the zone where grouting is required.
The proposed delay period between tunnel bore and the lining construction
will be used to observe and monitor groundwater inflows. A combination
of grouting works and tanked construction will be adopted over areas
of high flows to ensure the flows will satisfy the owner’s
requirements.
Groundwater
Modelling. Groundwater inflows to the tunnels were assessed
at pre-tender stage using an analytical method similar to that used
by Heuer (1995). We took into account the packer results, groundwater
head and geological/topographic features and our local experience
on major tunnels. A recharge mechanism from rainfall was assumed
to maintain the existing water table levels within the rock.
Groundwater modelling to predict tunnel inflows and assessment of
settlements adopted Groundwater Modelling Guidelines developed
for the Murray Darling Basin Commission, New South Wales. PB’s
John Ross was a principal author of these guidelines, which are
recognized throughout Australia as a defacto national standard on
groundwater modelling.
We developed a three-dimensioned groundwater model for the Lane
Cove River crossing and underground service facility site to simulate
groundwater flow and estimate distributions of drawdowns, and to
allow design of grouting and ground treatment in alluvium and fractured
sandstone where water pressure (packer) testing had given results
ranging from less than 0.5 Lugeon units to over 200 Lugeon units.
The U.S. Geological Survey’s Groundwater Simulation Model,
MODFLOW, was used to simulate groundwater flow in three dimensions
in steady-state and transient modes. The inflows predicted by both
the empirical (pre-tender) and three-dimensional analyses were within
10 percent of each other, considered good agreement.
Construction Methods on the Civil and
Rail Systems Contract
The TBMs are planned to travel at about 120 m to 180 m (394 feet
to 590 feet) per week through the sandstone. Initially a road header
will be used from the Waterloo Road site in Macquarie Park to excavate
the new Macquarie Park Station, and a second road header from the
Wicks Road/M2 site will excavate the new Delhi Road Station. TBMs
will then be launched from the Wicks Road site and head west towards
Epping.
During this time, a cut-and-cover tunnel will be constructed under
Lane Cove River using dewatered cofferdams that will cross the river
in two stages. This technique will enable the river to flow at all
times, minimising environmental impact. Once the Wicks Road to Epping
section is completed, the TBMs will be brought back from Epping to
the Wicks Road site, where they will be launched east to Chatswood.
All underground and surface construction work will be closely monitored
to ensure that the project meets the Environment Protection Authority
(EPA) criteria for noise, vibration, air and water quality and complies
with the conditions of planning approval.
The four stations, Epping, Macquarie University, Macquarie Park
and Delhi Road, will be about 30 m (98 feet) deep. Underground (mining)
excavation methods will be used to minimise impacts to the surface,
and surface works will be limited to entrance and service structures.
The station cavern “brain” shape was designed to minimise
excavation volume while incorporating architectural requirements.
The main platform cavern will have a maximum cross-section of 22
m wide by 16 m high (72 feet by 52 feet), and an adjacent concourse
cavern that is 18 m wide by 12 m high (59 feet by 39 feet). These
spans will be among the largest of their type in the Sydney region.
Work in Progress
A design stage geotechnical and hydrogeological investigation is
in progress to allow the design team to address some of the key
issues and risks identified during the tender design. This testing
will include in-situ stress testing for tunnel and cavern design
and pump testing of the Hawkesbury Sandstone aquifers to quantify
water inflows and requirements for ground treatment. Detailed design
has been ongoing for a number of months and is scheduled for completion
in late 2003.
Construction started in February 2003 on the first station cavern
at Macquarie Park (Waterloo Road), where our geotechnical staff
is providing geotechnical mapping for the 200 m (66 feet) long caverns,
and in march 2003 on Lane Cove cut-and-cover tunnel. Epping dives
began in May 2003 and Delhi Road began in June 2003. The first TBM
is due to start assembly in July 2003 with boring of the 12.5 km
(8.5 mile) long tunnel due to start in September. Completion of
works is expected in 2007.
The incorporation of PB expertise worldwide was vital to our success
on this project. The contributions of Los Angeles-based Tim Smirnoff
to tender designs of the tunnelling work proved critical to developing
a winning bid. New York-based Bill Kennedy’s high-level ventilation
design assistance further strengthened the effort, as did the continuing
contributions during detailed design from New York, Singapore and
Los Angeles. The value of such readily available global expertise
cannot be over stressed. |
Jim McNamara is a Principal Structural Engineer
and PB’s design manager for tender and subsequent detailed
design for the civil and rail works contract of the Parramatta Rail
Link project. Jim has been involved in numerous mass transit, tunnel
and marine projects in Australia, Asia, the Middle East and the
UK.
Paul Hewitt, a Principal Geotechnical Engineer, was responsible
for the tender stage geotechnical and tunnel design of the Chatswood
to Epping section of the Parramatta Rail Link. He has worked on
several projects involving tunnelling in Sydney; and continues to
work on the PRL project.
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