The so-called southern structures section of the new railway was a 1km length of tunnel involving a mixture of difficult crossings and structures. Included were access shafts for the TBM driving the tunnels in other sections, a crossing of the Cook’s River and crossings under a six-lane highway and four tracks of the Illawarra railway line.
There were even historical works to consider, with one of Australia’s earliest homestead sites, the 1832 Tempe House, lying on the route. It will be incorporated into later development of the Wolli Creek station.
The southern structures were planned, designed and constructed as cut and cover tunnels; although soft ground conditions enabled the use of a TBM eastward from Tempe Reserve, ground conditions here were variable and the depth of the cover over the tunnel progressively reduced. This southernmost section of the NSR was particularly complex because of: the confined space for construction; serious environmental and heritage issues; a large amount of services and utilities to relocate; very poor ground for cut and cover work; and the height of the water table.
Arup was commissioned for the design of these works following the signing of the design-and-build contracts in early 1995 by Australian/French joint venture Transfield Bouygues. Arups did design and documentation of cut-and-cover tunnels between Tempe Reserve and Turrella.
Much of this work involved highly complex interaction between new and existing structures, utilities and operating rail lines. The joint venture drew Arup into much of the temporary work design and coordination, as well as permanent works because of the complexities: track possessions at the railway crossing, for example, had to be booked around 12 months ahead.
Most complex of all was the river crossing. The Cook’s river is a flowing river sensitive to flooding, which imposes limits on any constricting works.
To achieve the crossing, Arup looked at rectangular steel sheetpile coffer dams and tension coffer dams as options for cut and cover – the only tunnel method considered. Eventually it suggested the pioneering use in Australia of interlocking circular cofferdams, a technique thought to have been used before in Canada, although not well known, says Hugh Muirhead.
Eleven circular cofferdams were used with excavation taking place within each of these to a depth of 16m below water level.
Design input for this needed to be high because the cofferdams were more complex structures than the permanent works and required several design
iterations, with input from the contractor, to achieve the desired constructability.
The water of Cook’s River is quite polluted from industrial areas upstream, although it is slowly improving. Contaminated silt was therefore initially dredged from the riverbed and treated on site, using a centrifuge system. All contaminated material in the tunnel’s path was removed ahead of the erection of the cofferdams.
After excavation, the tunnel section was constructed inside the cofferdam in 14m segments. The excavation was then backfilled with clean material so the riverbed was restored to its original level and the sheetpiling for the next cofferdam could begin.
The tunnel line passes approximately 172m from the TBM assembly trench at Tempe Reserve to the south bank of the Prince’s highway bridge over the Cook’s River. Depth of excavation varies from 18.7m at the eastern end to 14.8m at its western end, providing a constant three per cent rising grade to the west. The minimum cover to the top of the tunnel is about 2m below the river bed.
The site geology is marine and estuarine sediments, mainly soft clay and loose sands, which overlie alluvium of stiff clay and dense sand. These overlie again a veneer of residual soil, most of which had been scoured by previous river action on top of bedrock, a horizontally bedded sandstone with an unconfined compressive strength ranging from 2MPa to 12MPa.
Variability on site – both in terms of horizontal extent of these strata, and also vertically – caused much concern. The rockhead was terraced which meant some of the sheetpile toe levels were above dig level, while others were not in rock. The design had to have flexibility in it to allow for these variations.
The two lines of 11 overlapping circular cofferdams, each 22m in diameter, were constructed one after the other in order to limit the constraining effect on flow in the Cook’s River. Work progressed from the centre out on one face at a time only.
An additional advantage was that sheet piles and struts could be re-used for each successive cofferdam. The contractor accessed the cells from a temporary bridge built alongside and used a barge mounted crane and pile driver.
A twelfth cofferdam was designed by others because it was among temporary works for a different construction contract.
Design of the cofferdam had to be integral with the design of the tunnel section because of the manner in which later sections of the tunnel were constructed. This was achieved by overlapping the cofferdams, which meant elements of the permanent structure needed to be designed as components against which to seal the sheet piles. This part was part of the already constructed and reburied section of tunnel.
The base of each sheet was fixed into a curved collar to achieve a waterproof seal. The sheets were propped back on to the roof of the constructed box until the waler was installed. A clutch welded to the “ears” projected either side of the box segment engaging with a sheet to form the lateral closure on to the box. Beneath the constructed segment, cut off is achieved by sheet pile or a slurry trench between rock head and the base of the box.
Structural design of the tunnel longitudinally was heavily influenced by the variability of the ground conditions. The tunnel is founded partly on rock, partly on alluvium. Conditions beneath the river generally comprise 1-2.5m of base soil overlying sands and clays, with sandstone bedrock varying from 20m at the eastern end, to 10 m at the western.
On top of this the method of construction meant that sections of sheet pile wall would remain below the tunnel soffit and act as ‘hard spots’. Longitudinal design was therefore undertaken as a beam on an elastic foundation with discrete support points.
A complexity in the temporary works design was the lack of lateral support for the toes of sheet piles, particularly at the eastern end where the rock level was well above tunnel soffit.
The main design issues with the cofferdams was the toe embedment depths, the struts and the method of construction and re-use of the sheetpiles and struts. The piles were driven where possible to rock for hydraulic cut-off.
Toe restraint was not a concern in the central section, where adequate embedment in the soil was achieved. Additional boreholes were drilled to determine more accurately the rock head levels plus a geophysical survey was carried out in the main central portion of the crossing to correlate borehole results.
The critical design was the toe embedment: The cofferdam stability depended on adequate toe restraint. Embedment was also required against piping failure.
This was particularly important where the dig level within the cofferdam would be below the toe of the wall. In these cases an additional concrete toe ring beam was cast. The rock toe-in was required in the temporary condition prior to the last strut being installed. The toe-in was not required during the permanent construction works of the tunnel box. Early in the design, shear pins, grouted toe and pre-splitting were considered, but not incorporated into the final design.
Four to six levels of struts, predominantly fabricated steel “I” section rings, were installed dependent on the depth of the cofferdam. A few concrete ring beams were used in special locations. Top struts were typically 3.5 to 4m centres, and the lower struts 1.8 to 2m.
Most construction lessons were learned on the first two segments. From there on the operations were fairly repetitive. Special conditions were encountered according to the level of rock head relative to the base of the excavation, but the crew became practised.
Optimisation did occur in the method of releasing the walers on completion prior to backfilling. The planned sand jacks did not provide sufficient play to achieve release, and a wedge piece was introduced to supplement them. Close monitoring was specified using inclinometers and piezometers and Arup regularly analysed the information from these to ensure movements were within safe limits.
Arup carried out extensive design calculations and sensitivity analyses of the cofferdam stability, using its own in-house Oasys software package FREWÆ to analyse the construction stage moments and movements.
A comparison was carried out with a FLAC model due to concerns about the toe behaviour. Moments and deflections showed good agreement. Generally FREWÆ determined lower moments in the initial stages (1-2), very close agreement during the dig and strutting stages (3-8) and produced bigger bending moments during the backfilling stages (9&10).
Final design was based on Eurocode 7 case B for SLS condition and case C for ULS condition. Each cofferdam design was checked for a range of rock levels. The FREWÆ analysis showed that the stability was sensitive to small changes in soil strength parameters in the SLS condition, indicating marginal stability of the piles. The analysis confirmed that the rock toe/additional bottom strut was vital to stability of cofferdams.
For all elements of the design there was a continuous interaction between the designers and the contractor. For example, says Hugh Muirhead, development of the cofferdam construction sequence and the way the temporary and permanent works interacted was fed with ideas from both sides.
When the freeplay was insufficient to allow release of the steel waler rings having released the sand jacks, a wedge solution was used to increase release movement.
“And when the constructor wanted an alternative to the sheetpile cut off walls beneath the base of the segment, we designed a slurry trench,” adds Mr Muirhead. When the construction programme had to be changed to accommodate a temporary works failure on the adjoining Princes Highway Undercrossing section, Arup redesigned the linking box section.
This twelfth cofferdam, a rectangular shaped box designed by others, unfortunately did fail catastrophically underlining the complexities of the job. Fortunately there was no loss of life and the effect on the project from this incident was able to be managed by TBJV without delays to the project as a whole.
For the tunnel itself there were many joints to be sealed because of the manner of constructing the segments. This was achieved using a combination of rearguard and centre bulb water stops. Movement was provided for by incorporation of ‘Stafix’ dowelling technology, distributed around the cross section of the tunnel.
Extending westward from the Cook’s River, the NSR tunnel passes under the six lanes of the Princes Highway just south of the bridge abutment of the highway’s bridge over the river. In this section, the tunnel is entirely founded on sandstone and the excavation for tunnel construction was substantially in sandstone.
This enabled Arup to introduce economies in the cross-section of the tunnel, taking advantage of the reduced lateral loading as the tunnel walls could be cast directly against the vertically excavated sandstone. However, it was necessary to also reduce hydrostatic pressure around the tunnel achieved by a combination of waterproof membranes, longitudinal sealing of the walls and a drainage blanket below the tunnel soffit.
Highway works also meant relocation of major services and provision of support bridges to carry them over the tunnel excavation. A major construction task was to stage the works so as to avoid disruption of traffic on the Princes Highway.
Illawara Railway and East Hills railway undercrossings and ramp formed the westernmost part of the NSR project and were required to be constructed without disruption to the operation of the railways.
To do this and fit in in with the shifting of operating tracks, the simple rectangular cross-section tunnels were constructed in stages. This helped accommodate the new platform linked to Wolli Creek Interchange Station and the spreading of the existing East Hills tracks to accommodate the NSR ramp.
At the station, Arup developed its own earlier work done for the Rail Access Corporation on preliminary design and planning.
The project consisted of a new station at Wolli Creek at the intersection of the East Hills, Illawarra and NSR lines. The station features a high-level island platform, concourse booking hall and low level through platform.
Beyond the railway undercrossings, the NSR ramps to the surface in an open box and its walls progressively reduce in height. With this area of the track being susceptible to high water tables, the structure had to be designed to resist the effects of buoyancy. Additional resistance against flotation was achieved by external extensions of the base slab in order to increase the dead weight and passive resistance.
Close relationships were maintained between the Arup design team and the construction teams via the TBJV design management team during works. Arup Acoustics was a member of the Rail Services Australia team undertaking the laying of track in the tunnel and the fitout of the railway systems through the railway.
All major design was completed by mid-1999, and the project itself was opened in May this year.
Related Files
Cross section of the river crossing cofferdam sequence
Plan of the river crossing crossing cofferdam sequence
