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Landing Craft

Jet grouting has helped support an important Victorian sewer during construction of Sydney Airport’s new runway extension. GE reports.

Sydney International Airport has just undergone a A$100M (£61M) upgrade to comply with new international safety requirements.

As part of this upgrade, the main east-west runway has been extended to create a new runway end safety area (RESA) on a piece of land adjacent to the Cooks River estuary.

The project involved building a 90m x 90m land bridge spanning the heritage-listed main Sydney Southern Sewer (SWSOOS), the M5 motorway, and a new underground perimeter road for the airport.

The RESA site was originally low lying, but had undergone major alterations to the ground contours during previous construction works.

As a result, when the RESA project started, the soil strata sequence comprised fill up to 4m thick, reclaimed via earlier dredging works, over alluvium predominantly of marine origin.

The subsoil below sea level consisted of soft clay deposits, 8m thick, overlying 2m of sand and underlain by stiff residual clay.

Construction of the underpass for the perimeter road below the SWSOOS required excavation to a depth of 6m below sea level, and was mostly in the soft clay.

The project’s managing contractor, Baulderstone, brought in Menard-Bachy to help deal with the ground conditions, and the firm proposed jet grouting to stabilise the ground and facilitate construction of the cast insitu perimeter road underpass.

Temporary works involved constructing a buttressed cofferdam of interlocking jet-grouted columns, designed to prevent water ingress and provide ground retention around the underpass structure.

The base of the cofferdam was treated by jet grouting, which allowed the resulting, improved ground to provide a brace for the excavation.

The base of the cofferdam was treated by jet grouting, which allowed the resulting improved ground to provide a brace for the excavation, as well as enhancing the properties of the soft clay material, creating stable ground for the construction of the base slab, drainage lines, pit and pump station.

The first stage of the temporary works involved creating a shallow excavation beneath the SWSOOS, while it remained supported on its old foundations of driven concrete piles, to facilitate the installation of an underpinning prestressed concrete structure.

Jet grouting columns were then installed down to the sand layer, to support the formwork of this structure before the load was transferred to four external large bored piles and the old SWSOOS piles removed.

Stringent requirements limited movements in the sensitive SWSOOS structure and the piles supporting it to few millimetres.

This limitation - plus complex staging, difficult ground conditions and a number of site constraints - dictated that a very detailed and comprehensive design had to be finalised before the work could start, supported by finite element analyses using Plaxis Modelling.

Inclinometers were installed to measure ground deformations during grouting, providing daily monitoring of vertical and horizontal SWSOOS movements, with post-installation verification undertaken by sampling and testing the grouted columns.

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The very complex jet grouting scheme included around 1,600 columns, ranging in length from 1.5m to 13m and varying in diameter from 1m to 2.5m. About 500
of these columns were installed under the SWSOOS and in between its supporting piles.

With headroom limited to just 2.7m in these areas, a mini-rig was used for the jet grouting.

These highly demanding jet grouting works, tightly nested with other construction activities, were carried out in two main stages, namely before and after installation of the SWSOOS supporting structure.

They were all completed by 20 December 2009, allowing the final excavation under the SWSOOS to start on 7 January 2010.

As a result, the contractor was presented with a dry cofferdam, the bottom of the excavation was stable, and there was less than 2mm of horizontal movement in the excavation side walls.

Furthermore, prop settlements recorded during the pour of the SWSOOS supporting structure were less than 8mm, 1mm less than predicted.

The new structure is now complete, and is capable of supporting nearly 600t, the weight of a fully laden Airbus A380 aircraft, as well as emergency and rescue vehicles.

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