M1 extension to Raymond Terrace Temporary Works Jetties

The challenge.

The works formed part of the Black Hill to Tomago package of the M1 Pacific Motorway extension to Raymond Terrace and included temporary infrastructure within the Hunter River to support bridge construction activities. The jetties were founded on very soft alluvial soils subject to tidal variation, settlement and potential scour. Significant crane and construction plant loads were required to operate close to the river edge, while the footprint of the temporary works needed to be contained to suit environmental constraints and works boundaries. The temporary works needed to remain stable and serviceable for the full construction duration and be safely decommissioned at completion.

The approach.

FSG undertook detailed geotechnical assessment and temporary works design for modified rockfill jetties which included a sheet pile wall supported by driven steel tube “kingpost” piles. Site specific ground models were developed using available borehole and CPT data and bearing capacity, global stability, settlement and wall performance were assessed using limit equilibrium and finite element analyses under critical crane and surcharge load cases. An optioneering process was undertaken for the retaining system, resulting in selection of sheet pile walls supported by river side tubular piles to control deflections and avoid constructability risks associated with tie backs. Scour protection measures, staged construction sequencing and an observational monitoring regime were incorporated to manage uncertainty in ground behaviour during construction and operation.

A key challenge also included estimating the volume of rockfill required, based on predictions of the embedment of the rockfill into the soft riverbed sediments. This was done through a combination of bearing capacity theory, modelling and reference to published literature.

The outcome.

The final temporary works solution delivered stable and constructible jetties capable of supporting bridge construction across the Hunter River. Rock fill volumes, settlements and wall deflections were constantly monitored during construction and operation and remained within predicted tolerances. The temporary jetties enabled efficient construction of the permanent bridge structures while managing geotechnical, environmental and operational risk in challenging ground conditions and project constraints.