The Great Northern Highway (GNH) Fortescue Metals Group (FMG) Solomon Road over rail bridge, is a single span arch bridge, carrying the GNH over the existing Solomon Railway Works and includes realigning associated sections of existing road. This infrastructure is critical as it helps to facilitate FMG’s T155 expansion project in the region.

BG&E provided civil engineering services, including the detailed design and documentation of the following:

• three railway bridges over waterways in concrete composite girders and concrete substructure designed for extreme flood events;
• one rail-over-rail overpass consisting of a steel through-girder superstructure with an in-situ concrete substructure, designed to resist the impact loads from heavy haulage rail trains; and
• one road-over-rail overpass on the GNH, designed as precast and prestressed concrete beams with a composite in-situ deck.

The rail bridge design featured modular steel-concrete composite girders on piled foundation for the river bridges, including design for heavy haul rail loads and for significant scour depths. The river bridges comprised five-spans, four-spans and two-spans with a common modular span length of 25.6 metres. The rail overpass bridge featured a steel through-girder bridge with deep fabricated plate girders and fabricated cross girders supporting a longitudinal in-situ deck slab. Each of the rail bridge spans were designed to be delivered to site almost fully complete, with only the handrails and reinforcement to be placed on site, on the ground prior to lifting. The only site work once the beams were lifted into place was pouring the deck slab and place ballast mats. This approach to modular design highlights BG&E’s consideration of constructability and our understanding of optimal construction techniques in remote locations.

The waterways investigations included detailed hydrological analysis such as application of regional methods, run-off routing, flood frequency studies of gauged study and assessment of rainfall runoff on 2D models. The hydraulic modelling also included 1D and 2D models for the bridges. BG&E designed culverts for a 40-kilmetre section of rail where the railway crosses the Fortescue River Valley, west of GNH. We deployed a variety of 1D and 2D models on broad and local scales to assess the interconnected behaviour of the streams and culverts. Our team also designed multiple culvert banks in the central part of the valley to cater for the total flow across the wide floodplain and to achieve minimal water shadow and environmental impact.

The GNH overpass part of the project included designing the overpass bridge, realignment to GNH and associated local access roads and intersections. The bridge was designed to AS5100 with reference to relevant Main Roads Western Australia (MRWA) SES Circulars, contract specifications and guidelines. The bridge consists of tee off precast prestressed beams with an in-situ composite deck. The bridge supports are subject to rail impact loads from the FMGL heavy haulage railway below. The pier and abutment consist of in-situ concrete walls on piled foundations and are designed for rail impact loads.

BG&E worked with MRWA and FMG to determine the appropriate bridge width, clearance and span length, allowing for future expansion of the rail network and access tracks. The GNH roadworks were designed in accordance with MRWA standards and guidelines for geometry, signage, lines, drainage and barriers.

The geotechnical investigation was undertaken by Golder Associates (Golder), including boreholes and test pits at the rail bridges, supplementary investigations for the GNH overpass bridge, and detailed sampling and testing of earthworks and pavement materials for the GNH roadworks. Golder provided complete factual and interpretive geotechnical reports and undertook the relevant phases of the investigation in accordance with MRWA guidelines. Our team worked with the regional office to confirm the most suitable surfacing treatments and to assist the client and contractor in applying these on site.

The project was completed within the design and construction budget. The bridge and roadworks were opened to traffic on the required date to suit the road traffic and FMG rail traffic requirements.