This week, the Victorian State Government announced that their Level Crossing Removal Project had reached the half way mark with 25 out of 50 level crossings now gone for good. Douglas Partners is proud to have played a part in many of these. Douglas Partners have been engaged by the main construction consortia to undertake major geotechnical investigations, geotechnical design and construction phase services for eight of these level crossing removals around Melbourne.
Beginning in 2016, Douglas Partners contributed practical solutions to complex geotechnical challenges for the first phase of crossings, the most challenging being McKinnon Road in McKinnon.
The North-McKinnon-Centre (NMC) package involved replacing approximately 2.5 km of at grade track with three rail cuttings up to 8 m deep at each of the bridges which replaced the level crossings. The cuttings are retained by piled retaining walls and during the main construction occupation approximately 250,000 cubic metres of soil was removed. Douglas Partners was the geotechnical consultant to the project alliance through a subcontract with KBR. The alliance included representatives of the State Government together with John Holland and KBR.
Design and construction of the NMC package presented significant challenges due to the location in residential suburbs and a tight rail corridor and rail occupation timeframe. As a result, careful design and innovative construction techniques were necessary to limit impacts on the community. Douglas Partners addressed a range of geotechnical challenges which generally fell into one of three areas; earthworks, structures and groundwater.
The primary earthworks challenge for the project was moving the 250,000 cubic metres of soil in a 10 day window, however, from a geotechnical viewpoint, obtaining a stable subgrade on which to operate the necessary construction plant in a short time frame, with constrained site access and limited scope to import materials, was the key. Whilst closely watching the performance of the dewatering system, Douglas Partners worked with the contractor to develop a range of contingency plans to cope with any difficulties.
The structures comprised three bridges, stations, base slabs and around 4 km of piled retaining wall. The piles included driven steel sheet piles and Continuous Flight Auger (CFA) piles. Geotechnical challenges that Douglas Partners addressed included assessing the temporary and permanent stability of the walls, particularly as the lead time for the sheet-piles required locking in the design at a very early stage. Pile driving involved the use of some innovative techniques to minimize vibrations. We were also deeply involved with assessing the structural base slab with screw pile system that was adopted for two of the three cuttings that extend below the groundwater table to resist the long term hydrostatic uplift pressure and cyclic loading from trains.
Construction dewatering was also a major issue with two of the three cuttings extending below the water in a silty sand aquifer and the possibility of significantly impacting on the wall stability and construction earthworks. Silty sands are difficult to dewater at the best of times and the space and time constraints added difficulty. Douglas Partners’ engineers and consulting staff played a major role in developing and analysing an innovative horizontal directional drilling system that was adopted for well installation.
A combination of experience, technical innovation and practical solutions allowed this complex and challenging project to be completed successfully within a very tight time frame.
We have also made a major contribution to the geotechnical aspects of the Mernda Rail Extention Project, including investigation, design and construction phase services. This project, which is scheduled to start running trains shortly, provides a rail link to Melbourne’s fast growing northern suburbs. This project involved construction of three new stations and 8 km of new rail, all grade separated from the road network.
In addition, we have provided services, such as laboratory testing and working platform assessments to suppliers and sub-contractors for another ten crossings.