COP Planning for practice

Project Management

The Expressway project is managed by Opus International Consultants for joint principals Transit NZ and the Tauranga District Council (TDC). Funding for routes P, J and K comes from TDC while Western Bay of Plenty District Council is a minor funding contributor. As a result – perhaps indicative of things to come nationwide – route K is a toll road, with a user-fee payable each way.

The construction contract is the largest single roading contract ever awarded in New Zealand. Construction is being undertaken by a joint venture between SmithBridge and Fulton Hogan Construction, to conforming designs from Beca and Opus. From the outset the client partners and the joint venture were interested in innovative alternative solutions to the not inconsiderable technical difficulties they faced. Accordingly, Holmes Consulting Group and Tonkin & Taylor have been involved with the joint venture partners from day one, and as a result approximately 70% of the project is now to an alternative design. While the separation is not absolute, generally Holmes Consulting have worked with Mt Maunganui-based SmithBridge on alternative designs for bridges, with Tonkin & Taylor handling the geotechnical issues.

COP Brief development

By far the most difficult engineering problem the project has faced is the soil conditions encountered on the three routes. Route K, which will be the last to be completed, crosses an area of estuarine swamp bordered by higher ground consisting primarily of complex multi-layered volcanic deposits. The swampy soil in places is up to 18 metres deep. Routes P and J are through predominantly complex volcanic deposits that were laid down over a long period, undergoing a prolonged weathering process. The challenge therefore was two-fold: to create a suitably stable roadway through the swamp, and to stabilise the volcanic structures to support stable roadway, while forming and securing the various cuttings required.

The complexities of the volcanic deposits and the swamp were further complicated by the boundaries between the various sub-strata interacting on a micro scale. On all three routes it was found that within a few tens of metres the underlying soil type and structure could run through several significant variations, with markedly different settlement and deformation characteristics. The challenge therefore was to develop a scheme to stabilise these different soils, to ensure that the roadway would maintain a stable profile. Previous roads constructed in these conditions have typically undergone uneven settlement, becoming “wavy".

COP Outcome development and evaluation

To measure the actual settlement profile along the routes a Profileometer Pipe was laid across numerous key sections of roadway. A pressure altitude transducer was regularly pulled through the pipe to develop a settlement profile for particular areas. The basic settlement profile for estuarine soils follows a logarithmic progression, with a rapid initial settlement progressing over time to a much-reduced rate of settlement, suitable for roading. Compressing the soil involved overlaying the swamp with around 180,000 m3 of hard fill. On much of the swamp area the ground was overloaded to accelerate the consolidation process. This results in a slight spring-back when the additional load is removed but not enough to cause problems.

To assess the soil structure in the volcanic deposits Cone Penetration Testing was used extensively. While it yielded no great surprises the information gathered was vital for finding ways of stabilising the soil within the cuttings and under the roadway and bridge embankments. Again the complex nature of the soil boundaries and the different volcanic deposits complicated the design of several of the bridge embankments; marked changes in the underground topology required careful attention.

The unstable and varied volcanic deposits saw the extensive use of soil nails on several of the deeper cuttings. At up to 20 metres deep, the bigger cuttings transect multiple layers of volcanic deposits, all with the potential to collapse. One deep cuttings on Route K was further complicated by proximity to a significant archaeological site, which mandated the use of a steeper cutting face than might have been ideal. Here more than 200 soil nails up to 12 metres long, all post-tensioned and grouted in place, were used to stabilise the cutting faces.

Pumice is used as a lightweight fill to construct bridge embankments. However, the swampy conditions meant that the ground couldn't sustain the weight of earth embankments higher than seven metres. As some of the bridges needed 11-metre embankments, an innovative solution was to use polystyrene as a lightweight fill. While pumice is heavier than the plastic alternative, pumice at 1.3 t/ m3 is considerably lighter than either rock (1.9 t / m3) or brown soil (1.6 t / m3). Under an alternative design however, polystyrene would be used rather than pumice.

COP Outcome development and evaluation

In the original conforming design the route K overpass was two separate bridges. However it was found possible to combine them into a single 415-metre bridge, which was not only simpler from an engineering standpoint but also saved time and costs.

With the exception of the route K overpass, which has one joint, all the bridges are continuous of construction. The 600mm shell piles were driven on average 28 to 32 meters, with a maximum depth of 44 meters. A coupled pile-driving analyzer and Hiley capacity testing were employed to monitor the progress of the pile-driving operation, to verify the load-bearing capacity of the piles and to confirm that none of them had been damaged in the driving process.

Two partner clients, construction undertaken by a joint venture, and input from two further engineering consultancies into major ongoing redesign of the project – the potential for friction in the project management team was always there. Some lessons were learnt, and it was recognised that perhaps a less complex management structure would have been desirable. But much credit for the smooth running of the project is attributed to the establishment of a joint project office on site. This allowed personnel from all the parties to get together to work through the issues on site, face-to-face rather than office-to-office as is more common.