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Highways: Modelling the future

ground improvements works. image courtesy of nx2 group.

Work on a new motorway scheme has been enhanced through use of 3D ground models from the tender stage and is now aiding work on the construction phase.

When completed in 2021, the Pūhoi to Warkworth motorway project will provide a better connection between New Zealand’s biggest city, Auckland, and the neighbouring Northland region. With an estimated cost of more than NZ$700M (£380M), it is a key investment in the region’s infrastructure to improve the safety, reliability and resilience of the state highway.

Design of the new motorway has been optimised from the outset through use of software to create 3D ground models. Two years into the construction phase and the 3D models are still paying dividends for the project teams.

The scheme is being delivered under a public private partnership over a 25 year period by Northern Express Group with the construction work sublet to a joint venture of Acciona Infrastructure and Fletcher Construction and design undertaken by a joint venture of Beca and Tonkin and Taylor (TT).

The route of the new road cuts through steep hill country with numerous steep sided valleys, which are often filled with soft alluvial sediments. The final design required several significant road cuttings and embankments to be created, with more than 7M.m3 of earth to be cut and 5M.m3 to be filled. The project also requires seven bridges to be constructed, three of which are large viaduct type bridges. A suitable project-wide ground model was required as a basis for geotechnical design of the proposed earthworks and structures.

the 3 d subsurface model encompasses the full length of the 18

the 3 d subsurface model encompasses the full length of the 18

The 3D subsurface model encompasses the full length of the 18.5km alignment

The design joint venture used Seequent’s Leapfrog software in the tender phase of this project to create a 3D geological model of the route. The 3D geological model was then imported into the Open Roads motorway geometrics modelling software.

According to the design team, having the geometric design model incorporated with the 3D geological surfaces allowed the slope profiles and cut and fill quantities for different alignments to be quickly and easily compared. This allowed optimisation of the geometric model to be undertaken to balance the earthworks mass haul for the project, to assess the most cost-effective alignment for the motorway.

Understanding the material makeup of the cut and fill balance on this type of project was seen as crucial to saving money and time.

Environmental considerations were a key aspect of the project as the alignment traverses greenfield land, some covered in native forest. Approximately 162ha of vegetation are to be cleared and followed by a significant tree planting programme. Minimising the cut and fill footprint was important to ensure only a minimal number of trees are removed to reduce the impact of the work.

Detailed design started in October 2016 and is ongoing.

cut and fill slopes vary according to the geology

cut and fill slopes vary according to the geology

Visualising the ground investigations alongside the cut slope design and geology in 3D

“Leapfrog really helped us on what has been a significant and challenging project,” says TT senior engineering geologist Stuart Cartwright. “The length of the proposed motorway and its alignment through such steep topography made the ground model development challenging. The contact surface between the weathered Pakiri Formation soil and underlying unweathered rock, was critical for assessing likely cut slope profiles and excavation footprints.”

TT engineering geologist Chris Monk adds: “There were three areas of focus for our geotechnical model: north, which showed low-lying topography; central, which has significant cut and fill embankments and; south, which contained two viaduct structures.

“It was important we could use a modelling tool that worked flexibly to work around the different geology and surface types to give accurate outputs. We were able to continuously update the model as new investigation data was produced.

“We modelled 210 CPTs and brought in data from 420 boreholes, 355 hand augers and 220 test pits. Having a dynamic model that evolves as new data is provided has saved the team time from not having to re-create a new model every time, leaving us more time to focus on the analysis.”

The design joint venture has said it was able to produce more accurate 3D surfaces as a consequence of using Leapfrog Works. “The more accurate the interpretation of the geological model, the better the outcome of the design,” said a spokesperson for the joint venture. “The team were also able to better highlight the risks and uncertainties around the model to the other project staff.”

Cartwright adds: “By being able to show the model in 3D and cut sections at any desired location instantaneously enabled others to visually understand the geological conditions of the site with much better clarity. In the past we would have gone with paper sections, but the 3D model outputs and graphical interface changed the way we communicated and collaborated.”

According to Cartwright, as major infrastructure projects become increasingly large and complex with multiple stakeholders, having a 3D ground model to support the understanding of the geology allows geotechnical teams to improve efficiency of design. “Easily maintaining a dynamic model over the course of the proposal and design is transforming the way ground engineers are working,” he says. “This is a real step forward to enabling this industry become more responsive in an increasingly digital world.”

 

 

 

 

 

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