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Ground improvement works for the Puhoi to Warkworth motorway extension are well underway

3D modelling plays important role in design of Puhoi to Warkworth motorway

Design Joint Venture (DJV), a collaboration between engineering consultants Beca and Tonkin+Taylor, has highlighted the power of Seequent’s Leapfrog Works 3D geological modelling software in the design of the Puhoi to Warkworth motorway extension.

With an estimated cost of more than $700 million (including maintenance over 25 years), the 18.5 km Puhoi to Warkworth motorway extension is the first section of the Ara Tuhono Puhoi to Wellsford project. It comprises a road corridor that cuts through steep hills and valleys and the creation of seven bridges, including three viaducts.

Combine this with the cutting of 7 million cu m of earth (and the filling of 5 million) and the challenge of soft alluvial sediments, and it’s easy to see why a project-wide ground model is essential as a basis for geotechnical design, and to mitigate project risk.

Meeting population demands

As the population of Auckland reaches over 1.5 million, and the demand on the Northland area increases, the region of Warkworth is classed as a growth centre. The number of cars travelling the route daily is set to rise from 19,700 in 2012 to 31,300 in 2026. The motorway extension will provide a better connection with all of the associated safety and traffic flow benefits.

The design work has been subcontracted to Design Joint Venture (DJV), and Seequent’s Leapfrog Works, which is specifically designed for the civil engineering and environmental industries, has been used as the 3D geological modelling software to more accurately define the geology.

“There are three areas of focus: the north, which shows low-lying topography; central, which has significant cut-and-fill embankments; and south, which contains two viaduct structures,” explains Chris Monk, engineering geologist at Tonkin+Taylor. “It was important we could use a modelling tool that would work flexibly around the different geology and surface types to give accurate outputs. We have modelled 210 cone penetration tests (CPT) and brought in data from 420 boreholes, 355 hand augers and 220 test pits.”

Working in dynamic 3D

Detailed design started in October 2016 and is ongoing. The team started collecting ground investigation data and inputted this into their 3D geological Leapfrog model. Leapfrog Works’ dynamic and continuous modelling has transformed the way the geotechnical team work.

Using Leapfrog Works means geological surfaces can be mapped by a geologist, rather than engaging a CAD technician to work alongside a geologist, resulting in a smoother workflow and faster turnaround times. As project engineers need sections, they are able to go straight to a single point of contact to quickly create the desired section – saving time and reducing the effort in having to reproduce work.

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

“Leapfrog has really helped us on what has been a significant and challenging project,” says Stuart Cartwright, senior engineering geologist at Tonkin+Taylor. “The length of the proposed motorway and its alignment through such steep topography has made the ground model development challenging.”

The geotechnical team have been able to leverage the great visualisation of Leapfrog Works to bring together and better communicate across a wide range of project stakeholders, including the construction joint venture (CJV) team, quantity surveyors, surveyors and geotechnical engineers, and bridge designers.

“By being able to show the model in 3D and cut sections at any desired location, we have instantaneously enabled others to visually understand the geological conditions of the site with much better clarity. We have collaborated much more as a whole project team as we have been able to hold informal review sessions/workshops to show progress. In the past we would have gone with paper sections, but the 3D model outputs and graphical interface have changed the way we communicate and collaborate,” says Mr Cartwright.

Minimising the cut-and-fill footprint

Leapfrog was initially used by DJV in the tender phase to create a 3D geological model of the route. The model was then imported into Bentley’s OpenRoads civil design software. This allowed the slope profiles and cut-and-fill quantities for different alignments to be quickly and easily compared, and for optimisation of the earthworks’ mass haul to assess the most cost-effective alignment.

Understanding the material makeup of the mass-haul balance is crucial for minimising the cut-and-fill footprint. This not only reduces waste, but being able to understand earth composition means that extracted earth can potentially be used to fill in another part of the site, saving time and money. Environmental impact such as tree removal can also be minimised.

Cut-and-fill slopes vary according to the geology

“The contact surface between the weathered Pakiri Formation soil and underlying unweathered rock is critical for assessing likely cut-slope profiles and excavation footprints. Leapfrog made this process much easier to complete,” says Mr Cartwright.

Looking forward

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. This is a real step forward to enabling the civil engineering industry to become more responsive in an increasingly digital world.

“Having a dynamic model that evolves as new data is provided has saved the team time from not having to recreate a new model every time, leaving us more time to focus on analysis,” says Chris Monk.

The motorway is expected to take five years to build and be open to traffic in late 2021.

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