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Visualizing geological conditions

Visualizing geological conditions

3D modeling plays important role in design of New Zealand motorway.

Ground improvements works. Image: courtesy of NX2 Group

The Pūhoi to Warkworth project is an 18-kilometer motorway extension to provide a better connection between New Zealand’s largest city, Auckland, and the neighboring Northland region. With an estimated cost of more than NZ $700 million (over 25 years), it is a key investment in the region’s infrastructure. The aim is to improve the safety, reliability, and resilience of the state highway.

In May 2015, the NZ Transport Agency announced that it would proceed with construction under a public-private partnership (P3). Under the P3 contract, the Northern Express Group (NX2) will finance, design, construct, manage, and maintain the Pūhoi to Warkworth motorway for the 25 years that will follow the expected five-year period to build the motorway. Full ownership of the highway will remain with the public sector.

NX2 subcontracted construction to a Construction Joint Venture (CJV) between Acciona Infrastructure and Fletcher Construction. In turn, the CJV is subcontracting design work to a Design Joint Venture (DJV) of two Australasian engineering consultants, Beca and Tonkin & Taylor.

Both Beca and Tonkin & Taylor have a strong presence in New Zealand and understanding of local conditions. Detailed design work started in late 2016 and the motorway will be open for traffic in late 2021.

The setting

The 3D subsurface model encompasses the full length of the 18.5-kilometer alignment.

The population of Auckland has grown to more than 1.5 million, while the Northland region is expected to reach a population of 171,000 by 2031. Warkworth is classed as a major growth center of the Northland area; 19,700 cars a day were using the route in 2012, but this is set to rise to 31,300 cars a day by 2026. With this growth, the road to and from Auckland requires a faster and more economical route to support it.

In addition, the safety factor is also a key consideration as several fatal crashes have occurred between Pūhoi and Warkworth in recent years, some of which were head-on collisions. Therefore, a separated motorway with a central median barrier and improved road design will greatly improve safety.

The DJV originally used Leapfrog in the tender phase of this project to create a 3D geological model of the route. The 3D geological model was imported into OpenRoads software to model motorway geometrics. Having the geometric design model simply incorporated with the 3D geological surfaces allowed the slope profiles and cut-and-fill quantities for different alignments to be compared quickly and easily. This allowed optimization of the geometric model to balance earthworks mass haul for the project to assess the most cost-effective alignment for the motorway.

The 18-kilometer extension of the motorway is a large and complex project where the road corridor cuts through steep hill country with numerous steep-sided valleys, which are often filled with soft alluvial sediments. The final design requires several significant road cuttings and embankments to be created, with more than 7 million cubic meters of earth to be cut and 5 million cubic meters 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.

Understanding the material makeup of the mass-haul balance on this type of project is crucial because earth extracted from one part of the construction can be used to fill in another part of the site if it is of sufficient quality. The aim is not only to reduce the amount of wasted material but to understand that material’s composition and therefore how it can be used appropriately, 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 162 hectares of vegetation are to be cleared and then a significant tree planting program is taking place. So, minimizing the cut-and-fill footprint is important to ensure the minimal number of trees are removed to reduce the impact to the surrounding environment.

The response

The DJV turned to Seequent’s Leapfrog Works as its modeling tool of choice. Leapfrog Works is an implicit 3D geological modeling solution that enabled the DJV to more accurately define the geometry of the cuts.

“Leapfrog really helped us on what has been a significant and challenging project,” said Stuart Cartwright, senior engineering geologist, Tonkin & Taylor. “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.”

Cut and fill slopes vary according to the geology.

Detailed design started in October 2016 and is ongoing. The team started collecting ground investigation data and used this to input into the model. Chris Monk, engineering geologist, Tonkin & Taylor said, “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. So, it was important we could use a modeling 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,” Monk said. “We modeled 210 cone penetration tests 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 recreate a new model every time, leaving us more time to focus on the analysis.”

The outcome

The DJV has been able to produce more accurate 3D surfaces because of using Leapfrog Works. The more accurate the interpretation of the geological model, the better the outcome of the design. The team was able to better highlight to the other project staff the risks and uncertainties around the model.

Leapfrog Works has transformed the way that the geotechnical team has worked. This solution has meant that the geological surfaces have been able to be mapped by a geologist, rather than engaging a CAD technician to work alongside a geologist, so it has been a smoother process.

As project engineers needed sections, they were able to come straight to a single point of contact to quickly create the desired section, rather than have to draw something, then request a CAD technician to create this afterwards. This has saved time and reduced the effort in having to reproduce work.

The geotechnical team was able to leverage the great visualization of Leapfrog Works to bring together and better communicate across such a wide range of project stakeholders, including the CJV, quantity surveyors, surveyors and geotechnical engineers, and bridge designers.

“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,” Cartwright said. “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.”

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 this industry to become more responsive in an increasingly digital world.

Information provided by Seequent (www.seequent.com), developer of data visualization software and collaborative technologies.