Joining industry peers at the 14th Australia and New Zealand Conference on Geomechanics, Cairns 2023 (ANZ2023), Tetra Tech Coffey specialists Raathiv Shanmuganathan, Associate Engineer and Ching Dai, Senior Principal, presented their co-authored paper: ‘Design and construction of a contiguous pile wall at an existing soil nail slope founded on soils susceptible to liquefaction’.
Here Raathiv shares insight into the paper, sharing insights in the technical challenges in delivering The Waikato Expressway (Hamilton Section).
Tell us Raathiv about the project
The Waikato Expressway is one of New Zealand Transport Agency’s (NZTA) seven roads of national significance. The 102km four-lane highway reduces travel times between Auckland and Tirau by upwards of 35 minutes, and as each section has opened it has significantly reduced the number of fatal and serious injury crashes. These roads are aimed at unlocking economic potential through the increased movement of people and freight between the country’s largest population centres.
Tetra Tech Coffey were part of the City Edge Alliance (CEA) consortia comprising Tetra Tech Coffey, Fletcher Construction, Higgins Contractors, Beca, and Waka Kotahi NZ Transport Agency, with Hick Bros as a sub-alliance for the earthworks.
The design and construction challenge
As part of the project, the CEA was required to design and construct a retaining wall at an existing soil nail slope location that needed to cut through the existing soil nails.
The retained slope was up to 14 m high with more than 6 m vertical cut height. The geology consists of loose granular materials which are susceptible to liquefaction. Planning and constructing the new steel soil nails between the existing fibreglass soil nails required accuracy and increased complexity of the work.
Unexpected, perched groundwater behind the wall reduced the holding capacity of four of the new soil nails at a localised area and created significant water flow through the drilled hole.
The technical paper outlined our design and construction of the contiguous pile wall against static and dynamic loads, the actions taken to strengthen the soil nails affected by the perched water table, and other construction difficulties faced during the construction phases.
What were the core challenges you had to address on the project?
The complex site on the Hamilton section of the project, where a contiguous pile wall was constructed at an existing soil nail slope. Steepened batter slopes were constructed to avoid encroaching beyond the expressway designation at the crest of the slopes. Here, we had to address two technical design challenges:
- Geotechnical design challenge 1 – liquefiable loose ground and seismic criteria for lateral spread.
- Geotechnical design challenge 2 – complexity of design and construction solutions.
Project outcomes
The seismic analysis identified that the ground layers were potentially liquefiable/susceptible to cyclic softening under design seismic condition. The results of the analysis indicated that the toe of the retaining wall should not terminate in the liquefiable zone, but rather extend below to the non-liquefiable ground. To suit the complexity of ground condition and construction, multiple combined support solutions were designed to match each case requirement. To suit the bridge abutment, CPW was designed, and the piles were driven in between the existing soil nails. To strengthen the liquefiable embankment toe, RD and CFA piles were designed.
A trial-and-error back-calculation concept was successfully adopted to re-design the soil nails in the disturbed (saturated) zone. The combined numerical analyses were undertaken to assess the stability and formulate deformation predictions. Instrumentation points along the cut slope and the designed walls recorded deformations within the expected magnitudes at the time of writing, thereby confirming the observed stability and successful performance both during and post construction.
Leading with Science® – a collaborative approach
A number of Tetra Tech Coffey staff assisted on this project, contributing to the successful completion of the work. It was great to be able to collaborate and consult with my colleagues throughout the project.
For more information in relation to this technical paper, contact the authors.
Connect with Raathiv Shanmuganathan Associate Tetra Tech Coffey, Auckland New Zealand, s.raathiv@tetratech.com
Connect with Ching Dai Senior Principal, Tetra Tech Coffey, Sydney Australia, ching.dai@tetratech.com
Note from the authors: It is also acknowledged that all Alliance partners, including Fletcher, Brian Perry Civil and Beca, supported the detailed design and construction. The authors also wish to thank the New Zealand Transport Agency for their permission to publish this paper.