Deep soil mixing for seismic liquefaction pretreatment, Napier Port, NZ

The Cutter Soil Mixing (CSM) method has been used for ground improvement to provide seismic resilience in multiple infrastructure projects in NZ since 2019. The CSM equipment was developed around 20 years ago using rectangular panels to form insitu continuous walling, to replace the traditional methods using rotary drilling rigs that use overlapping secant piles or mixed columns to form walls. The CSM’s mechanical cutters provide construction flexibility through variable soils, with down hole sensors, and onboard computer system, to record key performance indicators. Extensive testing has been undertaken over multiple projects in NZ aimed at gaining a better understanding of the properties of the insitu mixed walls, leading to future optimization of designs, specifications, and construction, to deliver a more efficient and economical outcome. Lowering the cost base of the works can commonly be achieved by lowering cement content, minimizing waste and improving productivity. This paper sets out the design and testing processes for the upgrade of Napier Port, in a zone of significant seismic risk, and with underlying recent alluvial deposits causing a high risk of liquefaction requiring extensive ground improvement. Soil mixing was the best approach with fill and alluvial layers varying in particle size including silt, sand and gravel, overlying siltstone. The depths of pre-treatment ranged from 15m to 17m deep, using lattice walls in a closed cell system. The lattices provided approximately a 20-25% area replacement ratio. Equipment used comprised a Cutter Soil Mixing (CSM) rig for deeper mixing to 17m depth, with a batch plant supplying slurry to the rig. This paper details a practical solution to liquefaction risk and management of the wide range of variables effecting the in-situ mixing, the sampling and testing difficulties, and analysis of results. Collectively these factors all influence QA/QC outcomes, and we investigate how to provide sufficient ground improvement, without over delivering on the various quality metrics.