Lattice-Shaped Jet Grouting Configuration to Prevent Soil Induced Liquefaction at Christchurch Town Hall

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The study evaluates lattice-shaped jet-grout columns as a means of reducing earthquake-induced liquefaction and ground movement beneath Christchurch Town Hall, which suffered substantial damage during the 2010–2011 Canterbury earthquakes. Post-earthquake surveys recorded foundation settlements of approximately 240–630 mm, alongside evidence of liquefaction and lateral spreading.<br /><br />The proposed treatment uses overlapping jet-grout columns to create a stiff cellular block that limits soil shear strains and deformation. The authors modelled the site using PLAXIS 2D effective-stress finite-element analyses, including the UBC3D-PLM cyclic soil model. A recorded Christchurch earthquake motion was scaled to represent a design event with magnitude 7.5 and peak ground acceleration of 0.44g. Model predictions were compared with CPT-based liquefaction assessments.<br /><br />Four layouts were assessed: cells with clear spacings of 8 m, 6 m, and 4 m, and 7 m cells with an additional 2.5 m central column. The 8 m and 6 m layouts were ineffective, with predicted excess pore-pressure ratios indicating that liquefaction could be triggered. The 4 m layout and the 7 m layout with a central column performed better, generally keeping ratios in much of the susceptible layer between 0.5 and 0.7. These configurations reduce liquefaction risk, though some settlement may still occur. A supplementary model found that a low-permeability layer beneath the treated block could further improve performance, producing pore-pressure ratios around 0.5 or lower.<br /><br />The treatment was intended to stabilize the building footprint, not prevent movement of the nearby riverbank. The authors conclude that lattice jet grouting can improve seismic soil response, but its effectiveness depends on cell geometry and subsurface permeability.

Keywords

lattice jet grouting

liquefaction mitigation

Christchurch Town Hall

Canterbury earthquakes

jet-grout columns

ground settlement

effective-stress analysis

PLAXIS 2D

pore pressure ratio

seismic soil improvement