Liquefaction Mitigation for Critical Water Treatment Infrastructure

A 2017 geotechnical assessment identified liquefaction risks at the site, and further analysis in 2018 determined resin injection to be preferred option for mitigating the effects of ground liquefaction.

The existing structure included concrete floor slabs, concrete beam construction, concrete strip footings and basement areas beneath portions of the building. The key challenge was to improve the liquefaction resistance of soils beneath the reservoir portion of the plant without causing unacceptable movement to the structure, surrounding services or adjacent assets.

The trial also had to account for complex site constraints, including the proximity of the rail corridor, basement walls, internal pipe structures and strict heave tolerances. For the South Trial Pad, heave monitoring was particularly stringent, with 2 mm cut-off limits applied to pipe plinths and the rail corridor kurb, and a 5 mm cut-off limit applied to the basement wall.

Mainmark undertook resin injection trial works across two 60 m² trial areas to assess the suitability of the methodology in the site-specific soil conditions. The North Trial Pad was located in the asphalted car park, while the South Trial Pad was positioned partially beneath the structure. Resin injection was performed to a depth of up to 10 m, with pre- and post-injection ground investigations used to measure the extent of improvement achieved.

The proposed full-scale methodology used Teretek® Resin Injection, a targeted ground improvement technology delivered through small 25 mm penetrations in the structure’s footing and floor slab system. The process uses proprietary segmented injection tubes, hand-held equipment and a self-contained material production plant, allowing works to be carried out with minimal disruption and reduced need for excavation, soil removal or disposal.

During the trial, the methodology was refined in response to actual site behaviour. The North Trial Pad was initially injected from 2 m to 10 m below ground level, while learnings from this area informed the South Trial Pad design, where injection was targeted between 6 m and 10 m to focus on liquefiable sandy layers and better control heave. Angled injections were also introduced to reach obstructed areas around walls, foundations and permanent internal features. Surveying and heave monitoring formed a critical part of the methodology. Mainmark used laser monitoring, automatic levels, survey pins and scale tapes to track movement before, during, and after injection. An external surveyor was also engaged to provide peer review of survey measurements during the trial works.

The resin injection trial successfully demonstrated that Mainmark’s methodology could improve liquefaction
resistance in the site’s sandy soils while maintaining controlled movement. The trial showed a reduction
in liquefaction potential across the majority of critical layers, with post-improvement soil strengths in the
South Trial Pad assessed as being above typical liquefaction-triggering ranges for all considered design
cases.
In the North Trial Pad, resin injection achieved an average 23% improvement in Qc1Ncs values for target
soil behaviour types, with shear wave velocity improvements of up to 30% and stiffness improvements of
up to 68%. Expected vertical settlement was reduced by more than 40% for lower PGA values up to 0.2 g.
The South Trial Pad achieved an average 36% improvement in Qc1Ncs values for the targeted sandy
layers, with expected vertical settlement reductions of up to 36% for PGA values higher than 0.3 g. Heave
was successfully controlled, with a maximum of 6 mm recorded around the external perimeter of the pad
and only 3 mm within the adjacent basement area.
The trial provided valuable data to refine the injection design for the full-scale works, including how resin
quantities, injection depths, and soil behaviour types could be balanced to achieve target improvement
while managing heave. The findings supported a more informed detailed design process for improving the
seismic performance of the reservoir structures.