Protecting critical infrastructure from the ground up

How can you make an earthquake-prone structure more resilient? This was the question posed recently by operators of a waste water treatment plant in Wellington, New Zealand, as they sought proactive measures to protect the asset following the introduction of The Building (Earthquake-prone Buildings) Amendment Act 2016 (EPBA) on 1 July 2017.

Wellington lies along an active seismic fault with the area recording approximately 31,300 earthquakes since 2017 alone[1]. While the 2010 and 2011 Canterbury earthquakes were not New Zealand’s largest seismic events, they caused the most damage and significant loss of life, leading to a renewed focus on earthquake engineering and the strengthening of structures.

Waste water treatment plants are categorised as Importance Level 3 of 4 (IL3), meaning that they must remain operational post a seismic event. Wellington’s water infrastructure facilities cater to around 146,000 residents and a large number of industries. The plants must be built to the highest engineered standards to ensure they are able to withstand seismic conditions and prevent wastewater from escaping into the ocean.

The decision by the asset operators to undertake seismic strengthening through ground remediation is a smart move. While there has been growing awareness about the interaction between building structures and foundation ground during an earthquake, there is only brief consideration for Soil-Foundation-Structure Interaction (SFSI) in current New Zealand guidelines. This is despite much of the structural damage that was experienced in the aftermath of the Canterbury sequence being due to poor performance of the ground rather than the strength of the buildings.

Following the Canterbury earthquakes, Mainmark’s ground improvement research trials in the Christchurch Red Zone showed that new resin injection techniques can demonstrably improve the density and stiffness of earthquake affected ground and increase the resistance of soils to liquefaction. This has led Mainmark to release Terefirm® Resin Injection, a proven, non-invasive, ground improvement and liquefaction mitigation technique that can be easily applied beneath existing structures.

Validated by geotechnical testing, the Terefirm™ Resin Injection method is now included in New Zealand’s Ministry of Business, Innovation and Employment (MBIE) Module 5: Ground Improvement of Soils Prone to Liquefaction. This follows the internationally peer reviewed research conducted in partnership with the Earthquake Commission (EQC) and the MBIE, with the full research report now available on the New Zealand Geotechnical Society online library.

Terefirm® Resin Injectionis applied with surgical precision in a non-invasive process to densify the soil and increase liquefaction resistance. Geotechnical field testing is undertaken prior and post application to validate the ground improvement performance outcome.  Mainmark tailors the testing in line with the project requirements and the engineer’s specification.

During injection of the treatment zone, the low viscosity resin both permeates the soil to a limited extent, and also penetrates under pressure along planes of weaknesses within the soil profile. The material reacts soon after injection, rapidly expanding to many times its original volume. The expansion of the injected material results in compaction of the adjacent soils, due to new material being introduced into a relatively constant soil volume and thereby better protects structures from damage as a result of liquefaction in future earthquakes.

While the waste water treatment plant is the first asset of its kind to undergo proactive ground remediation using Terefirm® Resin Injection, this solution would be highly beneficial for many other critical infrastructure sites located in New Zealand’s high risk seismic regions.

[1] https://www.theguardian.com/cities/2017/dec/11/earthquake-proof-city-christchurch-japan-colombia-ecuador