3D Rockfall Modeling Exercise: Christchurch's Post-Earthquake Recovery

Following the catastrophic earthquakes that affected Christchurch, New Zealand in 2010 and 2011, over 16 miles of slopes were affected, many of which followed or sat above the two primary lifeline routes in and out of the city. Collaborating with Urban Search and Rescue, Geovert led a mammoth effort to secure these lifeline routes, while the entire country of New Zealand was under a declared state of national emergency.

Following this effort, a large-scale rockfall modeling project was initiated. This project, conducted by Geovert for the New Zealand Government with our partners Freefall and international experts from the University of Milan, was the largest and most complex 3D rockfall modeling exercise undertaken globally at that time.

Project Overview

The Canterbury Earthquake Recovery Authority (CERA), a New Zealand Government Agency, engaged Geovert to develop a comprehensive solution for assessing and mitigating rockfall risks in the earthquake-affected areas. The primary objective was to comprehensively scale rockfall risk to inform the Christchurch Port Hills rebuild process of best practice solutions including time and cost to implement, implement and therefore assist in decision-making regarding Red Zoning and Section 124 Notice determinations.

Rockfall Modeling Methodology

Geovert employed both 3D and 2D modeling techniques to provide a thorough understanding of the rockfall risks. The 3D modeling presented the lateral extent of the issue and helped define areas exposed to risk, while the 2D analysis calculated energies and impact velocities to determine the level of risk in identified areas.

In the 2D analysis 16 ¼ miles of length were analyzed, with on average, 15 to 20 cross-sections per mile, with variations depending on the risk presented at individual locations. To ensure high-level expertise, Geovert collaborated closely with our partners Mr Hannes Salzmann from Freefall AG (Austria) and Professor Giovani Crosta from the University of Milan (Italy). These specialists assisted in conducting the large-scale rockfall modeling using the Hy-Stone program, which was not commercially available at the time.

Key Components of the Analysis

The project encompassed a wide range of activities, including inputting extensive field work, data collection from the major consultancies, and analysis. The team analyzed historical and earthquake-induced rockfall inventories, as well as topographical, geological, and land use data. They prepared a database for buildings and population distribution and attributed rockfall onset susceptibility based on available geotechnical studies.

The modeling process involved calibrating the 3D Rockfall Model for each subarea and conducting modelling for different rockfall scenarios, considering various block sizes and shapes. The team also implemented and analyzed mitigation structures in both 2D and 3D models.

Outcomes and Significance

The results of this extensive modelling exercise were instrumental in guiding the Christchurch Port Hills rebuild process. They informed Red Zoning decisions and Section 124 Notice determinations. Additionally, the project provided a cost-benefit analysis of rockfall protection solutions and developed detailed design-build solutions for rockfall protection systems.

Implications and Future Applications

The Christchurch rockfall modelling project represented a significant advancement in rockfall risk assessment and mitigation strategies. By combining advanced large-scale 3D modeling techniques to identify areas to focus 2D modeling on, in collaboration with international expertise, this project set a new standard for large-scale geohazard analysis.

The methodologies developed and insights gained from this project have potential applications beyond Christchurch. They offer valuable tools for urban planning and safety measures in other earthquake-prone regions, demonstrating the important role of advanced geotechnical engineering in disaster recovery and prevention.

Ultimately, this project showcases how technology and expertise can be effectively combined to address complex environmental challenges, providing a model for future large-scale geohazard assessments worldwide.