Calibration studies for landslide quantitative risk assessment

Executive Summary

Geotechnical practitioners in the past have relied heavily on qualitative judgement to assess whether the stability of a slope is adequate to allow development. complemented by the use of numerical techniques such as the factor of safety concept. Quantitative risk assessment (QRA) techniques that have been applied to areas such as the nuclear and hazardous waste industries and dam safety can also be used to quantify the uncertainty associated with the stability of slopes. These techniques use both numerical methods and subjective judgement to express numerically the risk attached to any particular slope. Although procedures are well established. the use of QRA to evaluate slope stability risk is very much an emerging concept overseas and still in its infancy in New Zealand.

One of the recommendations arising out of a previous research study for the Building Research Research Association of NZ (BRANZ) was that a series of case studies should be carried out to assess the usefulness of QRA techniques, "with emphasis on the probabilistic characterisation of landslide hazard and quantification of the risk". The principle objective of this publication is to present the use of QRA in some New Zealand slope stability case studies to highlight its use for evaluating risk acceptability, and to describe its advantages and limitations.

The basic framework and terminology for slope stability QRA are reviewed.

Four case studies were selected; one each in Tauranga and Queenstown and two in Nelson, The Tauranga study involved a rural residential subdivision where, following a period of prolonged rainfall a landslide was activated on an upslope property. The failure resulted in destruction of a detached garage and minor damage to a dwelling. Subsequently. civil proceedings against the territorial authority, the land developers and their engineering consultants were initiated on the basis that the land was of inadequate long-term stability. Due to these proceedings a large amount of information about ground conditions was gathered, which has proved to be suitable for use in QRA. Rainfall records were also evaluated. The QRA analysis showed that there was a high probability of a landslide and that the risk of death to an occupant of a dwelling in the subdivision was unacceptably high, and that also the risk of damage to buildings would be thought unacceptable.

The Queenstown example involved a substantial slope failure during excavation of building platforms for townhouses on a sloping site. The landslide affected much of the site and encroached onto two adjoining properties. Study of the schist rock showed it to contain defects (foliation shears and joints) that were predisposed to cause failure. There was a commercial risk as the site is no longer suitable for residential development and a potential health and safety risk to site workers. QRA showed that while the risk may have been acceptable in commercial terms, it would have highlighted the risk to the developer prior to landsliding, allowing preventative measures to have been carried out.
At a site in Nelson heavy rainfall triggered a landslide from a natural slope that narrowly missed a house. Subsequent site investigations showed the slope was unstable and the house was removed to prevent possible total loss. Several older landslips were evident on the slope above the site. QRA involving rainfall event return periods and review of existing ground conditions showed that the risk of death to an occupant of the building was unacceptably high, and that the risk of damage to the building would also have been considered too high.

Another site in Nelson was also affected by the same rainfall event, which caused a cut slope to fail and debris impacted on the rear of the house. Site investigations found that a clay layer within gravels, exposed by cutting of the slope, had acted as an aquitard, and formed a sliding surface. QRA showed that removing the toe of the natural slope considerably increased the risk of landsliding, and therefore the risk to the occupants and damage to the building. Lastly the study looked into whether QRA could be a useful tool in assessing "imminent loss" claims, The Earthquake Commission Act (1993) allows the Commission to consider whether a property is at risk of "imminent loss" from landsliding. and to fund remedial works, rather than to wait until damage occurs. In order to assess such claims a geotechnical practitioner has to try to predict the timing of a future landslip event.

While it is not possible to predict the exact timing of a landslip event. it was found that ORA provided a useful framework to better understand the vulnerability of a site to landslip damage as well as the risks and consequences involved in failure. QRA can also be used to assess the expected costs of remedial works versus the cost of repairing damage at some future date.