1. Introduction

This overview outlines the geotechnical slope risk assessment for the large landslips which occurred on Mount Spec Road during the extreme rainfall event of February 2025. This information was utilised by the Department of Transport and Main Roads' (TMR) Northern District in planning to reinstate and manage traffic on the road following the event.

The large upslope landslip at chainages 10.88 km and 14.3 km have residual hazards of significant volumes of unstable ground and boulders present. These hazards were not feasibly able to be treated during the disaster's emergency work period. Of the many landslips which occurred on Mount Spec Road during the extreme rainfall event, these slides were identified as the most critical with respect to risk mitigation requirements for public safety.

It is noted that there has previously been a landslide related fatality along Mount Spec Road in 1997.

2. Overview of the Risk Assessment

Due to the scale of damage at these locations, a site-specific analysis using the Australian Geomechanics Society (AGS) 2007 method was used to assess the magnitude of risk to road users.

The risk assessment concluded that precautionary measures are necessary to manage risks effectively. This includes reducing the traffic volume from the current annual average daily traffic of 149 vehicles per day (VPD). These measures are aimed at lowering the risks to a level considered tolerable for the ongoing design and reconstruction period for the slope restoration works.

The risk analysis determined that, with mitigation measures in place, such as barriers at the base of the slipped road segment and enacting complete road closures during heavy rainfall, a daily traffic limit of 70 VPD would achieve the desired level of risk reduction. This limit accounts for potential variations in daily traffic, including return trips, and allows for occasional spikes of up to 85 VPD. However, if traffic exceeds 85 VPD, the risk level increases and falls into the high-risk category for road users.

3. Concluding remarks

The risk assessment and nominated mitigation measures were designed to support TMR in making decisions and planning for the reinstatement of public vehicular access along Mount Spec Road until the necessary slope restoration works can be carried out.

Further impacts from weather events (e.g. rainfall, drainage, erosion), existing site conditions, effectiveness of mitigation measures and the progress of restoration works may influence future risk assessments and potential risk management strategies for Mount Spec Road.