Background

RoadTek, facilitating the works program, required a considered solution to assess the safety of the busy hinterland roadway. The road winds above the rainforest of the Mount Tamborine National Park, connecting a popular tourist resort and providing the local community to necessary amenities, such as a local school.

In 2022, more than 1200mm of rain fell on parts of southeast Queensland between January and March. This caused widespread urban flooding and local landslips in mountainous areas, with a combined estimated damage bill of $7.7 billion.

Local road engineers were aware that the downpours had triggered deformation and cracks in Tamborine Mountain Road’s surface but were unable to determine the detailed slope movement. Urgent analysis was required to determine the stability of the road; one traversed by locals and a school bus twice a day. Understanding the nature of the movement was crucial so a geotechnical design could be made to stabilise the slope with sufficient ground support, thus optimising the cost of ground support.

Geobotica’s GeoLidar was brought in as a cost-effective solution to monitor the slope.

An independent geotechnical engineer working with RoadTek confirmed the deformation in the road was due to geotechnical forces, and not from wear and tear. This meant the department could seek financial assistance for repairs under the jointly funded Commonwealth-State Disaster Recovery Funding Arrangements. 

Outcome

From the very first scan, the geometry and slope angles of the site were revealed, showing the natural 3D formation of the slope and its movement. The data’s sub-millimetre precision quickly captured the state of the road, including that of the tension crack zone.

The crack zone was approximately 60m and it was determined that a seven-metre portion was moving at a much greater rate than the remainder of the road.

It also revealed that a zone with a natural buttress of soil, on the widest part of the shoulder, was approaching stability. The area that was moving did not have additional material on the shoulder and downslope to hold it in place.

The tension crack in this seven-metre zone was pulled further apart, and the ground dropped at a total of 14mm in the most-affected location. The trend of the data was very important and showed that despite the large movement observed, it was unlikely to pose immediate risk of collapsing.

The rate of acceleration was inputted into the well-known inverse velocity collapse time forecast algorithm developed in the 1980s by Fukuzono. This determined that a collapse was unlikely to occur, and if the current trend continued without an external event, such as more rain, the addition of more groundwater or another external influence, it would be anticipated the earliest a collapse could occur, may take nine to 12 months. Likewise, it may also decelerate and find a new stable equilibrium in that time too.

The rest of the area was shown to be deforming at the slow rate of only 0.5mm per week, in a trend generally accepted as ‘background creep’ on the slope. This meant any repair of that area was not only less urgent, but required less work, time, and cost to support that section of the road.