Client name: Queensland Department of Transport and Main Roads

Duration: 2013 - 2014

Location: Toowoomba, Queensland, Australia

Forward planning, detailed risk analysis and close collaboration on this complex project meant Coffey was able to assist the department in reducing rock fall or landslide.     

Highway passes through geologically complex area with known landslides and rockfalls 

The Warrego Highway climbs the Great Dividing Range as it enters Toowoomba. With complex geology, several high and steep embankments and cuttings, the area has had a history of regular small to large scale landslides. This has caused traffic risks and has often closed the Highway for periods of time. 

There were several steep cut slopes (60º) up to 25m high in variable geological conditions, including multiple lava flows with intermediate lower strength ashfall layers.

Without intervention, ongoing rockfall and instability was expected, creating a risk to vehicle traffic. Constant, ongoing maintenance was required to manage the associated risk and asset.

The 1200m long realignment of the highwayabove Redwood Park includes an embankment up to 30m high and crosses over the main drainage gully for the site. For this area, the drainage capacity needed to be upgraded, whilst managing concerns over the stability of the highway embankment.

The stability of the embankments across gully areas was marginal and work was needed to reduce the risk of landslide.  

The standard approach of widening the road or excavating and placing stabilised fill was not going to work due to a lack of space and narrow road geometry.

Risk based approach, field mapping and geological ground model 

Coffey worked with the client to develop several design options for consideration. The final design used was developed using a risk based approach, which considered both the likelihood and consequences of rockfall and landslides to users of the highway.

Whilst in the last stages of construction, the expectation is that the occurrence of rockfall or landslide will decrease considerably, reducing both the risk to road users and the associated maintenance costs.

Our team undertook detailed field mapping to develop a geological ground model and assessment of the subsurface drainage. Stability analyses showed that for the embankment and natural surface geometry, layers of geogrid were required to enhance the embankment’s stability. 

We worked with the client and drainage designer to provide geotechnical design solutions for a 6m wide x 80m long drainage batter chute anchored to the slope to enhance the drainage capacity. This required temporary slope stability assessments and on site involvement for the earthworks portion in order to ensure construction did not further damage the embankment.

Our team developed a solution that placed a buttress of fill against lower embankment on one section. An A-frame Ischebeck micropile design was developed to further enhance the stability on the other sections. Our solution, while reducing the risk of landslide, also had minimal impact on the highway during construction – meaning less inconvenience to traffic.

Project met all the nominated risk criteria

Coffey’s collaboration with the client and its construction contractor in a practical risk based approach has meant this project has met all its desired outcomes. Construction was performed with minimal disruption to traffic and within budget. Most importantly it has reduced the risk to traffic from rockfall or landslide to meet the nominated risk criteria for the project.