The Use of Micropiles in an A-Frame Arrangement to Stabilize Dalrymple Road on the Eungella Range, Mackay, Queensland Australia

This paper details the investigation, design, construction, and monitoring processes performed in the employment of a micropile solution to restore a failed section of Mackay Regional Council’s Dalrymple Road, located near the township of Eungella. The road asset was restored using micropile in an A-Frame arrangement. The restored site comprised a 140 m long section of Dalrymple Road where unstable upper soils of colluvial origin were encountered at depths of up to 7 m within the road corridor restoration extents. The road had collapsed in late January/early February of 2019 following the area experiencing a week of inclement weather with rainfalls in the order of 970 mm. Queensland Transport and Main Roads (TMR) have been historically cautious in the use of small diameter piles (micropiles) as a piled support option for managing stability issues associated with their road assets. Caution is advisable where deep seated failures are involved, as micropiles may not be well suited to mobilize the large resisting forces required to ensure stability, or where there is potential for loss of material downslope on the passive side of a micropile remediated site, in which case the effectiveness of a micropile solution is lessened due to their small diameter. Concerns regarding the use of micropiles in such situations are overcome through rigorous investigation, design, and construction processes. During construction, the site was instrumented for long-term performance monitoring purposes. Monitoring commenced in 2020 following the completion of construction works and is continuing annually. Monitoring carried out following about a 1000 mm of rainfall in the week of January 2023 indicated a total cumulative lateral displacement in the order of 10–15 mm.