Remediation of caves below Princes Highway West, Yambuk, VIC
This paper presents a hazard assessment, construction methodology, and investigation for remediating an extensive shallow cave system within a Pleistocene calcarenite geological unit that partially closed the A1 Princes Highway West in Yambuk, South-western Victoria. In January 2023, a road grader unintentionally punctured a 0.5 m thick roof slab of a previously unidentified shallow karst cave with a vertical cavity dimension of 3 m during road widening works on the A1 Princes Highway West. The cave posed a safety risk for the highway, which is a freight route, requiring an extensive sub-surface investigation to assess the extent of the cave and to develop a de-risking rehabilitation (or remediation) methodology. The investigations revealed a ‘domed’ fanning cave which formed part of a multilevel chamber system and an extensive network of corridors. Void volumes affecting the highway exceeded 3000 m3. Close inspection and collaboration with the contractors allowed for excavation of the cave roof, deemed necessary due to the collapse caused by the grader. A first principles approach was adopted to predict the consequence of cave subsidence and helped delineate the excavation area prior to implementing a ‘cave-plugging’ remedy. The remedy included the compaction of engineered fill. Additional cost-effective treatments adopted included compaction of pockets of the excavated calcarenite and infilling the caves with mass concrete through 300 mm diameter ‘entry’ cored holes to avoid large volumes of excavation. Drainage blankets and geotextile layers were used to minimise the impact on subsurface hydrogeology and prevent wash-out of the plug. The construction approach was considered cost-effective and lessened structural impacts to the surrounding cave structures.