Stabilisation of a fill embankment using soil nails

Drilled and grouted soil nails have been successfully used to stabilise a section of fill embankment on the Warrego Highway near Toowoomba, Queensland that failed as a result of heavy rainfall in January 2011. Instability occurred due to groundwater rise within the fill materials resulting in a tension crack developing at the traffic lane edge and an approximate 300 mm displacement within the outer portion of the embankment. Reinforcement of the unstable fill slope with soil nails was the selected remediation method. The soil nails were installed through the existing granular fill materials and grouted into the fill and underlying basalt. Soil nail holes were drilled using equipment fitted to excavators to typical lengths of 10 m to 12 m with temporary casing used to prevent hole collapse during drilling. A reinforced shotcrete facing was constructed over the face of the embankment to provide the necessary nail head restraint and prevent erosion. Soil nails were installed in a prescribed sequence to manage the risk of construction plant trafficking the marginally stable fill embankment. Full time monitoring of construction was carried out in accordance with an action plan developed specifically for the site. The coordinated approach to the design and construction of the works resulted in a successful implementation of the remedial works.