2016 – Investigation of the Foundation, Sub-surface Drainage and Slab Anchor Degradation of a Concrete-lined Spillway: Fairbairn Dam, Queensland

Peter Simson, Deryk Foster

Fairbairn Dam is an earth and rockfill embankment dam with an ungated, concrete-lined, spillway, located at AMTD 685.6 km on the Nogoa River, approximately 16 km south of Emerald in Central Queensland.

Following the flood of record in 2011 it was decided to repair a number of areas of spalling concrete which uncovered a collapsed transverse drain and a large void beneath the chute floor. The spillway chute is designed with subsurface drainage system of floor slabs consisting of alternate strips of concrete footing and gravel bed to aid in the control of uplift. The gravel was flushed from under the spillway floor into collapsed earthenware pipes of the drainage system resulting in an unsupported floor slab. Further investigation was carried out using Ground Penetrating Radar (GPR) which identified additional locations of possible voids. Concrete coring was carried out at selected locations to confirm the voids with some being over 250 mm in depth.

Investigation of the sub-surface drains was carried out using CCTV and showed many of the open jointed earthenware collector pipes had cracked and/or collapsed causing the drainage gravel and founding sedimentary rock to be scoured out by spillway flows entering the system through open contraction joints.
Following the discovery of the foundation scouring it was decided to expose a number of anchor bars in the chute floor to undertake a pull-out testing program. Of the ten anchor bars that were exposed, six were found to have corroded completely with the remaining four noted to be partially corroded and subsequently failed under loading.

A geotechnical investigation of the foundation materials was planned to determine the condition and strength of the founding sedimentary rock. In addition, the investigation also included sampling of seepage and reservoir waters to characterise the hydro-geochemistry and its contribution to the deterioration of the anchors.
Artesian conditions also occur within the spillway area, driven by the reservoir, with water passing through an extensive network of pervasive defects in addition to permeable flat-lying strata.

Coal seam gas is also known to occur, providing a further contribution to aggressive water geochemistry.