Chaffey Dam, Australia: Reinforced Earth® Walls to Double Capacity, Increase Safety Standards

Chaffey Dam is located 43 km south-east of Tamworth on the Peel River, NSW, Australia. The dam provides water for irrigated agriculture, industry, town water supplies, river flows and domestic requirements, flood mitigation and environmental flows.

Prior to the upgrade the dam could be described as a 54-m-high, 430-m-long clay cored rockfill embankment with a combined morning glory spillway and outlet works.

The objectives of the Chaffey Dam Safety Upgrade and Augmentation Project was to increase the dam’s storage capacity from 62 GL to 100 GL and to upgrade the dam to comply with ANCOLD and NSW Dam Safety Committee (DSC) safety standards regarding flood mitigation and stability of the structure.

Stage 1 of the project was the construction of a 35-m auxiliary spillway with release plug which was completed in 2011.

Stage 2 and augmentation involved raising the dam wall by 8 m and raising the morning glory spillway bell mouth crest by 6.5 m.

John Holland Pty Ltd (JH) was appointed as the Principal Contractor of Stage 2 by WaterNSW, and URS Australia Pty Ltd (now AECOM) designed the dam raising, The Reinforced Earth Company Australia (RECO) was subcontracted by JH to design and supply the Reinforced Earth® structures and Fusion Civil Pty Ltd installed the walls.

The scope of works for the upgrade included: Firstly, the installation of a vertical Reinforced Earth® wall structure with an attached 1.8 m parapet wall on the crest of the existing earth and rockfill embankment to raise the dam wall by 8 m; and secondly to raise the water level by 6.5 m by duplicating the existing bell mouth crest of the morning glory spillway structure at a higher level.

RECO was contracted to design and supply 6950 m² of Reinforced Earth® back-to-back walls. In addition, on the top of the upstream wall is a 1.8-m-high parapet wall and at the base is an L-shaped wall with a handrail. This L-shaped walkway creates walkway access along the upstream of the wall.

RECO also designed and supplied a temporary Reinforced Earth® wire wall, which created a cranage platform from which the morning glory spillway bell mouth crest was raised.

The design engineers were required to assess stresses and deformations under flood surcharge loading conditions, evaluate earthquake response of embankment and vertical crest raise and evaluate stresses at the base of vertical crest raise for assessment of the bearing capacity for load cases (static, flood, earthquake loading cases).