The rock mass beneath the proposed Shannon Creek Dam was interpreted as valley bulging caused by unloading of the strata due to erosion of the valley with a number of defects, including bedding, joints, veins, and suspected shears/faults in the valley base. Grouting and Foundation Works Australia were appointed to carry out the foundation grouting. This required drilling of holes up to 65 metres deep and the mixing and placing of over 60,000 bags of GP cement.
Shannon Creek dam site is located 9km due west of Coutts Crossing (south-west of Grafton, NSW) and provides a storage of 30,000 megalitres at a cost of about $90 million. The dam consists of approximately 1,150,000 cubic metres of earth, clay and sand excavated from within the inundation area creating a storage area of approximately 210 hectares. The dam wall is approximately 47 metres high and 400 metres long.
The dam is founded in sedimentary rocks of Triassic – Jurassic age belonging to the Kangaroo Creek Sandstone and the Walloon Coal Measures, consisting of sandstone and interbedded sandstone and siltstone/claystone/shale, with minor conglomerate and coal. The rock mass contains a number of defects, including bedding, joints, veins, and suspected shears/faults in the valley base. In the geometric centre of the valley, the rock mass is interpreted to be affected by valley bulging caused by unloading of the strata due to erosion of the valley.
Grouting and Foundation Works Australia were appointed to carry out the foundation grouting. This required drilling of holes up to 65 metres deep and the mixing and placing of over 60,000 bags of GP cement. During the work the condition of the foundation required an increase in the grout curtain from a single line of grout holes to a maximum of seven parallel lines of holes including blanket grout holes and an upstream and downstream grout curtain in the central section. GFWAust provided additional resources to meet the requirements of this increased scope of works and maintain the construction program including three grout stations and up to three drill rigs.
