The Karuah Bypass: A case-study in the engineering geology of the Southern New England Fold Belt

Stephen Fityus, John Gibson, David Bax and Tim Rannard

The recent construction of the Karuah Bypass provided an excellent opportunity to improve the state of knowledge of a poorly documented geological sequence in the southern New England Fold Belt and to evaluate some of its engineering characteristics. The 2500 m thick, fault bounded, sequence of rocks transected by the Bypass has been previously consigned to the western Myall Block of the Tamworth Belt. This work has established that it spans an interval that begins with the Johnson’s Creek Conglomerate to the west, includes the McInness and Booral Formations and terminates with the Karuah Formation, which is probably truncated by a fault. These predominantly terrestrial formations are dominated by thickly bedded sandstones and conglomerates, with a variable tuffaceous component (mostly siliceous) and minor shales and rare coal seams. They contain several significant volcanic units, which despite having considerable thickness, were generally not encountered in excavations along the selected road alignment. Residual soils derived from these formations are almost exclusively clays, ranging broadly from low to high plasticity. Where encountered, rocks were mostly of high to very high strength, with some units retaining very high strengths in close proximity to the surface. Due to the presence of extensive localised and regional faulting, outcrop in some areas of the alignment was very poor and the depth of mottled, residual clay rock was considerable. Anomalous conditions encountered along the Bypass include pyrite-bearing dacitic volcanics with acid sulphate potential, layers of pedogenic silcrete that impeded pile driving, numerous deeply weathered basaltic dykes and bedding-parallel thrust faults that show small displacement.