Soil Nailing in Dublin Boulder Clay

Extensive areas of Dublin City are underlain by a very stiff to hard lodgement till, known locally as Dublin Boulder Clay. Over the past number of years there has been increasing use of soil nails mostly to support temporary slopes for excavations, road widening and cut and cover tunnels. Although guidance for nailing in stiff clays exists, the very high strength and stiffness of the Dublin soils mean that it is conservative to use these established techniques and further optimisation may be possible. This paper presents data from the Dublin Port Tunnel project for a large set of short term pullout tests carried out on nails from a slope some 12 m in height together with slope monitoring data. The significant influence of drilling technique is discussed. Optimisation of temporary soil nails in Dublin Boulder Clay is appraised. The relationship between bond strength and actual nail performance is discussed drawing on data from a heavily monitored slope. Measured data on the vertical and horizontal movements of the slope together with data from piezometers confirm the excellent behaviour of the soil nailed slopes.