Performance of soil nails in Dublin glacial till

Soil nailing is being used in many projects in glacial tills in Ireland, particularly to provide temporary support to steep slopes. Little design guidance is available for such materials and it is known that application of design procedures developed for other material is conservative. Detailed nail instrumentation and field monitoring during large scale soil nailing works for the Dublin Port tunnel project has been undertaken. It was found that the short-term behaviour of nails was the reverse of that assumed in current design methods. Most load was induced due to drilling and nailing the lift immediately below the nail being monitored, rather than due to excavation induced stress relief. The highest forces were developed in the upper nails, where the largest ground movements occur. This is the reverse of most current design methods where the highest soil-nail bond is assigned to the deepest nails. It would seem that the observed short-term, pre-failure behaviour of nailed slope is governed more by the deformation pattern of the slope rather than by large scale development of failed wedges. Current design procedures should be reviewed. Despite this the trial confirmed that the currently used procedures are highly conservative for Dublin glacial till.