MHB recently carried out a soil nailing design for a site at Ballybracken in Northern Ireland.
An existing road cutting 4.5m high constructed a few years ago was found to be over steep (31 degrees) and was required to be strengthened to meet current design standards. Due to the site layout it was not possible to cut the slope back to a shallower angle, as this would have involved land purchase. Instead a design was developed using soil nails to increase the factor of safety of the slope.
Following analysis using Slope/ W software, 32mm diameter self drilling soil nails from Dywidag were adopted as these permitted installation in the mixed ground without the use of temporary casings. A grid of soil nails at spacings varying from 1.5 to 1.75m was adopted, with nail lengths from 4-5m long and galvanised for the last 2m closest to the ground surface to ensure durability over the 60 year design life.
The surface of the slope was faced with Macafferi rockfall netting to span between nail heads. Sub-horizontal slope drains were installed along the toe of the slope to reduce water pressures.
Our client was Albion Drilling who also installed the soil nails on behalf of Lagan Construction.
The works were successfully completed on site in July 2017.