THE USE OF LIVE WILLOW POLES FOR STABILISING HIGHWAY SLOPES

Shallow slope failures are a widespread and costly maintenance problem which affects highway earthworks, particularly slopes in overconsolidated clays. Typically, stabilisation of highway slopes may be undertaken using a variety of proven hard engineering approaches. An alternative technique makes use of live willow poles driven into the slope. The poles provide an immediate reinforcing action and subsequently grow to provide the longer term benefits associated with established trees. This report describes the successful installation and establishment of 900 live willow poles on four highway slopes in the UK. Installation, of poles, took place in 2000 and 2001 and results from the trial sites indicate that by the summer of 2003 approximately 91% of the installed poles had survived. Exhumation of selected poles at the trial sites and the use of a mini-rhizotron, identified the presence of roots at depths of 1.5 to 2.0m. These findings indicate that root growth will be acting to increase the benefit in terms of mechanical stabilisation provided by the poles themselves. As such the results of these trials have illustrated the potential of the technique as a viable preventative slope stabilisation measure. Guidance on the use of the technique is also provided in the form of a draft Advice Note. This note gives further details on species selection, design, installation and maintenance issues associated with the technique. (A)