Soil nails and rock bolts: construction challenges

Soil nails and rock bolts are a cost effective method for long term stabilisation of slopes or near vertical excavations where staged, top-down construction is advantageous compared to other traditional methods of slope retention. This method of ground support can provide near-instant active or passive support against ground instability. This paper presents the challenges encountered on a recent Melbourne infrastructure project during the installation of soil nails and rock bolts, and the solutions adopted to overcome them. In these case studies, passive soil nails and post-tensioned rock bolts were installed in residual clay soils, weathered siltstone rock, and basalt rock across two sites in Melbourne as part of a transport infrastructure project. The challenges encountered throughout the construction process, in particular the variability of ground conditions and construction timeframes shows the need for supervising engineers to be flexible in the development and implementation of the designs. Where designs have been modified additional checks should be completed to ensure the design intent is achieved. Sacrificial testing required as part of the design was not undertaken to failure of the grout to soil bond so the reported results only provide an indication of the mobilised bond stress. Occasionally this was because the test load required to cause failure would exceed the tendon yield stress, however typically the nominated test loads were not high enough for pull out failure. Whilst the testing confirms adequate bond stress for project-specific requirements, the results may not provide information on the ultimate geotechnical performance of the materials being tested.