Excavation and rockfall protection adjacent to live rail

Rock cuttings up to 12 m in height adjacent to a live railway needed to be widened without interruption to rail operations. This required the authors to develop an excavation methodology and design temporary rock fall protection structures. Photogrammetry was used to generate a digital terrain model, to obtain geological structural data, accurate slope geometry and rock mass characteristics. Boulder masses of 0.5 t and 3.0 t were considered. In consultation with the contractor, the authors developed initial rock fall protection structures and excavation methodologies. These were based on the results of the modelling and proposed excavation equipment. The slope geometry for one cut was modified in the modelling to reflect the geometry that would be ‘seen’ by a 3 tonne boulder. It was considered that such a large boulder would not be affected by batter roughness (i.e. ledges) smaller than approximately 0.4 m, but would be affected by the overall batter shape and roughness. Based on the modelling, one cut required minor modification to the existing slope to be carried out during rail possessions before bulk excavation. Both cuts were successfully widened without impact to the rail envelope or train schedule. There was one recorded instance during construction where a large boulder was released from the crest of one of the cuts and impacted the rock fall protection structure during excavation.