Annual inspection reports
We have assessed the condition of the rock face on the A890 Stromeferry Bypass since it was opened to traffic in 1970. Much of the rock face has been netted to mitigate the risk of falling rocks reaching the road and railway. We are currently in the third year of a 5 year contract with the geotechnical consultant AECOM Infrastructure and Environment Ltd, the inspection contract will run until 2020. AECOM has been advising on the rock slope condition since 2012. The detailed annual inspections reports undertaken by AECOM complement the daily and monthly roadside inspections which the Council undertake to ensure the effective management of risk associated with the rock face.
We use the reports to plan future rock stabilisation works and we aim to prioritise areas which are most in need. This can include new rock netting systems, replacement of the existing protection system or new rock fall protection works (e.g. removal of the hazard, rock bolts, catch fence). In the last two years we have undertaken works to three of the highest risk sections of rock face on the Stromeferry Bypass and will continue to identify any areas which require improvement to reduce the risk.
In the 2018 annual report four slopes, in addition to the slope currently being stabilised (Autumn 2018), were identified as very high risk.
It is anticipated that the works for 2019 will include these four areas. The exact programme for these works is subject to the detailed design, acceptance of method statements to safely carry out the works, agreement of the traffic management proposals and access to Network Rail land and all the associated track works and signalling. A road to rail bypass will be required at one of the slopes and the traffic management system with the enhanced rail signalling operated in October 2018 will be used. It is currently anticipated that the works to two of the slopes can be undertaken whilst maintaining traffic flow using a stop/go system, stop periods would be in the order of 20 minutes. The works to the remaining slope are likely to require up to three weekend full closures, as a road/rail bypass cannot be constructed due to level differences between the road and railway.
We undertake daily drive through, monthly walked inspections and annual specialist geotechnical inspections and reports to mitigate the risk to road users from rock slope instability. Additional specialist inspections are carried out when the Council's daily or monthly inspections identify any issues that require geotechnical advice. The current level of monitoring is deemed an appropriate mitigation measure, but this will be constantly reviewed by the Council and AECOM as works are programmed and progressed.
The reports below represent the risk assessment approach which is used to assess the condition and suitability of each rock face and identifies interventions which may be required. The risk assessment approach considers the size of a potential failure, how much material may reach the road and the sightline on the road at that location. Given the number of variables involved (e.g. rainfall, wind, frost, vegetation growth, drainage, rock structure, animal activity), the timing/probability of a failure occurring cannot be determined. However, if an issue was identified during any inspection that was deemed to be at risk of imminent failure and that could reach the road, this would be addressed and remediated as emergency works.