SLOPE STABILITY & MONITORING
WSP’ s rockfall hazard management strategy for Los Bronces
WSP recently implemented an innovative rockfall hazard management strategy for Anglo American’ s Los Bronces copper operation in Chile, encompassing 130 km of roads in complex geographic conditions. This was based on the implementation of a Rockfall Hazard Rating( RHR) system.
Managing rockfall hazards over extensive road networks is complicated by the impracticality of analysing every potential source and the inconsistency of qualitative assessments. This may result in a reactive hazard management strategy where areas known to be problematic but not necessarily with greater risk, receive priority. The aim of this work was, on one hand, to provide a semiquantitative assessment of rockfall hazards for extensive road systems, by means of a simple rating system that can be reliably applied with a modicum of specialised training.
A cumulative frequency analysis of rockfall hazard ratings was carried out to determine hazard classifications based on percentile thresholds for RHR( that is, the rating before accounting for barriers or other controls). RHR values were classified as“ high” hazard, as these cells with highest RHR correlate to the highest qualitative hazard values in the calibration stage. On the contrary, RHR values below the 30th percentile were classified as“ very low or
negligible” hazard, as these are cells where few contributing factors were identified and no rockfall events are known to have taken place. The remaining middle range was divided halfway at the 55th percentile rating, resulting in the moderate hazard classification( 55th to 80th percentile RHR’) and low hazard classification( 20th to 55th percentile RHR).
After a year one field program was completed, the LBRHR classification was mapped over the road network and the most critical sectors were identified and prioritised by Los Bronces. Some of these areas were earmarked for either immediate mitigation( such as placement of concrete barricades) or subjected to further analysis such as rockfall simulations, to both confirm the likelihood of rockfall reaching the road and to establish barrier requirements. As a result, several of the areas initially deemed high hazard were reclassified as moderate or low hazard when revisited in year two.
The LBRHR map is also made available in handheld digital devices as part of a comprehensive geological hazards management platform developed for Los Bronces, providing field personnel with an updated rockfall hazard assessment, and providing means for feedback in the form of field reports which are in turn used to update the hazard map.
Although the scale of the problem initially presented a significant challenge,
as an analysis of the hazard ratings map shows that half of the evaluated roads have a very low to negligible hazard rating and, on the opposite side of the spectrum, 11 % of those 134 km presents a LBRHR classification of“ high” with current controls in place. Although this totals nearly 15 km of roadways, a detailed breakdown by sector reveals that these higher hazard areas are concentrated in the STPPipeline and Riecillos sectors.
One aspect that now bears mentioning is that, in its current form, the LBRHR does not account for expected exposure or through traffic intensity, which is highest in the primary access to the Los Bronces mine area( ie route G-245 and internal roads). This means that the most sensitive areas, where resources should be prioritised, are concentrated in 2.5 %( approximately 3.4 km) of the Los Bronces road network.
The rockfall hazard rating system developed for Los Bronces prioritises objectivity and adaptability to a variety of geological-geotechnical conditions in the Los Bronces district, and it has been found to generate traceable and reproducible results. The main value of the LBRHR lies in its ability to differentiate sectors under homogeneous and consistent evaluation criteria, to assess and manage the most critical sections where mitigation will have the greatest impact on overall risk.