Rockfall scaling ranges from simple scaling of loose rock mass with hand-held tools such as “gwala” to more advanced processes such as the sensitive blasting with low energy rock-breaking cartridges.
Rockfalls presents major hazards in excavations for open pit mines, highways and railways in mountainous terrain. A rockfall event is generally initiated by climatic and biological event that causes a change in the forces acting on a rock. In a mining environment, rockfall usually occur due to blasting induced vibration. Rockfall in general range from minor falls to larger falls which may affect the safety of people and may also cause damage to infrastructure. Rockfall may also include substantial slope failures that cause serious damage to mining, critical public and private infrastructure.
A rockfall protection system refers to the structural elements used for the mitigation of the potential hazards resulting from un-controlled falls of ground. Rockfall hazards mitigation systems aim at minimising the potential effects of rockfall events through the design and installation of rockfall protection systems. Rockfall protection systems are engineered to be durable, effective and to save lives.
However, in many instances the structural elements used for rockfall hazards mitigations are technically limited; and do not guarantee the required protection. In this instances, rockfall scaling or re-sloping is considered. However, the biggest problem with rockfall scaling is the highly variable and unpredictable nature of the associated costs. Hoff Geotechnik has developed a set of basic factors to be considered by transportation departments, property developers and mining companies when making decisions related to budget approvals for rockfall scaling projects. Here is a brief summary of cost estimates and factors to be considered.
The technical factors include the following:
1. Quantity of the materials to be scaled down
Regarding quantity consider volume or area covered. Volume refers to the total physical quantity of the all individual rocks to be scaled down from a given area, usually measured in tons. Area covered refers to the total area requiring rockfall scaling.
A small area is classified as an area covering less than 10 000m2. A large area is classified as anything above 10 000m2. For both these cases the cost of setting-up will be more or less the same. However, the cost variation between the two areas is related to the amount of time required for rockfall scaling for each respective area size or volume of materials to be scaled down.
Simple scaling for an area covering 10 000m2 could range from $12 000,00 to $95 000,00 depending on complexity of the site.
2. Size of the individual rocks/blocks needing to be scaled down. For rockfall purpose the size of the individual rocks may range from small rocks of less than 0,5 ton to larger blocks of over 300 tons. The small rocks will be much easier to mobilize down the slope compared to large rocks which may require additional drilling and blasting before mobilizing down the slope. Areas requiring drilling and blasting (re-sizing of the blocks) will cost a lot more. This cost will be largely driven by the number and size of the boreholes to be drilled; and quantity, type and cost of the explosives to be used for re-sizing.
Drilling and blasting of the 300 tons mass block into smaller pieces may cost between $10 000,00 and $80 000.00 depending on the complexity. Re-sizing a rock mass block twice the size of 300 tons will increase the cost by less than 50% because most of the cost is on setting up for the task.
3. Complexity of the terrain and setting up requirements for the anchorage points.
Consider the slope angle and height; as well as the number of anchorage points required. Drilling and blasting a 600 ton rock mass on a 60deg slope at 450m below or above ground level costs between $60 000,00 and $200 000,00 depending on complexity. Drilling and blasting a 1200 ton rock mass in the same conditions as above would result in a duplication of the cost as the task requires exactly double the amount of effort.
4. Equipment required for the task
A simple rockfall scaling task requiring only hand-held tools costs over 50% less than the more advanced rock fall scaling with pneumatic jacks or drilling and low energy explosives for the same areas.
Non-technical factors
1. Economic factors of the site
The economic factors will include factors like salaries and cost of living in a certain area. Doing the job in an area with a high cost of living will result in a higher cost of the project, this is common sense. Doing work in remote regions of the world will also result in higher cost due to the cost associated with hiring expert and skilled technicians.
Other factors may include legal and contractual factors, as well as insurance requirements.
Prepared by
Ernest Rankhododo (MSc. Eng).
