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The Foidel Creek Mine currently extracts coal using 245m wide longwall panels, between 250m and 275m below ground level. The
Twenymile Sandstone outcrop forms a prominent cliff between 15 and 70m high, running for many miles through the mining district. With a coal extraction thickness of some 2.4m, subsidence analysis predicted
that sections of the cliff would fail during undermining.
Fieldwork was carried out during the summer of 1994, and involved geotechnical assessment of the cliff and the foreslope. The critical factors to be
assessed were the range of potential block-sizes which could be released from the cliff and the nature of the foreslope, in particular its topography, roughness, and frictional and restitution coefficients.
Block-sizes were
assessed using scanline mapping to provide statistical data on discontinuity spacings and orientations. The probable ranges of block volumes and key dimensions were calculated from these distributions using the simulation
program @RISK. This analysis indicated some very large potential block sizes.
Rockfall hazard analysis was carried out using the Colorado Rockfall Simulation Program (CRSP3) developed jointly
by the Colorado Transportation Institute, Colorado Department of Transportation, and the Colorado Geological Survey. CRSP3 uses Monte Carlo simulation to predict the probable size, mass, velocity and height of
falling rocks at any user-specified point on the slope. Input includes the slope cross section, the range of block sizes and shapes, and the slope surface characteristics assessed during site investigation. The output
can be used directly in the design of barriers, catch ditches and other protection measures, should the analysis indicate these are needed.
Some twenty critical sections were identified and analysed
using CRSP3. For each section a thousand rockfall scenarios were analysed to provide the probabilistic output. Back-analyses of known rockfalls were used to
calibrate the models, ensuring the slope surface characteristics were appropriate. The results were presented as a hazard map contoured at specific risk levels. Where the map
indicated unacceptable risk, rockfall protection was designed. In view of the potential size of the blocks involved, the use of conventional rockfall protection fences was ruled out. Instead, a system of rock traps,
consisting of trenches and earth berms, was designed. The depth, width and face angles of these traps were checked using CRSP3, and modified as needed until the program indicated a extremely low probability of boulders escaping.
As a result, approximately 915m of 6m-deep trench was constructed between County Road 27 and the Twentymile Cliff to intercept and contain falling boulders. In addition, a 460m section of the road was realigned
to enable construction of the rockfall protection measures. The total cost of the works was about US$1M. |
