Editor:
To Town council (April 2, 2013):
The following is said without prejudice and based on facts as I know them. I should have explained the Factor of Safety as it applies to Canmore’s friable coal.
Golder Associates discloses that research published by Mark and Barton (1999) suggests that the in-situ mass strength of coal should only be derived by reducing the measured lab values using the Holland and Gaddy (1957) formula when the coal is blocky. For friable coals, the size effect is much less pronounced over non-existent. The highly sheared coal found in the Canmore area can be considered friable.
The laboratory value for in-situ plate load testing to estimate the in-situ uniaxial compressive strength of said Canmore coal as reported in 1975 (Golder Report – Three Sisters Creek subdivion 2002)… results gave a range of UCS values 3.5 to 8.5 MPa and pillar stress ranges from 3.0 to 5.8 MPs with Factor of Safety (FOS) = an average pillar strength/average stress.
Maximum FOS = 5/3 = 1.67, minimum FOS = 5/5.8 = .86, giving a mathematical average of about FOS = 1.27.
Golder discloses that a FOS greater than 1.5 is commonly assumed to indicate long-term stability.
If actual testing of Canmore coal at the Riverside Mine has minimum FOS of .86 and an average FOS of about 1.27 and a maximum FOS of 1.67, how can Golder justify a FOS of 3.0 for this same area (peer reviewed references as disclosed by Golder states “that the size effect in determining pillar stability may be non-existent for friable coal)?”
Perhaps this will clarify my concern of FOS as it relates to Canmore coal.
Further, to mayor and councillors (March 18, 2013):
The precautionary principle is now part of the Canadian Environmental Assessment Act, 2012. In layman’s terms, the principle embodies the “better safe than sorry” approach. When in doubt, it’s best to err on the side of caution.
Two guiding principles are: The scientific evidence required for a precautionary decision should be relative to the chose level of protection, and, precautionary measures should be reconsidered in light of evolving scientific knowledge.
These two principles would seem to override the Town of Canmore’s undermining guidelines when it comes to FOS in determining the strength of coal pillars as used by Golder, 2002; i.e. FOS of 3.0 without uniaxial compression strength test on cores.
Undermining Guidelines (2.3 page 8) – at least one core should be obtained of each coal seam in order that the uniaxial compressive strength of uncrushed pillars may be determined for evaluation of the current and long-term stability of coal pillars. This is to determine a FOS on Canmore’s undermined lands.
The evolving scientific knowledge of determining the in-situ strength of coal pillars has evolved to the point there is, A Unique Approach To Determining The Time-Dependent In Situ Strength Of Coal Pillars, by K. Biswas, C. Mark and S. Pang… a borehole penetrometer (BPT) is used to measure the strength profiles in coal pillars. The recorded failure pressure is then converted by a formula to determine the uniaxial compressive strength (UCS) at the location in the borehole.
The BPT’s great advantages are that rock strength is tested in situ and multiple tests can be conducted within a single borehole. Stress or Strength (MPa) can be plotted against Time resulting in a graphy that can predict when a pillar will fail.
I believe that in light of the precautionary prinicple being accepted and referred to by the Supreme Court (the Hudson case) and the fact there have been mitigation failures (parkway collapse and sink hole) that the Town of Canmore should at a minimum have the proponent do in-situ strength testing of each coal seam as stated in the undermining guidelines.
Raymond Haimila,
Canmore
