SUBSIDENCE CONTROL

STANDARD OF REVIEW

According to the CBJ Mining Ordinance (49.65.135(a)(4)), the CBJ shall require that:

• "the operator conduct all operations...so as to minimize to the extent reasonably practicable safety hazards and to control and mitigate adverse impacts on the public and neighboring properties, such as from ...surface subsidence..."

Also, CBJ 49.65.135(b) requires, as a feature of mine reclamation:

• "...adequate pillaring or other support to prevent subsidence or sloughing..."

The objective throughout should be to eliminate hazardous conditions or any degradation of environmental quality caused by surface subsidence.

ANALYSIS

According to the Final EIS (1992), the Kensington ore body can be described as a steeply dipping, irregular tabular mineralized zone, varying in width from 22 to 165 feet and extending at least 1,500 feet horizontally and 2,800 feet vertically. There is also considerable variation in the richness of the ore; the limit of recoverable ore is an economic one, rather than a sharp boundary between mineralized and unmineralized rock.

The mining plan calls for extracting up to 90 percent of the ore body. After all the ore is mined from a stope, a mixture of dewatered tailings and cement will be placed in the open void and allowed to set up to form a weak concrete. This tailings backfill will prevent the walls and roof of the stope from caving, and it will allow the subsequent mining of the pillars of ore that are left between the stopes. Tailings for backfill will be processed at an underground plant located high in the ore body. The tailings will be delivered to this plant as a slurry from the ore flotation plant at the surface. Dewatered tailings will be mixed with concrete and delivered to backfilling locations via pipeline, with gravity providing the motive power in most cases. Some of the planned stopes, including all stopes above elevation 2050, are beyond the effective range of gravity power, and will be left open after mining.

The applicant is planning to use only about a quarter of the tailings for backfill, with the rest going to the dry tailings facility. Plans also call for some stopes to be backfilled with waste rock not required for dry tailings facility construction. Practical experience and economic considerations will determine whether the proportion of tailings going to backfill can be increased. However, current plans anticipate approximately half of the stopes, particularly all those within 900 feet of the ground surface, being left open after mining. In those areas, rock walls and ceiling will be allowed to cave into the void left by ore extraction.

Caving is a progressive process in which material falling into the open stope exposes fresh rock surfaces in the walls and ceiling, which in turn caves and exposes still more fresh rock. However, as caving continues the available volume of open space into which the caving material can fall becomes progressively smaller, owing to the way the caved rock rubble bulks in volume because of the air gaps between individual fragments of caved rock. After caving, the mass of caved rock takes up a greater volume than it did as in-place ceiling and wall rock. This process will continue until the caving reaches the surface of the ground, unless there is sufficient rock remaining above the stope to allow the bulking effect of the caved rock to completely fill the void space and support the remaining rock walls and ceiling. To prevent caving from reaching the surface and causing subsidence, some of the ore nearest the surface outcrop is left in place as a "crown pillar." The deeper the crown pillar, the more rock is available to fill voids left by mining. The pillar size will vary to account for variation in the width and volume of the mined-out stopes below.

The subject of crown pillar size and stability has been addressed in a variety of ways in the course of planning and permitting of the Kensington Mine:

• Responding to an inquiry from the U.S. Forest Service in March 1991, the applicant stated that the mining plan "...provides for crown pillar height of 250 to 500 feet..." and that "...no subsidence will occur with existing mining plan." (See Attachment 1)

• According to the Final EIS (1992), page 2-5, "...subsidence at the surface is not expected due to the bulking effect of the broken material and the minimum 150 foot crown pillar which would be left to ensure stability."
• Responding to CBJ's request for crown pillar design analyses reflecting the revised (partial backfill) mining plan, the Applicant provided Golder and Associates' October 28, 1996 Geotechnical Assessment of the Kensington Project. In the section dealing with crown pillar design, Golder assesses the stability of a proposed 80-foot crown pillar, concluding that long-term stability is ensured by the use of cemented backfill in all stopes. (See Attachment 2)

The mining plan has been substantially changed from what was contemplated in 1992; such features as 375-foot high stopes and mass blasting of pillars have been eliminated in favor of partial backfill of smaller stopes with cemented tailings. Despite these changes, there appears to have been no comprehensive reassessment of subsidence. The Amended Plan of Operation (August 1997), the Reclamation Plan (August 1997) and the Final SEIS (August 1997) do not address subsidence in the context of the new mining plan. Treatment of the subject in the Amended Application for Large Mine Permit (February 1997) is as follows: "The use of backfill combined with a permanent crown pillar with a minimum 150 foot thickness will result in a geotechnically stable environment and will prevent surface subsidence". According to the application, however, backfill is not planned above elevation 2050 although the highest stopes, and those nearest the surface, reach planned elevations approaching 2800 feet.

These apparent contradictions can be resolved if there has actually been no significant change in the mining plan since 1992 for that part of the orebody lying between the ground surface and elevation 2050. Given the understanding that the change to backfilling with cemented tailings applies just to the deeper parts of the mine, it leaves unchanged the pillar design factors used when the permanently stable crown pillars were sized for a minimum 150-foot thickness.

However, if the mining plan above elevation 2050 were changed, concerns about subsidence would necessitate up-to-date information regarding where permanent pillars would be left to prevent subsidence, and the updated geotechnical analysis used to size the crown pillars.

Subsidence, if it were to occur, would follow the surface outcrop of the ore body. For the ore blocked out for the Kensington, that would mean that subsidence would be confined to the high steep slopes of Lions Head Mountain several hundred feet above the valley floor. From the standpoint of risks to human safety, surface subsidence on a large scale in that very rugged area would only marginally increase the risks from falls. The same would seem to apply to the effects on wildlife.

Subsidence is of concern since it would lead to the interception of surface water which would flow down into the mine and through the caved mine workings, potentially leaching sulfide minerals in the broken rock and contributing acid rock drainage to the mine effluent. Although the applicant maintains that the ore and waste rock testing and analyses they have performed effectively rule out any potential for acid rock drainage, this risk needs further evaluation.

Two types of tests are used to measure the potential for acid rock drainage: static testing which measures the relative amounts of acid-generating and the acid-neutralizing minerals, and kinetic testing which assesses the rates of acid generation/neutralization under simulated field conditions. The static testing of the ore body examined hundreds of drill core sections and found that between a quarter and one-third of the ore zone has a neutralizing to generating potential ratio less than 3, the threshold below which further kinetic testing is typically required. The static tests yielded ratios less than 2 for many areas within the overall ore zone.

The only kinetic testing done to date was a humidity cell test conducted on a composite sample of flotation concentrate and tailings which itself was produced from a single 4-ton bulk sample from one part of the ore body. By relying on this one sample, the testing fails to address the heterogeneous nature of the ore body with respect to acid-generating potential. Logs of core drill holes show that the ore zone is comprised of sections exhibiting high acid-generating potential contained within the larger rock mass which has little or no potential. There is commonly a ten-fold disparity in acid-generating potential between the two, so they should have been treated as separate geologic units for sampling and testing. See Draft Acid Rock Drainage Guide, Volume 1, a report to the British Columbia Acid Mine Drainage Task Force by Steffen Robertson and Kirsten (SRK, 1989).

Another problem with the kinetic testing is that the sample that was used in the test consisted of finely ground material. This guaranties that all potential acid-neutralizing mineral is available for the reaction and thus risks overestimating the degree of neutralization that will actually occur under field conditions. Rock tends to break along naturally occurring planes of weakness and the resulting surfaces often expose disproportionately large amounts of some minerals and correspondingly less of others. Realistic testing requires the use of coarse grained sample material exhibiting more natural fracture surfaces. See Mine Rock Guidelines-Design and Control of Drainage Water Quality, SRK, (1992).

STAFF FINDINGS

• 1. Is the application complete with respect to surface subsidence impacts?

  (CBJ 49.15.330(e)(1)(B))

  Yes. The information in the large mine permit application indicates that surface subsidence will not occur.

  2. Will surface subsidence endanger public health or safety? (CBJ 49.15.330(f)(1))

  No. Subsidence in and of itself would not endanger public health or safety due to the rugged, remote nature of the affected area.

  3. Will the operator conduct mining operations so as to control and mitigate adverse impacts on the public and neighboring properties from subsidence?

  (CBJ 49.65.135(a)(4))

  Yes. The applicant's mine plan calls for the use of a crown pillar sufficient to prevent surface subsidence.

STAFF RECOMMENDATION

The staff recommends that the large mine permit contain the following condition:

• 1. The operator shall maintain at least 150 feet of crown pillar to eliminate the possibility of subsidence.

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