1. Retsof Salt Mine Collapse Mark Ferguson Manos Adoniadis

2. Retsof Salt Mine ► Largest salt mine in North America, second largest in the world. ► In operation from 1885- September 1995.

3. Pillar and Stall Mining ► Used for deep mining of thin beds by partial extraction. ► 10-40% left in place to form pillars to support roof.

4. Geological History of the Genesee Valley Bedrock Shale and limestone Limestone Salt Layer Glacier Sand and Gravel Sand Sand, gravel and silt Sand Coarse Gracel Aquifers Shallow floodplain aquifer Upper Aquifer Basal Aquifer

5. Timeline of Events ► November 1993 – Room 2-Yard South abandoned; ► March 12 th 1994 – Earthquake, magnitude 3.6 Richter scale centred near Cuylerville, Room 2-Yard South collapses – 150x150m section; ► April 6 th 1994 – 60m diameter sinkhole appears directly above room 2-Yard South, 6m deep, Two weeks later expands to 180m; ► April 8 th 1994 – Roof collapse in room 11-Yard West; ► May 25 th 1994 – 60m diameter sinkhole appears directly above room 11-Yard West, 15m deep, Over time diameter increased to 250m; ► September 2 nd 1995 - Operations ceased.

6. Collapse Sinkhole Sinkhole above room 11-Yard West Sinkhole above room 2-Yard South

7. Collapse Sinkhole Sinkhole above room 11-Yard West Sinkhole above room 2-Yard South

8. Collapse Sinkhole Formation

9. Water Ingress ► Room 2-Yard South collapse – 19 cubic metres/minute, 27,000 cubic metres/day ► Room 11-Yard West collapse – increased to 85,000 cubic metres/day

10. Additional Effects ► Diminishment of potable water supplies – reduction of quality and quantity; ► Reduced air quality – escape of methane and hydrogen-sulphide; ► Short- and long-term land subsidence

11. Could this have been avoided? ► Better evaluation of geophysical surveys and historical data;  Pressurised brine and gas pools. ► Backfill of excavated areas; ► Grouting of affected areas.

12. Thank you for listening Any Questions? Mark Ferguson and Manos Adoniadis