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y ~ • III IIIIIIIIIIIIIIII • ~OG, tY6~vrl.E~ OIjJEC.l~r~s GXVL~~~i~ Mr. R. Demers, Technical Manager General Chemical Canada Ltd. P. O. Box 2000 Amherstburg, Ontario N9V 2Z6 Via Fax (519) 736-8915 & Mail May 23, 1997 Dear Mr. Demers: Dr. Leo Rothenburg 188 Mohawk Ave., Waterloo, Ontario N2L 2T3 Re: Report on rock mechanics design of a new brine field and overburden stability issues. The purpose of this letter is to answers several technical questions that have arisen following your review of my report on design of a new brine field at Amherstburg. As far as I understand from our telephone conversation, this review went well although there were some issues related to stability of overburden strata that needed a clarification. The overburden stability above the proposed brine field has always been the main design issue. The report puts emphasis on the stability of the Bass Island Dolomite member as it is the stiffest stratum overlaying the proposed brine field. Its integrity is critical to the global stability of the entire field. Analyses of the proposed design solution suggest that no tension will occur in the Bass Island Dolomite stratum in the area over the field for more than a century. Some tension may develop at the top of [he Bass Island Dolomite over abutments after 50 years. These estimates relate to an unlikely situation that the overburden will be undermined over the entire field area. The report also carefully addresses the integrity of other overburden strata. Of particular relevance here is the Section 7.1 of the report (p. 30) that discusses geologic peculiarities of brine fields in the Windsor-De[roi[ area. The emphasis of this section is on the fact that the Unit F of the Salina formation in the area of the Amherstburg field contains no salt. This is a very important and a positive observation as creep of confined sal[ induces horizontal tension in adjacent strata. In this respect, the geology over the Amherstburg field is much more favorable compared to other brine fields in the Windsor-Detroit area. This is demonstrated through an analysis [hat is discussed in the last paragraph on p. 63 of the report and illustrated in Figure 35 (p. 65). Figure 35a illustrates horizontal stress that develops after 100 years in overburden strata above the cavity center.