1993-06-07_REVISION - M1977378 - Laserfiche WebLink

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TR-14 Revision Page 3 By analysis Mr . Abel shows that the degree of irregularity (ie. intact rock irregularities) sufficient to prevent shear sliding from the hydraulic load is 4% of the surface area (using a conservative rock-concrete bond strength) or 10% of the surface area with no allowance for bond strength. An irregularity of 0 . 15 in. is sufficient to develop the design strength of the concrete in radial compression and a high frictional resistance to sliding (ie. wedge effect) . In view of the relatively small percentage of gross perimeter area intact rock irregularity required (4-10%) to equal or exceed available concrete shear strength and cause concrete shear strength to be the controlling shear parameter (bending strength was the controlling design parameter) and the size of the irregularities that will fully develop the concrete strength in radial compression, SGC believes visual confirmation of irregularity will be adequate. If there is any question of the irregular perimeter area not exceeding 4%-10% by visual inspection a confirmation by detailed measurement will be made. If detailed measurement does not confirm sufficient irregularity, then shear pins into the rib sides will be designed and installed . SGC is confident that this will not be required as the sites were selected to avoid any slick- wall type geologic features and normal rock breakage mechanics (drill-blast type mining) will provide the irregularities required. Mr. Abel provides a discussion (p. 4 - p. 11) incorporating case studies, book values of rock shear strengths and a comparison of rock shear strength to concrete shear strength to answer the DMG' s concerns related to the need for additional shear strength data on the rock mass. In summary, Mr . Abel provides reasons why additional rock shear strength data is not necessary. Quantification of potential for failure through the rock mass was not done by Mr . Abel because there are no potential failure paths available for the rock mass to fail (ie. the concrete in the tunnel cross-section will be the preferential shear path) . The reason for this is that the bulkhead sites were selected such that broken ground or intersecting parallel and cross faults coupled with outside opening tapered tunnel dimensions (that might result in a path for a rock mass failure) do not exist at the sites. A site visit is probably the best way to provide the DMG with confidence in the competency of the selected sites and lack of potential rock mass failure planes. Item 11. Ultimate Life of the Bulkheads Mr . Abel addressed the difficult issue of bulkhead life by listing known bulkhead installations at other sites, their impoundment pressures and their service life to-date (p. 13 , Table 2) . Using this information for comparison, he states, "Given even this limited history for bulkheads it seems reasonable to predict a minimum bulkhead life of 50 years" .