2003-09-29_REVISION - M1977378 - Laserfiche WebLink

Home Browse Search

Mogul Mine bulkhead design Page 3 July 19, 2003 water (range from 600 to 900 ppm). See Table 9,3.1 from ACI 318-95 for cement requirements. No. 1 structural grade, Douglas Fir, or equivalent, is required for the timber form structural members, 8 x 8 posts and sill and 3 x 12 lagging. The lagging will have to be cut to fit the back, ribs and floor profile. The 3-in schedule 90 (standard weight) pipe braces are indicated on the drawings, as are the L 6 x 4 x 1/2 angles and #8 Dywidag anchor dowels. The 1/2" thick plywood, or particle board, is to be nailed against the inner form face of the 3 x 12 lagging (form planks). The plywood, or particle board, will have to be cut to fit the crosscut back, ribs and floor profile. The upper half of the concrete/rock contact should be low pressure grouted (>60 and <165 psi) with Type II neat cement grout through drill holes angled upward and outward through the contact to prevent leakage around the bulkhead. See attached drawings. The drilling and grouting of the concrete/rock contact should wait until after the bulkhead concrete has set for a minimum of 7 days. The contact grout holes should directed toward high spots in the tunnel back near the center of the bulkhead. It is difficult to completely fill all high spots in a tunnel roof when pumping concrete. Garrett and Campbell Pitt indicated that pressure grouting of the concrete-rock contact would permit pressure gradients of 163 psi/ft without obvious leakage. Applying a factor of safety of four produces a design pressure gradient of over 91 psi/ft when the concrete-rock contact was pressure grouted. The indicated benefit from pressure grouting the concrete-rock interface is an eight fold decrease in bulkhead length required to prevent unacceptable leakage. Chekan (1985) analyzed Garrett and Campbell Pitt's pressure gradient data and produced a graphical version of their data, which is attached. Sincerely, ~~ ) John F. Abel, Jr. Colorado P.E. 5642