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Koehler Tunnel bulkhead design Page 3 July 31, 2003 <br />BULKHEAD DESIGN <br />The specified bulkhead concrete strength is 4000 psi in <br />compression. The concrete controls the bulkhead design because it <br />is weaker than the rock. The bulkhead was designed on the basis <br />of 3000 psi compressive strength because of the difficulties <br />inherent in pumping and placing concrete underground. Type V <br />sulfate resistant cement is recommended because of the sulfate in <br />the water draining from the Koehler Tunnel (range from 800 to 5500 <br />ppm). See Table 4.3.1 from ACI 318-95 for cement requirements. <br />No. 1 structural grade, Douglas Fir, or equivalent, is required <br />for the timber form structural members, 8 x 8 posts and sill and 2 <br />x 12 lagging. The lagging will have to be cut to fit the back, <br />ribs and floor profile. The 2-in schedule 40 (standard weight) <br />pipe braces are indicated on the drawings, as are the L 6 x 9 x <br />1/2 angles and #8 Dywidag anchor dowels. The 1/2" thick plywood, <br />or particle board, is to be nailed against the inner form face of <br />the 2 x 12 lagging (form planks). The plywood, or particle board, <br />will have to be cut to fit the tunnel back, ribs and floor <br />profile. <br />The upper half of the concrete/rock contact should be low <br />pressure grouted (>_95 and <265 psi) with Type V neat cement grout <br />through drill holes angled upward and outward through the contact <br />to prevent leakage around the bulkhead. See attached drawings. <br />The drilling and grouting of the concrete/rock contact should wait <br />until after the bulkhead concrete has set for a minimum of 7 days. <br />The contact grout holes should directed toward high spots in the <br />tunnel back near the center of the bulkhead. It is difficult to <br />completely fill all high spots in a tunnel roof when pumping <br />concrete. Garrett and Campbell Pitt indicated that pressure <br />grouting of the concrete-rock contact would permit pressure <br />gradients of 163 psi/ft without obvious leakage. Applying a <br />factor of safety of four produces a design pressure gradient of 41 <br />psi/ft when the concrete/rock contact is pressure grouted. The <br />indicated benefit from pressure grouting the concrete-rock <br />interface is an eight-fold decrease in bulkhead length required to <br />prevent unacceptable leakage. Chekan (1985) analyzed Garrett and <br />Campbell Pitt's pressure gradient data and produced a graphical <br />version of their data, which is attached. <br />Sincerely, <br />~~ ~ , <br />John F. Abel, Jr. <br />Colorado P.E. 5642 <br />cc. Larry Perino <br />