1993-04-27_REVISION - M1977378 (2)

Home Browse Search

L.]�. Perino Page 4 Al 27, 1993 Minor irregularity of the tunnel walls, sufficient to introduce only about 4 percent of minimal shear strength (1150 psi) intact rock along an otherwise smooth and circular cross section with a 70 psi bond strength (f.) , is needed to prevent shearing (FS 1.00) , as follows: X - proportion of 1150 psi shear strength rock necessary to prevent sliding Minimum bond area = n(13)25(144) = 147000 in2 147000 in2 - rock-concrete bond area for 13-foot diameter circular cross section Resisting Force = Thrust Concrete Intact Rock (1 - X) 10300000 + X(147000) (1150) = 16300000 10300000 - 1030000OX + 16900000OX = 16300000 15870000OX = 6000000 X = .0378 (3.78 % intact rock, perimeter area) As indicated at the bottom of page 6 of the list of concerns the Division of Minerals and Geology requires that the degree of irregularity of the final bulkhead walls be measured. The purpose of this measurement program is to verify that there is sufficient ' intact latite to prevent shear failure along the concrete-rock interface. In the absence of any bond strength and using the circular tunnel cross section and minimal 1150 psi rock shear strength assumptions, approximately 10 percent intact rock is necessary to prevent shear sliding, as follows: X - proportion of 1150 psi shear strength rock necessary to prevent sliding Resisting Force = Thrust Concrete Intact Rock (1 - X)00000000 + X(147000) (1150) = 16300000 00000000 - OOOOOOOOX + 16900000OX = 16300000 X = .0964 (9.64 % intact rock, perimeter area) In the absence of a downstream opening taper for the full bulkhead length at the bulkhead locations the percentage of intact rock will vary from zero to 100 percent, averaging 50 percent. The worst-case zero percent intact rock condition, i.e. 100 percent concrete, was covered in the detailed design for the concrete bulkhead. All other irregularity intact rock percentages result in greater resistance to thrust. The concern raised at the top of page 7, "The Division believes that direct shear tests should be performed on sawed and surface ground samples of the tunnel wall rock and on samples that contain the 'healed' fractures with the associated white mineral described in Appendix D of the March 12, 1993 Simon Hydro-Search