RC Tunnel Liner Grouting Pressure

[LicensedToPEe]

I am designing a 20ft DIA RC tunnel liner 335' down in bedrock. The hole in the bedrock will be blasted which means the annular void between the rock and outer surface of the liner will need to be grouted. The minimum grouting pressure needs to be at least that of the external hydrostatic pressure. At 335 feet, the water head will be 335’ x 62.43pcf x (1/1,000) = 20.91ksf = 145.21psi. So, let’s say I will require at least 150psi of grouting pressure. If I space out the grout holes in a 5’ by 5’ grid (typical in practice), what is the recommended procedure to check the flexural capacity if the concrete liner?
If I use an equivalent 1ft ring and 5 foot spacing, the following is used:
150psi = 21.60 ksf/ft distributed over a 5 foot length (spacing of grout holes). I have modeled the liner as a 20 foot circular segment with radial and tangential rock “springs”. The load is applied center-top, radially over the length of 5 feet (for D=20’, C=63’). The resulting positive moment (at L/2) is approx. 43 kip-ft [or wL^2 x (1/12)]. For a typical liner thickness of 12”, this moment kills my design (#8@12, ea way w/ 4” cover).

Is my assumptions/approach too conservative? Is using 150psi (12.60ksf) too high of a load?

Thanks!

 

[cvg]

335' of (static) head assumes you have groundwater all the way to the surface? Unless you are boring through a swamp, this will not be the case. It also assumes you have hydraulic connectivity to the top of the groundwater without any headloss. If you have weepholes or drains installed in your liner, you can relive some of that pressure. Of course, as you grout, some of the grout will also come out the drains, displacing the water.

 

[LicensedToPEe]

I understand your reasoning CVG. However, the static head requirement is a desing criterion that comes from the Reclamation District and that is simply a loading taken as is by me as a structural eng. If I just use 150psi grouting pressure (forget where the number comes from), original dilemma remains. Does the grout nozzle pressure of 150psi translate into a 21.60 ksf/ft of loading on 5 ft segment of the liner?