tunnel lining

[par060]

I am designing the final lining for construction under a lake. In operation the tunnel will be full of water. My controlling case will be during the construction period the lining will support the hydrostatic load. The head has been determined from the flood level of the lake. The liner should be unreinforced concrete and the tunnel is in rock.
To this point I have used the .55 strength reduction factor (ACI318 9.3.5) and I applied a 1.6 load factor to the hydrostatic pressure. The load factor is 1.6 rather than 1.4 because there is not a control of the flood level. The construction period is over 1 year.

Am I being to conservative?

 

[TTK]

I am not at all experienced in the design of tunnel liners, so please keep that in mind as you read my post.

Using a strength reduction factor of 0.55 and a load factor of 1.6 equates to a factor of safety of 2.9, which is high for temporary works.

An approach that is used in cofferdam design is to set a design flood elevation and only allow work in the cofferdam when the water surface is below this elevation. If it goes higher, vents (cut-outs) are used to flood the temporary works automatically so as not to fail the cofferdam.

You could design for a 2-year, 5-year, or 10-year storm and cleary note this on the plans. If the water level exceeded this level, the tunnel might have to be flooded until the water level receded.

With this approach you should be able to use the lower load factor as the unknowns would already fe factored into the design.