Mike, no explosive materials; just a mechanical valve gate. Sorry, my description was vague. And, the geotech report says groundwater is below the proposed structure. And, from what I read in the soils report, the site has no inherent corrosive or reactive substances. Plus, the crappy highly organic virgin soil around is being replaced entirely by CR-6 backfill.
If one approaches it from the IBC or ASCE definition of usage groups, it gets really sticky. The ASCE 7-05 seems like it could go either way as far as being low or high risk. It is a 54" valve (quite large!)on a line carrying potable water. Does this constitute a essential use building? If it fails, what are the consequences?
In the 'doomsday' situation, one could argue this is a critical structure and needs to remain in operation during crisis' for water access due to its size. Maybe analogous to the lack of redundancy of a 2-girder bridge. I agree that it could be the case for this structure. However, there is little or no probability of eminent failure, as earthquake loads are eliminated from load combination equations and the vault can be easily inspected via an access hatch.
frv, I am not so sure about exposure as it is above the groundwater table. Everything else equal, we don't use the ACI 350 for general retaining walls, e.g. landscaping structures, residential foundation walls, etc. Seems like other than the structure usage, exposure isn't a controlling factor alone.
To me it boils down to crack control for the longevity/durability of the buried structure, and not a hazard issue. I used ACI 318 for the initial design which resulted in As min equal to t&s steel in nearly every part of the vault. Without fail... changes were made at 70% submission, requiring a redesign. I am concerned that even with the lowest possible environmental factor from ACI 350, it will look like extra steel for 'no good reason'.
In the design review, the water commission didn't comment about the design methodology, so they obviously don't see a need or are as ignorant as I.
"Structural engineering is the art of modelling materials we do not wholly understand into shapes we cannot..."...ah...screw it, we don't know what the heck we are doing.
