Detention Tank Design

[msquared48]

Got a design problem I have been working with a Geotech on with a detention vault.

The vault is 20 X 104 and about 18 feet deep with 2 to 5 feet of cover under a roadway. The north long wall is on the uphill side with the opposite side sloping away. The vault is situated toward the south side of a 30 foot wide access right of way with a 20 foot wide roadway.

The problem I see is that the Geotech recommends a shoring system using either tiebacks or internal bracing. I have a problem with using tiebacks due to the right of way imitations and the need for additional right of way agreements, and the internal bracing because it would have to be removed since it is a detention vault, meaning you are stuck with external cantilever soldier pile system, and very large pile. Also, since one side is sloping away with less resistance laterally, you are kidding yourself in thinking that the internal bracing system would actually be functional in compression.

Any ideas?

Mike McCann, PE, SE (WA)

 

[msquared48]

I am investigating soil nailing.

Mike McCann, PE, SE (WA)

 

[Splitrings]

Can the lid be used to provide top support to the walls? This way they act more like a simply supported member rather than a cantilever? The top would have to be installed before backfilling.

 

[msquared48]

That is usually the way I do it, but due to site access constraints during construction, the sloping cuts to install the walls cannot be done. We have decided to use soldier pile with tiebacks and deal with any easement issues later to get this submitted. This is a very non standard vault.

Mike McCann, PE, SE (WA)