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Concrete sunk caisson design

Concrete sunk caisson design

Concrete sunk caisson design

(OP)
I am in need of some good reference recommendations.

My project consists of a 10-foot diameter by 29-foot deep concrete caisson.  (To avoid confusion, this is not a drilled pier caisson, but is the kind where it is essentially a pipe installed vertically.)  The soil is stiff clay with a sand lense 5 feet below the bottom of the excavation.  Another sunk caisson exists on the same site adjacent to where this new one is planned, but not close enough to effect it.  I have calculated the pressures on the caisson for the various loading conditions and need help in the concrete design.  The caisson will house a steel pressure vessel below ground, so it should not be full of water unless the pressure vessel leaks.  The main loading is compressive forces due to lateral earth and water table pressures.

I tried applying PCA's Circular Concrete Tanks without Prestressing criteria, but it is geared toward large diameter tanks in relation to height.  Also, ACI 350 seems like it would be too restrictive since that would be for water tight structures from the inside.

I am in need of guidance on design of hoop reinforcement and wall thicknesses.

Thanks in advance for any recommendations.

RE: Concrete sunk caisson design

Might want to see if you can check out Indian Standard IS3955.  This is their standards on a "well founation" which is basically your situation.  You have indicated that the sand layer is 5 ft below the founding level - be careful until you get your plug in about uplift pressures - the unbalanced pressures in the sand versus that which might be in the well.  Just a thought as you don't want a blow-out.

RE: Concrete sunk caisson design

(OP)
Dewatering will be required to prevent a blow-out.  I understand from the construction of the other caisson on site that they did not dewater during construction and had a blow out after the caisson was submerged and they started to drive piles to support the test vessel.  As soon as the piles broke into the sand layer the water rushed in.

Thanks.  I'll check out IS3955.

RE: Concrete sunk caisson design

Do some research on tunnel construction, this is very similar to the design of a ventilation shaft for a highway tunnel.

RE: Concrete sunk caisson design

Todh,
I ran into the same situation as you during a caisson design.  Mine was 18' diameter and 110' deep.  The PCA Tables don't cover such an extreme height/depth ratio.  I used Analysis Group from IES.  It has a circular tank module that worked nicely.  It was able to calculate the vertical moment at the bottom of the caisson.  Most of the caisson is subject only to compression, but the very tip near the plug is subject to vertical moments.  

Are you using a tremmie plug?  

Another load to consider is differential movement from a seismic event.

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