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Underground Rectangular Concrete Tank - Deepened?

Underground Rectangular Concrete Tank - Deepened?

Underground Rectangular Concrete Tank - Deepened?

Can underground conventionally reinforced concrete rectangular tanks be 'deepened'.

The wastewater department has asked me if this is possible? I told them that I do not think it is. (or not without being crazy expensive.)

That was not the answer they were looking for.

I don't see how you connect the new section and get reinforcement developed, etc..

What do you think? See attached.

Thank you!

RE: Underground Rectangular Concrete Tank - Deepened?

Anything is possible if you throw enough money at it.
  • Consider that the bottom slab of this structure supports the walls. If you cut it away, you have short term and long term effects. Short term regarding the loads on the walls during construction. Long term has to consider the loads from both the soil and liquid inside.
  • Excavating inside the structure is tricky.
  • How do you form the lower walls? Are you expecting the soil to stand up straight like that?

  • I've been asked to do this several times. But it never materialized. It didn't price out for me and I don't think it will price out for you.

    RE: Underground Rectangular Concrete Tank - Deepened?

    If the walls are monolithic with the bottom slab as you show then I suspect those walls partially rely on the slab for some flexural resistance. Do you have any details of the reinforcement in the tank?

    Ian Riley, PE, SE
    Professional Engineer (ME, NH, MA) Structural Engineer (IL)
    American Concrete Industries https://www.facebook.com/AmericanConcrete/

    RE: Underground Rectangular Concrete Tank - Deepened?

    If you have the old drawing, you can recalculate it. The good thing is, the tank is pretty square. So your geometry helps. I would analyze the wall as if the load only goes horizontal wall. See if the existing rebar can take the positive and negative moment.

    If that does not work, you can probably install buttress/counterfort in the middle of the wall. This will break the spans in half. More than likely the existing rebar will work after you do this.

    RE: Underground Rectangular Concrete Tank - Deepened?

    Thank you all for the input. It was all helpful.

    RE: Underground Rectangular Concrete Tank - Deepened?

    If the existing walls were designed to span horizontally between the 4 corners, then you don't need the base slab and it can be removed so you can dig the additional 5 feet. However, you would probably still need to vertically support these walls so that they didn't drop when you dig 5' deeper. The walls could be supported by a few different methods - such as helical piers inside the tank at the base of the walls, driven or drilled piles outside and along the tank walls, underpinning, or by hanging them from beams and cribbing.

    If the existing, 4 walls were designed to get their support (cantilever up) from the existing base slab, then you have a bigger problem and the walls would need to be supported vertically, as described above, and would need to be internally braced, probably at two levels, across to the opposite walls.

    If the soil below the tank is not very stable or if below the ground water level, then you have more problems - dewatering and shoring of the additional 5' cut.


    RE: Underground Rectangular Concrete Tank - Deepened?

    What if you drove sheet pilling around the existing tank? Then, cut some of the bottom out, but save the rebar by just bending it out of the way of the work below, cross brace the tank as needed and excavated under the tank. Removing some of the fill btwn. the tank and the sheet pilling should allow the tank to settle. They often build large caisson in a similar way. Then build the new 5-6’ wall on top of the existing walls, in the dry, and recast the floor hole. Maybe you still need some new cross walls, with water movement holes, to stiffen the lower walls and bottom for the increased soil pressures.

    RE: Underground Rectangular Concrete Tank - Deepened?

    Assuming there are existing sewer lines tied into this structure, you will want to limit any movement or settlement to whatever the tie-in connections can tolerate.

    Get a good geotech involved to cover the soil stability aspects for you.

    RE: Underground Rectangular Concrete Tank - Deepened?

    Is this tank inside a building?

    RE: Underground Rectangular Concrete Tank - Deepened?

    There's a lot of good suggestions on how to do this. But you'll need a pretty sophisticated contractor to do this. And a smart contractor will price the risk of this work appropriately. Add that to the cost of just building a brand spanking new tank, which Joe 'I've got a Pick Up Truck' Contractor can do, and I think the choice is made.
    Our clients really think they're being budget wise when they re-use something. I've got 20 years of disasters hidden in the folds of my brain due to this trick. Valves, valve operators, cooling units, buildings, FRP tanks and on and on. It seems like a good idea, but it almost never is.
    Had one client who bought a bolted steel tank, disassembled it and moved it to his town and reassembled it. Guess what? It leaked! A crappy, cheap bolted steel tank. And the tank manufacturer is nowhere to be found. Probably saved a few hundred dollars.

    RE: Underground Rectangular Concrete Tank - Deepened?

    What about raising the top of the tank? Possible if being pumped into, but not if filled by gravity?

    Would be cheaper alternative, but you may need to add stiffener grade beam around the existing rim.

    RE: Underground Rectangular Concrete Tank - Deepened?

    Place a cofferdam frame with some small corner braces in the bottom of the existing tank to act as a base slab for support. Cut holes in the existing slab to drive soldier piles. Cut remaining portion of slab to excavate for proposed deepening. Cut the top of the soldier piles immediately below the existing slab. Cast propsosed deepening walls directly against the soldier piles and tie it into the existing slab. Abandon soldier piles.

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