Sewer Lift Station Detailing 3

[Deker]

I'm in the process of designing a sewer lift station and don't have much experience in this arena (typically do buildings). I think I've got a good handle on what's required by ACI 350, but I'd like to get some feedback from more experienced designers when it comes to detailing and construction practices for these types of structures.

1. I'm not sure whether to locate construction joints between the vault / wet well interface or start and stop the walls so that vault and wet well walls can be poured together. Any benefits to one over the other?

2. I plan on detailing 3/8" PVC x 6" wide waterstops at the base of the walls (and at horizontal joints) and hydrophilic waterstops at the tops of the walls. If the consensus is to use a vertical joint between the vault and the wet well, what type of waterstop would you suggest?

3. I'm scratching my head over how to approach supporting the vault, but of the three options below I'm leaning toward option (c). Anything I'm overlooking?

a) Neglect bearing pressure under the vault and design the vault to "hang" from the wet well...extend the wet well slab under the vault as required for overturning.
b) Assume the vault bears on soil and design the wet well for surcharge accordingly...not sure how feasible this really is given the potential difficulties with backfilling and compacting in what could be a pretty tight excavation.
c) Specify CLSM under the vault down to the bottom of the wet well slab...eliminates surcharge on the wet well and doesn't seem too costly given the small footprint of the tank.

 

[dik]

I'd probably do this...

I'd check to see if chemically there are any issues with using bentonite rod in a keyway for waterbar and use outside wall (surface) waterbars. I've encountered a few problems with the waterbars that are installed in a keyway.

Dik

 

[Deker]

Thanks Dik. Could you elaborate on some of the issues you've had with waterstops in the keyway?

 

[r13]

1. Suggest to provide vertical construction joint, with shear key and dowels, between the vault and wet well. The benefit is fewer joints, so you can make the wet well walls in one lift.
2. PVC water stop can be used, if the vault will ever get wet.
3. Approximate bearing check.

 

[dik]

Improperly attached... and collapsed on a few projects over the decades...

Dik

 

[dik]

Something like... I couldn't find a file with reinforcing in both faces...

Dik

 

[HTURKAK]

I will suggest you;

- Extent the length of vault to match with wetwell walls ,
- Provide just one CJ at the bottom of wetwell so you can make the wet well walls in one lift as MR. retired13 suggested,
- The waterstop SHOULD BE USED with kicker. The kicker (ht 4 in ) shall be poured together with base slab.( The waterstop shall not hinder the top rebars of the base slab )

During excavation of wetwell, the subgrade of vault will be disturbed. The analysis conservatively should based on that the vault is a cantilever to wetwell.

The connection of vault with with shear keys (the dowels will be recessed )

Around forty years ago.. i used the same concept. I could draw a sketch but searched the web and found ;

 

[JedClampett]

[ol 1]
[li]I'd not use keys. They're hard to form and do very little. That's an argument for another thread. [/li]
[li]I like the horizontal CJ above and below the upper slab. Much easier to place the waterstop. See below for waterstop recommendation. [/li]
[li]All 6 inch PVC ribbed waterstop 3/8 inch thick. No bentonite, hydrophilic or anything like that.[/li]
[li]At the base slab, you have two choices. Thicken the slab and hold down the reinforcing for 4 inch clear to place the waterstop or use a 3 inch starter wall. The starter wall should be as thick as the wall. If this is likely to be a contractor that's not used to building starter walls, than the first choice is easier. Just be prepared for comments on why the slab is so thick.[/li]
[li]Most important: Place slurry or CLSM or other cementitious fill below the higher slab, down to the bottom of excavation. The contractor will do a bad job of compacting that backfill and it will settle, creating a cantilever box. Don't get me wrong, I'd still design the box as if it has no support from the soil. [/li]
[/ol]

 

[Deker]

Thank you to everyone who contributed.

JedClampett: You touched on my concerns with relying on soil bearing under the vault slab. It looks like I'll be designing the vault to cantilever from the wet well and have the contractor slurry under the vault anyway. When you use 6" PVC waterstops at the top slab, do you typically raise the slab bars to accommodate or hold the wall pour 3" low?

 

[JedClampett]

Our company has details where the joint is held down a few inches at the top slab/wall joint. But I question myself on the need for waterstop there. If there's enough liquid pressure to push past an un-waterstopped joint, it's coming out the top hatches. Maybe you can tell me it's for odors, but that also seems unlikely.
Another subject is coating. I've made a lot of money on designing repairs for uncoated or poorly coated wastewater facilities. Don't neglect it.

 

[dik]

Jed: unless there are chemical issues with the bentonite... I've used it for the last several decades in lieu of PVC waterstops... generally easier and less costly. If the contents will prevent the bentonite from swelling... then don't use. I use keys to keep the waterstop and/or dowels in place, and I don't use 'elevated curbs' for the waterstop.

Dik

 

[r13]

I personally prefer HTURKAK's suggestion. It is the cleanest/efficient way to go. If there are equipment must be placed on current vault slab level, equipment platform could be considered.

Not against CLSM, if you willing to place it around the structure on all sides, or limit the placement using formworks.

Your original sealing plan for the top slab is good idea. Avoid placing water stop there, which would cause unnecessary problems.

 

[David RP]

I'm also a fan of what HTURKAK proposed. Bear in mind that the kicker joint should be high enough so as not to allow the waterstop to contact the reinforcing steel below it. A good rule of thumb is that the distance from the waterstop to the rebar is 1.5 times the size of the largest aggregate.

A=concrete clear cover
B=size of the largest aggregate
C=distance of waterstop from rebar (1.5 x largest aggregate size)
D=joint opening distance