huangyhg

precast manhole to footing connection
i want to get an idea of what others do in this situation.
i have a 25 foot deep (10' id) precast manhole. we are going to put "doghouses" in the bottom so that it can be placed directly over some existing 30" and 36" pipe. a footing will be poured beneath the existing pipe and the manhole will then be set.
my question:
how do others go about attaching the precast manhole to the cast-in-place footing??
additional:
we are thinking about "flooding" the manhole so we can neglect bouyancy. does this change how you attach the two elements??
i have details i use when it's all new (nothing to work around) but this is a little different than what i've done before. thx
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brandnew,
i have encountered this situation once before. with your situation it will actually work even better because you have an extremely deep manhole of a large diameter.
when casting the bottom of the manhole, allow for a keyway in the top surface. placing a compressible, watertight gasket in the keyway will create an excellent seal. the selfweight of the manhole alone will create a uniform and watertight seal around the entire perimeter. the keyway will also keep the manhole laterally stable on its base. it works quite well and its relatively inexpensive.
good luck.
are you talking just using the typical 2" keyway or a larger one??
i've always poured concrete up around the bottom of the barrel section (bottom foot or so) but this situation doesn't lend itself to that very well.
thanks for the response!!
check with the precast manufacturer to see what size of a key they typically provide on the bottom of their precast sections. you don't want to specify a key with a unique shape or dimensions. i would recommend using a keyway depth that is approximately 1/4 to 1/3 of the thickness.
on my project, i used a 16" thick bottom slab with a 4" key. i've also seen different key shapes: \_/ or |_|
i always prefer to use the first one if possible. cheers.
thanks
when casting the base of this structure make sure the manhole does not rest on the pipe. use a flexible sealer 1 -2 inches thick to keep the load off the pipe. using a bentonite rope under the manhole will be all the seal you need. also excavate all the way down past the bottom of the pipe to get bearing on native soils. you don't know if the original installation was subcut too much.

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