Manhole Buoyancy

[JDWright]

I am specifying some relatively deep 5' diameter precast concrete manholes (25'-35' deep) which are located in a floodplain (will be fully submerged during floods) and thus they will have water tight lids with MH rims at existing ground level. I have ran the standard buoyancy calculations and extended the MH footings base lip by 6" to provide for more total weight (concrete and soil weight) to offset flotation with a FS of 2.

My more theoretical question is why does the upper portion of the manhole not float up from the base? For all the precast details I have seen, there is never any mention of needing to ensure that there is not separation from the base section with the upper sections due to the water tight lid. Sr. engineers around here have no idea. All the models for analysis seem to show manholes that are assumed to be open at the top and tightly sealed at the base with the uplift buoyancy forces acting on the underside of the footing. Would a water tight upper portion of a MH float up independent of the base causing separation of the precast sections (or at least causing separation of the shallowest top section to relieve the pressure?

 

[cvg]

it makes no sense to design a watertight manhole lid and not make the rest of the structure totally watertight. so the shaft should be designed either without joints or with joint restraint, such as was previous recommended should be used. as with most watertight manhole lids, these should be bolted down and anchored into the shaft also.

"an inexpensive solution is to strap and bolt the sections together"

http://www.dot.ca.gov/hq/esc/oe/pro..._advertised/07-215934_plans-pgs-0201-0300.pdf