Wet Well Bouyancy
Wet Well Bouyancy
(OP)
Ok, am I missing something here? I'm told that this thing will not float and is with in a reasonable S.F. but my calcs say the S.F. starts to reduce after the water table hits 1517.28. The wet well is flush top = el. 1531 ; concrete= 145 #/ft3 outer diameter is 112" (96"+8"+8"). There is a hatch that is 4'x8' but I neglected the weight of the hatch from my calcs due to just rough calcs. What am I missing?
http://files.engineering.com/getfile.aspx?folder=a...
http://files.engineering.com/getfile.aspx?folder=a...





RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
Because of the sliding resistance of the soil, it is nearly impossible to float a lift station. Compare the force necessary to pull a fence post out of a swimming pool to the force required to pull a fence post out of the ground.
Use the manhole floatation analysis procedures prepared by the Concrete Pipe Association to check floatation:
http://www.concrete-pipe.org/pdfdd/DD_41.pdf
If you still have a concern, add a l2-Inch lip around the base.
RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
1. The worst case may occur during construction, before there is any backfill.
2. I practice in an area with clay soils that in dry weather may shrink and pull away fron the walls, reducing the friction.
Some additional concrete is a relatively small price for security.
RE: Wet Well Bouyancy
Concrete is cheap, failures are expensive.
RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
A safety factor is only appropriate if friction or cohesion or earth are the primary forces resisting flotation. The safety factor is used to account for the variability of the soil properties.
It is also not economical to add weight for floatation during the construction process. If the Contractor has concerns about floation, he can address that by adding wieght on his own dime. Construction (of the lift station) is considered to be "means and methods". The Engineer is not responsible for construction, the Contractor is.
In your example, the lift station is buried 30 feet in the ground. It would extremely unlikely to have 30 feet of clay soil. Note that you should have a soil boring for a project of this scope.
Another item missing in your analysis is the weight of soil on the bottom lip. The additional effective weight of the backfill in the cylinder above the lip can be added as an anchoring force. A conservative soil weight in the cylinder above the lip is 65,000 lbs.
RE: Wet Well Bouyancy
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Wet Well Bouyancy
Here is a link to a geotechnical report for a similar lift station project where the geotechnical engineer is using skin friction to resist uplift. See page 8.
http://www.houstonlwsforum.org/designCompetition/S...
RE: Wet Well Bouyancy
==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: Wet Well Bouyancy
RE: Wet Well Bouyancy
==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: Wet Well Bouyancy
As long as the contractor owns the site, dewatering and bouyant conditions are his bother. Don't make them yours!
f-d
¡papá gordo ain’t no madre flaca!
RE: Wet Well Bouyancy
However, these factors are subject to more variation than the simpler factors discussed above, and therefore an appropriate safety factor should be used.
I find it easier to simplify and use no F.S. On the few occasions I have checked, there is a minimal difference between the methods with a F.S. of 1.0 applied to the simplified method and an F.S. of 1.5 applied to the more detailed method. Concrete is cheap.
In my area, the groundwater level is typically very high, so the groundwater at ground level assumption is OK. In areas with low groundwater tables, this assumption could be excessively conservative.
RE: Wet Well Bouyancy
The the UK the codes state that the min. F.o.S. should be 1.1