rplayer
Geotechnical
- Jun 21, 2005
- 2
I am faced with a unique foundation arrangement and would appreciate guidance on how to analyze:
Location: Wastewater treatment plant
Project: Adding two large egg-shaped digesters and associated piping galleries. Plan view looks like a giant pair of Elton John eyeglasses - two round lenses with a large rectangle on top.
Soils: Site soils consist of medium dense to dense sand and gravel (<5% passing No.200) with occasional cobbles and boulders (Lake Missoula outwash gravels) N60 average = 25.
Existing grade is at elev 1710 feet. Top of floor slab will be at 1680 feet. An excavation 300 ft xt 200 ft will be opened up to build digesters and surrounding galleries. Outside edges will be backfilled to ~elev 1695, but inside will be open.
Digesters to be supported at their equators by large circular concrete stub walls that in turn are supported by 20-ft wide donut shaped spread footings. The stub wall diameter is 80-ft, so the diameter of the outside edge of the footing is 100 feet and inner diameter is 60 ft. Footings will be buried 5' 8' below finish grade.
The footings for the two digesters are 20-ft apart at their closest point. In addition, a 10x10 foot square column footing will go between them, leaving a 5-ft space between adjacent footing elements.
Required bearing capacity of the "donut" foundation is 8000 psf (big, liquid filled vessel).
The question is how to calculate bearing capacity (8 ksf should be doable) and settlement (<1 inch tolerable). Do the footing loads superimpose? How apart do they need to be to not superimpose?
What are some good references for complex, closely spaced footings? For "donut" foundations?
(I refer to them as donut foundations because they are so large, they don't seem to meet the ringwall criteria.)
Location: Wastewater treatment plant
Project: Adding two large egg-shaped digesters and associated piping galleries. Plan view looks like a giant pair of Elton John eyeglasses - two round lenses with a large rectangle on top.
Soils: Site soils consist of medium dense to dense sand and gravel (<5% passing No.200) with occasional cobbles and boulders (Lake Missoula outwash gravels) N60 average = 25.
Existing grade is at elev 1710 feet. Top of floor slab will be at 1680 feet. An excavation 300 ft xt 200 ft will be opened up to build digesters and surrounding galleries. Outside edges will be backfilled to ~elev 1695, but inside will be open.
Digesters to be supported at their equators by large circular concrete stub walls that in turn are supported by 20-ft wide donut shaped spread footings. The stub wall diameter is 80-ft, so the diameter of the outside edge of the footing is 100 feet and inner diameter is 60 ft. Footings will be buried 5' 8' below finish grade.
The footings for the two digesters are 20-ft apart at their closest point. In addition, a 10x10 foot square column footing will go between them, leaving a 5-ft space between adjacent footing elements.
Required bearing capacity of the "donut" foundation is 8000 psf (big, liquid filled vessel).
The question is how to calculate bearing capacity (8 ksf should be doable) and settlement (<1 inch tolerable). Do the footing loads superimpose? How apart do they need to be to not superimpose?
What are some good references for complex, closely spaced footings? For "donut" foundations?
(I refer to them as donut foundations because they are so large, they don't seem to meet the ringwall criteria.)
