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ghostbear (Geotechnical) (OP)
4 Aug 10 21:59
thread194-260175: Ring Foundation Design for Water Tanks

Hi, I am working on a project consists of the construction of a 30' x 90' cast in place concrete basin with a precast concrete screen wall. There will be 3 cooling tower cells located within the basin. Basin is 2.5 feet deep and is below the grade.

Tower diameter is 25 feet; tower height is about 35-40 feet. Groundwater is about 6-7 feet below the surface.  Site is underlain by 2 to 4.5 feet of SM soil, as fine sandy clayey soil, or silty sand.  The sandy soil SM is underlain by 50 feet of CL (sandy/silty clay) with thin CH; CL is more compact and less plastic with depth.
The soil boring and lab test (include unconfined compressive test on clay) will be underway in a week.

Is concrete mat foundation 30 ft by 90 ft beneath the water basin a good solution to support the three cooling towers? The mat foundation will be on properly compacted backfill.

What is the mat foundation bearing elevation? I am thinking its bottom elevation being about 6 feet below the ground surface or 3 feet below the water basin. Would bearing elevation near water table adequate?

If not, what is alternative, such as three trapezoidal concrete ringwalls, and sloped inside and covered with a geotextile? The concrete ringwall will be built within the 30 ft x90 ftx2.5 ft water basin.
 
Helpful Member!  SlideRuleEra (Structural)
5 Aug 10 15:51
Keep in mind that the fans and other mechanical gear in an industrial sized cooling tower usually have tight tolerances on differential settlement. Suggest that you confirm if this is an issue for your project. Unless the soils are very good, supporting the basin with driven piling / drilled piers may be necessary to prevent real problems during tower operation.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

BigH (Geotechnical)
5 Aug 10 19:15
He will be getting some relief due to the excavation below grade (load added to mat less soil pressure removed) - given the area of the mat, the likely added loading might be small - and hence the differential may not be all that critical - it does take some good thought, data, computations and judgment.
ghostbear (Geotechnical) (OP)
5 Aug 10 23:14
Dear Sir, So many thanks for your comment. The soil boring log and blow count shall be available tomorrow.

UC test and consolidation results will be back about a week. From the nearby boring logs, the soil is SM silty sand from surface to 6 feet below underlain by CL. I will check the settlement with new lab data and consider water loading replacing soil loading within the water basin .

Shall seismic design be concerned for the large mat foundation, adding driven piling / drilled piers?

I am wondering what is coefficient of friction between concrete and SM or CL soil ?

Is allowable foundation bearing pressure of 3,000 psf a little high?

Any reply is appreciated.
 
SlideRuleEra (Structural)
6 Aug 10 13:02
If your structure is in a seismic area, then designing for seismic loading should be performed whether you use a soil supported mat foundation OR piling / drilled piers.

If you do decide to use piling / drilled piers the allowable soil bearing pressure is not important for the completed structure; the piling / drilled piers should be designed to carry ALL (100%) of the vertical loads. It is a big mistake to have a portion of the loads from a structure supported by soil and the remainder supported by piles / drilled piers.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

ghostbear (Geotechnical) (OP)
8 Aug 10 17:12
Dear  SlideRuleEra Thanks so much for your great tips. I got more info for cooling towers.  They are 24 ft by 24 ft counter flow tower with concrete casing and FRP internals. These towers are field erected type. Yes, there are 60HP fans and a 1440 gpm pump. The water basin is 40 ft by 72 ft by 2.5 deep.  

The city 50 mile south of the site had an earth quake in 1947 scale 4.6, this earthquake damaged chimneys in that city, but was felt as intensity II near the site.  The boring shows SC (sandy clay) as the dominate soil, and no water table even at 40 feet depth.

 If piles are to be drilled, what is the depth? I am thinking to use steel H pile.  Hope to use Lpile software for a quick calculation.. the dead load is about 2600 ton.

The real water table is 70 ft below the grade, I am thinking settlement should not be a concern since it is the fully saturated clay stratum, in situ, that displays the lowest strength and greatest compressibility. Beside, the mat foundation is 72 feet wide. However, I will use lab's one dimensional consolidation test result for the settlement calculation.
 
SlideRuleEra (Structural)
9 Aug 10 10:56
ghostbear - IMHO, you should work closely with ;other geotechnical, structural, and mechanical engineers who have the experience with this location, type structure, and equipment. The specs to have a cooling tower work correctly are very exact and you really only get one chance to get it right. Engineering Tips is great for general questions, but to work out the exact details a high level of understanding and involvement with the project is needed. I don't believe that this can be done successfully by using just the internet. Good luck with your project.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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