Settlement of Water Tank
Settlement of Water Tank
(OP)
I'm trying to estimate the settlement of a steel water tank to be constructed over medium dense sany alluvium, water table at about 13 feet. The tank will about 55 foot diameter and 23 feet high, and will be set on a concrete ringwall foundation. I have a CPT profile to 50 feet. I don't think settlements will be excessive for these conditions, but want to get an estimate to confirm this.
I am considering using the Schmertman (70, 78) method of estimating footing settlement on sand with the strain influence approach and elastic theory, but this method is for a rigid footing, whereas the tank should be flexible.
Another simplified method I'm considering is by Myerhoff (65), modified by Duncan and Buchignani (76). This is a simple formula which uses blow counts and bearing pressure.
Any suggestions are appreciated.
I am considering using the Schmertman (70, 78) method of estimating footing settlement on sand with the strain influence approach and elastic theory, but this method is for a rigid footing, whereas the tank should be flexible.
Another simplified method I'm considering is by Myerhoff (65), modified by Duncan and Buchignani (76). This is a simple formula which uses blow counts and bearing pressure.
Any suggestions are appreciated.





RE: Settlement of Water Tank
www.angelfire.com/mech/slideruleera
Best Wishes
RE: Settlement of Water Tank
I would expect the settlements under the center of the tank to be several inches. You should also consider the effects of other groundwater conditions when comunicating with the owner.
RE: Settlement of Water Tank
RE: Settlement of Water Tank
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: Settlement of Water Tank
A specific example: During construction of a new power plant in 1981, the contactor constructed the ringwall and properly compacted the fill, both inside & outside the wall. The industrial waste tank was erected. During hydro, the tank/ringwall started leaning to one side. After about a week (with the hydro water still in the tank) the lean stopped, with both the tank & ringwall about 15 degrees out of plumb. We had the tank dismantled - under the tank bottom, the properly compacted fill had been contained by the ringwall and had also "tilted" 15 degrees.
The tank/ringwall had been constructed over an area at the edge of a (backfilled) 15 ft. deep excavation for the plant foundation. This was were the excavation had been sloped - one side of the ringwall was on 15 ft. of 6 month old fill (that had not given a high degree of compaction). The opposite side of the ring wall was on undisturbed soil.
My point is that the ringwall "took" the interior confined, compacted soil "with it" during this differential settlement "experience".
RE: Settlement of Water Tank
RE: Settlement of Water Tank
Several years ago I chaired a task group charged with the general revision of the foundation design appendix of a nationally recognized tank standard. There were about 10-15 members, about evenly split between major oil companies and tank contractors. This group had extensive tank foundation experience. It was interesting to note that there was a "use a ringwall" crowd and a "don't use a ringwall" crowd. Both had their reasons, motives and opinions... and there was no way to get anyone to repent of their approach. Needless to say, all types of tank foundations are addressed. Neither side should prevail as it would be foolish to adopt a position to always use a ringwall or never use a ringwall. Personally, I'm convinced that a ringwall is the better way to go, not for geotech purposes, but for maintenace purposes. Earthen foundations seem to have too many edge settlement problems, often related to site drainage.
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: Settlement of Water Tank
Ring wall or no ring wall?? Either works. A high modulus pad with sufficient extension beyond the walls works as well as a ring wall. One reason for using a ringwall is to minimize the undermining potential from erosion.
RE: Settlement of Water Tank
RE: Settlement of Water Tank
Thanks for the comments SteveBraune. It seems that in the various threads in the various forii here, I fell like the only one who questions the need for a ring wall.
For moe333 - don't forget that the centre of the tank will settle more than the edges so the tank should be constructed initially with a slight upwards camber towards the centre of the tank.
RE: Settlement of Water Tank
RE: Settlement of Water Tank
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: Settlement of Water Tank
RE: Settlement of Water Tank
RE: Settlement of Water Tank
The differential settlement due to settlement in the center and the edge must not exceed an allowable value. Slideruleera's case actually sounds like a shear failure of the soil rather than excessive settlements.
RE: Settlement of Water Tank
The bottom plate is normally 1/4" plate and the plate is extended 1+ inch beyond the face of the tank wall. This 1+ inch extensiion + the wall thickness + an equuivalent length of 1+ inch on the inside of the tank is used to carry and transfer the shell loading to the subbase structure. The structure should weigh around 85,000# and have a perimeter loading of 85000/55pi = 175 plf = 14.5 #/in resulting in a loding of approximately 7 psi.
As noted by Steve above, there are ringwall people and there are pad people. Reguardless, the center of the tank will settle more than the edge. We grade the granular backfill of the ringwall sloping from the center to the edge to account for the deflection. As a rule of thumb, we use a 1-inch in 12-foot grade. For your 55-footer, grade the center 2 1/4-inch higher than the edge.
Best Tincan
RE: Settlement of Water Tank
Regarding settlement equations, it seems to me that Harry Poulos compiled a book on theoretical settlement solutions. Here it is;
H.G. Poulos, and E.H. Davis, "Elastic Solutions for
Soil and Rock Mechanics", John Wiley & Sons, 1974.
This might help you, moe333. Hopefully, your library has a copy since it has settlement solutions for flexible circles, circular rings, rigid circles, etc. I don't have the book, but it might have even include flexible circles with rigid rings. However, be aware that while theoretical solutions are not too bad for flexible loads, they have this infinite stress "problem" at the edges for rigid foundations. The latter leads to a (possible) over-prediction of settlement for rigid foundations (or, at least, I've read complaints about the predictions, which may only mean that they are not bad). For semi-rigid foundations, the overall settlement will be somewhere between the max flexible settlement and zero. I would assume the same total displaced volume, as in a boat.
RE: Settlement of Water Tank
I have been reading with interest this valuable post and noted all interesting recommendations.
I have a problem of a different nature. I do not have any experience in tank design but this is definitely a very interesting subject. We have large process water tank, 26m diameter and 7m high, resting on 300mm thick ring wall. I do not have the design calculation of the structure and foundation and can not comment on the adequacy of the structural components, but assuming that they have adequate strength - being done by suitably qualified and experienced people.
The tank has three large water inlet pipes. The site people reported that they are experiencing significant. vibration in the tank when those inlet pipes, pumps are running. Those pipe and pump foundations are isolated from the main tank foundation. I am wondering if any of you have experienced such a situation and what type of action can be taken to remedy such a situation. Is it posssible that this may not be structural engineering related problem also!
Any input will be greatly appreciated.
Kind regards
AM
RE: Settlement of Water Tank
For am - what kind of soils are you on? You say that the pumps, etc, are isolated (i.e. separate) from the main tank foundation - still, the pump pad isn't big enough and is not damping sufficiently. The vibrations may not, then, be isolated from the main storage tank foundation. You may wish, then to truly isolate your pumps via trenching or other "cut-off" measures (See Richart Hall and Woods' book on Soil Dynamics).
RE: Settlement of Water Tank
Rumor has that a lot of tanks you may be referring to may have "bearing plates' under the sketch plate. Is that true from your experience?
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: Settlement of Water Tank
Thank you for your suggestion. Soil is well compacted granular soil, Ø is about 30°. I did not run any numbers yet, but looking at the pump foundation indicates they are quite substantial and will provide good damping. However I do not have the pump details yet and can not confirm my views with calculation. However I am trying to source the Soil dynamics book as per your suggestion.
Regards
AM
RE: Settlement of Water Tank