Ring wall and Tank Center Differential Settlement
Ring wall and Tank Center Differential Settlement
(OP)
I am designing a ring wall foundation for a 65 fr dia x 65 ft high tank. The soil is clay and bearing capacity value for flexible foundation is 200 KPA with 150 mm allowable settlement. However for ring wall bearing capacity value is 70 KPA.
My problem is that for the center of tank using the liquid height x by density of water, we are using flexible foundation bearing capacity having pressure close to 200 KPA and for the same having settlement close to 150 mm . The ring wall is sized to have bearing pressure less the 70 KPA allowed, therefore for the same the settlement is around 45 mm.
The differential settlement is around 105 mm which is quite high. The dia of the tank cannot be increased due to space constrains.
This is my first Ring wall design, can any one tell me what i am doing wrong. It may be elementary but there must be something i am doing wrong.
One of my seniors say's that the ring wall will settle the same as the center despite having a large base. Is it the case?
Regards,
Casa
My problem is that for the center of tank using the liquid height x by density of water, we are using flexible foundation bearing capacity having pressure close to 200 KPA and for the same having settlement close to 150 mm . The ring wall is sized to have bearing pressure less the 70 KPA allowed, therefore for the same the settlement is around 45 mm.
The differential settlement is around 105 mm which is quite high. The dia of the tank cannot be increased due to space constrains.
This is my first Ring wall design, can any one tell me what i am doing wrong. It may be elementary but there must be something i am doing wrong.
One of my seniors say's that the ring wall will settle the same as the center despite having a large base. Is it the case?
Regards,
Casa





RE: Ring wall and Tank Center Differential Settlement
By the way, 150 mm settlement is too high and may induce tilt. Your geotechnical engineer can run the settlement analysis for you. Ring wall will only settle the same amount as the center if you design it that way and not automatically.
RE: Ring wall and Tank Center Differential Settlement
Regards,
Casa
RE: Ring wall and Tank Center Differential Settlement
It would be nice if you had given the undrained shear strength; you have indicated a bearing capacity of 200 kPa (note k is not capitalized - capital K means Kelvin degrees).
Your ring wall will undergo more settlement than just the amount you compute for a footing on clay as the soil underneath the ringwall is also being consolidated by the flexible tank loading. What is the tank center settlement? (150 mm?). If so as you have indicated, the edge of the tank, using flexible elastic theory (Milovic) would be very roughly estimated at about 70 mm (about 40% of the center settlement).
Personally, I would put the tank on "tank pad" consisting of a well graded sand and gravel base (crushed road base course) - compacted to 95% modified proctor - say 2 m thick. Extend the material at least 3 m beyond the tank edge. - and do away with the ring wall. For a flexible tank bottom, differential settlement over the radius of say 80 mm/10,000 mm is approximately 0.008 angular distortion - which isn't that "large" for a flexible structure - remember that most angular distortions (differetial settlement) are for rigid or semi-rigif foundations or frame structures. At least this is my opinion.
RE: Ring wall and Tank Center Differential Settlement
Have you asked a tank supplier what kind of settlement their tanks can tolerate?
RE: Ring wall and Tank Center Differential Settlement
Steel tanks do have spec for max settlement. A good paper on the subject : "Criteria for settlement of Tanks" by W A Marr, J A Ramos and T W Lambe, ASCE Journal of Geotechnical Engineering, August 1982.
RE: Ring wall and Tank Center Differential Settlement
RE: Ring wall and Tank Center Differential Settlement
http://cedb.asce.org/cgi/WWWdisplay.cgi?41728
+1 on BigH's post too.
f-d
¡papá gordo ain’t no madre flaca!
RE: Ring wall and Tank Center Differential Settlement
To determine your road base or crushed gravel thickness, take your uniform applied stress and apply Boussinesq stress with your 65 ft circle diameter and see for example the stress at -4ft. Then get the consolidation characteristics of the soils and see if the settlement in the center is 1.0 inch. If not, keep increasing gravel thickness until you hit it the 1" mark. If your calculations show unrealistic gravel thickness, trick the soils. Use 2 ft of low strength concrete under the tank topping sand layer and then gravel below that for few feet before the Clay subgrade. This will create a 2 layer system whereby the upper layer is more than 10 times as stiff as the underlying layer & this will drastically reduce your settlement in the center. The low strength concrete layer will in effect bridge it. If you do this 2 layer system, use Westergaard stress distribution and not Boussinesq since the "soils" are no longer of uniform consistency.
You may try http://www.soilstructure.com/settlementanalysis.ht... or ask your soils engineer to assist you in settlement computations.
If the Clay thickness is deeper than 50 ft, then liquefaction and seismic settlement will not be a concern. However, if Clay is only upper 20 ft then check for dynamic or seismic settlement for the soils beneath the Clay. Good luck.
RE: Ring wall and Tank Center Differential Settlement
Regards,
Casa