tank foundation 1

[coopernik]

Hello,

I am dealing with foundation of a large oil tanks. Could somebody give me a little piece of advice regarding ring wall foundation for tanks? Ring wall should supported loads from shell and roof and prevent lateral displacements? So it transmit loads from construction to grade only? What is the advantage of ring wall type of foundation? How to prevent distortion from differential displacements of subgrade?

Regards

 

[SlideRuleEra]

Download the paper from my website that is referenced in thread256-163185

http://www.SlideRuleEra.net

 

[BigH]

Coopernik: It is a good paper, but read it carefully looking for his comments on the need for a ring wall. It seems to have become "standard" to use concrete ring wall foundations (see many threads on the subject at this site). Boberg points out that it isn't absolutely necessary (in most cases (my words) but he likes them because of a few points. I have been personally involved with nearly 30 tanks or more some as large as 50m diameter; I have reviewed several hundred tanks our company has done and have talked with a soft soil expert in India on the need for ring walls. I have never seen them used - in my experience or review. We usually just put the tank on a gravel pad some 5 ft thick or so, top it off with 6 inches of crushed fines for leveling and place the tank on it. This has been done successfully on hard soils, and on firm to soft soils. In one case, where we had variable fills, we did make a crushed stone ringwall - but not concrete.

 

[cvg]

I guess it comes down to local practice and conditions, as every steel tank I have designed or seen in Southern California does have a concrete ring wall. These all being water tanks ranging up to 5 MG.

 

[MRM]

BigH,
I'm interested in the soft soil sites you've mentioned. Have you got any (or know of any) relatively detailed case histories for the sites with large tanks on soft soils, particularly those with settlement monitoring, or pore pressure monitoring during loading?

 

[BigH]

Actually I know of several case histories - but have no access to them from where I am. I will try to scour a bit - but if I remember correctly one of the good ones was done by an Italian set of authors. Give me some time and I'll see - the proceedings was of some specialty or regional geotechnical conference. Our company had done a large one in Hamilton Harbour area of Ontario - it was quite soft soil. It was built and had no problems that I am aware of - but I doubt any monitoring was done. I suggest, as I have done in the past, Bjerrum's paper on a tank failure in Norway that was presented at the London (1957) conference of ISSMFE (or whatever it is now called).

 

[coopernik]

my questions comes from responsibility for decisions, I am a quite young geotechnical engineer (means experience from mathematical modeling field mostly). I wolud like to obtain answers consistent with real behaviour of construction.

at this particular site, I am involved in computation of settlements. It is a tank field. Four of them already exist without any problems or danger. Another two are under construct now. Contractor wants to put tanks on asphalt- concrete pad. More, he wants to put concrete ring wall adjacent (outside) steel shell to prevent lateral deformation.

Diameter is 50 m, volume 125.000 cubic m. Profile consist of fills (up to 3m, in the past it was a site of gravel/sand pit) then 2 m of sand and gravel, and then marl rocks. Fills are unsuitable for foundation purposes, so it were excavated and changed by more suitable fills. Marl rocks are highly weathered from the top.

I performed computation and the output are: about 15 cm of settlements around the center of the tank. (limits are 10 cm, so ups..I am in trouble) Settlements under the shell are smaller.

It is sound ok or not?

Thanks a lot for discussion.

 

[jdonville]

coopernik,

I am a bit surprised that the previous fill material was replaced before the settlements were computed and an appropriate foundation material/geometry could be selected.

Jeff

 

[coopernik]

jdonville,

from the in-situ test we obtain low value of deformation of fills, so it was better to replaced local materials by more suitable.

 

[BigH]

Sounds to me like your contractor is on a cost plus basis contract and he'll get paid for whatever he can talk you into. Three m of fill is peanuts for such a large tank - draw it to scale. I don't really think given the stratigraphy that you have presented that (1) you need a concrete ringwall to prevent lateral spreading (2) given that you have marl "rock" that your settlement will really be that high - and 10 cm is not to my way of thinking even close to critical as far as settlement of tanks go (3) you could, if you think it necessary, use a gravel ringwall under the shell replacing the fill - and if your fills are so settlement sensitive, excavate say half of it, pound the day lights out of the remainder with a 10 tonne vibratory roller and then replace using the excavated fill with proper compaction assuming that the fill isn't "garbage". As I say, I don't see 10 cm being 'critical' in the centre of the tank - remember that the centre of the tank will settle more than the outside edges (flexible foundation on elastic medium). Need, though, to use flexible connections to the tank for liquid filling/draining.