Tank Foundation recommendation

[geotech888]

Dear All,

I am facing a problem in recommending the foundation system for a tank, which was already in place since last 25 years, recently dismantled for changing the steel of tank due to corrosion. The theory and practical observations at site are contradicting each other.


Site Condition in Brief
1.0 Tank dia 30.0m , H=12.0m
2.0 Top 2.0m is good earth fill (filled up soil)
3.0 The subsoil (marine clay –SPT N=3 to N=7) from 2.0m to 20.0m, 18.0m thick followed by hard clay SPT N - >100).
The preconsolidation pressure (Pc) at depth 3.0m - 90 kPa, @ 4.5m - 80 Kpa and @ 9.0m – 68 kPa.
4.0 The hard clay (SPT N > 50) from 20.0m to 30.0m (termination of borehole)
5.0 Hydrotest load (120 kPa), maximum operating load (108 kPa) is considered for evaluating long term settlement.


Shear failure check with respect to J.M. Duncan paper was observed that Factor of safety (FoS) against Base shear failure is 1.6, whereas FoS for the edge shear failure is 1.4.

Highlight Points / experience of the site condition.

The subsoil strata are consistent throughout the depth and when compared with the second borehole drilled in other location of the terminal.
The stiffness of soil is increasing with depth (Refer SPT N value ranging from 3-7 borelog 1).
No catastrophic failure is recorded in performance of tank and similar tanks in the same location resting on earthen pad foundation since 1987.

Tank settlement profile is determined based on the following factors.

The immediate settlement is evaluated based on elastic theory and is worked out to be about 185mm at centre and 78mm at edge. Differential settlement of about 107mm.

Long Term Settlement
Settlement Centre Edge Diff
Total Settlement at end of 115 years / 90% Consolidation S (mm) 860 476 384 Bousinessq
Total Settlement at end of 25 years / 48% consolidation S (mm) 413 229 184
Total Settlement at end of 115 years / 90% Consolidation S (mm) 720 461 259 2V:1H
Total Settlement at end of 25 years / 48% consolidation S (mm) 346 221 124


Discussion
From above investigation, it is understood that the subsoil is weak and compressible in nature, same would have been still weak before 25 years ago, the site does not have any records of settlement observation in writing, but it is know that settlement used to happen and the tank bottom was lifted when hampered the serviceability condition. The piping connection was kept flexible due to anticipated settlement.

It may be noted that almost 70 tanks in the same terminal of varying diameter and two tanks adjacent to the tank in question is resting on the earthen pad foundation since last 25 years.

Now the question is what shall be the foundation recommendation
1) Earthern pad based on the experience from vicinity tanks. (Providing geogrids/ geotextiles)
2) Stone column foundation (which involves the complete shutdown of adjacent tanks for construction)
3) Pile foundation (Falls under severe Seismic zone, with PGA = 0.36g)


Request my fellow engineers to suggest the foundation recommendation for the given tank.

 

[Ron]

Historical performance is often a much better predictor of future performance than our analytical wild a$$ guesses; however, was the seismic condition considered 25 years ago and has the tank seen any level of seismic event? Based on the soil conditions, liquifaction is not an issue. Do you know if the structure was designed for seismic? When shell replacement was done, was there any upgrade to the tank.

It is going to be a hard sell to go with anything other than an earthen pad, since there is apparently nothing unique to this tank as compared to the surrounding ones. Your credibility will be questioned if you recommend something that is not practicable unless you can justify it through changes in codes or the probability of a future event that has not yet occurred and has to be considered by code now that was not considered at original design.

 

[FixedEarth]

If we take shear strength of the Clay as 5.4C, your allowable bearing pressure would be 65 kPa. The Hydro load test of 120 kPa would then be too high. Your F.S. of 1.6 with respect to shear failure is also what I found but it is rather low. Your calculated settlements also make sense and are too much. Consider drilled pier supported mat foundation.

 

[GeoPaveTraffic]

I mostly agree with Ron. Past performance is an indicator of future performance in this case.

If not for seismic considerations, A tank like the one that had been there for 25 years can be put back and expected to perform the same for the next 25 years. Acutally, performance should be somewhat better since the soils have been preloaded.

No the seismic loading is a different matter. Assuming the site has not seen a design level earthquake in the last 25 years. In this case you would need to see what the expected performance of the tank is under the seismic loading. I would calibraite my model based on the performance during the last 25 years.

All in all, I would be surprised if anything "special" were to be justified.

Good luck.

Mike Lambert

 

[geotech888]

Thank you all for your prompt responses.

Yes the site has experienced a earthquake of magnitude 7.5 on richter scale 11 years back, but no failure or extreme damage is reported at the site terminal.

Yes, as an geotechnical engineer,
I recommend for going with the past experience of providing earthen pad foundations. Though I added to also provide geogrids and geotextiles at base in layers to counteract differential settlement to certain extent.

Can any body help me how to calculate the settlement with geotextile/ geogrids provision ???