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
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.