tank foundation in fill

[carmoch]

Hi, I have a problem. We have to propose a solution in a area which is gained to the sea. There is a sector which is constructed 4 oil/water tanks (with a load of 1,2 and 2 kg/cm2, respectily).
The fill is composed of gravel and sand (USCS, GP-GM or GW-GP),mainly. It has a average thickness of 22 m, and have found areas with gaps due to poor compaction (less than 1 m).and the natural substrate is formed of sands (Nspt between 12 and 40).

Initially I suggested an improvement of the land with the construction of piles to natural terrain, but I think it could pose a foundation by micropiles and / or jet grouting (filling the gaps with injections of compensation). This treatment may leave the field or the field should always take natural?
On the other hand, I have other structures with the lowest load (0.4 to 0.7kg/cm2). Could raise a replacement ground only? Do you suggest?
Thank you very much.

 

[CarlB]

You should investigate in-place soil improvement, you will likely achieve satisfactory results at much lest cost than pile foundations.

I've seen some large tanks placed on reclaimed ground, that was first improved to 30 meters with sand-compaction piles (a standard Japanese technique, similar to stone columns but with sand). Deep dynamic compaction may be effective with those GP/GW soils.

Also deep vibratory compaction:

http://www.vibroflotation.com/

http://www.fellenius.net/papers/266 Deep vibratory compaction of granular soils.pdf

 

[BigH]

You haven't really indicated much in the matter of details of the fill - you indicate "gaps" due to poor compaction - any details on the N values in these "gaps"? What is the magnitude of the estimated settlement for the tanks given "the poorest conditions possible" and "the best conditions possible." Take a look and note if your tank design can handled such settlements. Tanks on built up granular tank pads have undergone fairly large settlements in past case histories and you can handle connections by the use of flexible connections. Given that the tanks are founded on free-draining granular materials, the settlements will be built out during the test filling of the tanks. Before recommending ground improvement, prove to yourself that it really is necessary.