Rebound of Sand after Surcharge - Storage Tanks
Rebound of Sand after Surcharge - Storage Tanks
(OP)
I have a project for planned aboveground water storage tanks. Subsurface materials primarily consist of SP soils with decent blows (varies but typically N = 10 to 20 range). Due to the depth of the stress influence zone, anticipated settlements are several inches.
To mitigate this settlement my design called for construction of a fill to apply a surcharge that will equal the pressure of the planned tank mat (w/ overlying fluid). Surcharge will then be removed after settlements have occurred and prior to construction of the tank. My inherent related assumption is that there will be no significant rebound in these cohesionless soils after removal of the surcharge load and that the final tank load will cause no significant settlement.
I'm experiencing some pushback, however, by another engineer who feels there will be significant rebound in the sands after removal of the load. I grant that the "elastic" settlement in sands is part permanent movement in the particles and part true elastic compression which would rebound (I view it like loading up a big pile of marbles). But my feeling is that the truly elastic movement and related rebound component is small. I've done some searches to find literature discussing the mechanics of settlement in cohesionless soils and rebound potential but haven't found much.
Does anyone have thoughts/information on how much rebound (i.e. % of total settlement) is likely under this scenario? Thanks.
To mitigate this settlement my design called for construction of a fill to apply a surcharge that will equal the pressure of the planned tank mat (w/ overlying fluid). Surcharge will then be removed after settlements have occurred and prior to construction of the tank. My inherent related assumption is that there will be no significant rebound in these cohesionless soils after removal of the surcharge load and that the final tank load will cause no significant settlement.
I'm experiencing some pushback, however, by another engineer who feels there will be significant rebound in the sands after removal of the load. I grant that the "elastic" settlement in sands is part permanent movement in the particles and part true elastic compression which would rebound (I view it like loading up a big pile of marbles). But my feeling is that the truly elastic movement and related rebound component is small. I've done some searches to find literature discussing the mechanics of settlement in cohesionless soils and rebound potential but haven't found much.
Does anyone have thoughts/information on how much rebound (i.e. % of total settlement) is likely under this scenario? Thanks.





RE: Rebound of Sand after Surcharge - Storage Tanks
RE: Rebound of Sand after Surcharge - Storage Tanks
I really don't know the timeframe for tank placement after removal of the surcharge. I think, however, that any elastic rebound would happen almost immediately after surcharge removal and is therefore something I have to plan on regardless of timing.
I might be misinterpreting your intent but I generally don't agree that the rebound will be recompressed during construction resulting in no significant long term settlement. If there really were significant rebound (for example say 2"), it will not recompress until the water is placed into the completed storage tanks. This recompression would then be excessive settlement affecting my structure. Again I doubt there is much rebound.....but if I'm wrong, it will have an impact.
Thanks for the input.
RE: Rebound of Sand after Surcharge - Storage Tanks
Terzaghi, as with all things geotechnical, addresses this. Check his book as well as some of the other classical texts such as Sowers.
RE: Rebound of Sand after Surcharge - Storage Tanks
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f-d
¡papá gordo ain't no madre flaca!
RE: Rebound of Sand after Surcharge - Storage Tanks
RE: Rebound of Sand after Surcharge - Storage Tanks
I agree there's a chance the settlements may not be a problem. I think my calculated totals are pretty conservative and will be less in reality (wish I had some dialtometer testing to better estimate). But my design was performed very early in the project when many specifics of the tank weren't finalized. Structural designer wasn't sure but thought it likely that a concrete ringwall would be used. My main concern was for differential settlement at the edges that could cause problems for the ringwall and connections between the bottom and sides of the tank.
The subsurface materials are free draining and therefore surcharge settlement will pretty much be immediate (so no significant construction schedule impacts). Plus they are bringing material on site for use as fill in other areas (so no significant extra material import costs). Basically my thought was "since there's not a significant cost or schedule impact why not temporarily stockpile the imported fill material over the tank location and help eliminate the potential for settlement problems". And especially given the uncertainties over the tank type, etc at the time of design this seemed to be a no brainer.
I did some more looking on the rebound issue and found a good discussion in Fang about stress history (he indicates if a granular material is loaded, unloaded and then reloaded, the reloading modulus can easily be 5 to 30 times that of the original loading.....which to me is the same as saying that there is little rebound). Mitchell also had some information discussing the topic which gave me the general impression the true elastic behavior is mostly at small strains and the larger settlements are primarily plastic type behavior.
Thanks again for the feedback.
RE: Rebound of Sand after Surcharge - Storage Tanks
RE: Rebound of Sand after Surcharge - Storage Tanks
RE: Rebound of Sand after Surcharge - Storage Tanks
Surcharging the site will (may) increase the modulus value, but it won't make it so elastic compression will become moot. If current soil conditions and imposed stresses calculate out to show 10 inches of settlement a preload that results in 10 inches of settlement will not justify zero settlement after the preload is removed.
Get the ASCE publication and learn about settlements below big tanks.
f-d
¡papá gordo ain't no madre flaca!
RE: Rebound of Sand after Surcharge - Storage Tanks
RE: Rebound of Sand after Surcharge - Storage Tanks
I'm really not too concerned that I'll have problems at my site. My calculated settlements (5"-6" range) are relatively small overall for a tank and likely I could get away without the surcharge at all. And I think these settlements are conservative (on other recent projects using dilatometer has cut our calculated settlements in half as compared to SPT correlations). But, considering the very small cost and time to surcharge, I think it's good insurance, helps limit potential impact of various uncertainties in the analysis and limits potential problems with utilities etc. So I was mostly just trying to get a handle on the amount of rebound so I could intelligently discuss with the reviewer.
Thanks also to others for their input.