Re-compression Settlement

[Dhoon]

Dear all,

At one of our project site tank is under hydro-testing. Tank was filled with water and settlements were measured for some duration. Then, tank was emptied for maintenance for one month duration and again filled up to the initial stage.

After filling of water up to the initial stage , settlement exceeded from its previous value by 20%. Since water was filled up to the initial level (same stress on soil as of previous), it was expected that re-compression settlement will occur almost same as of before. Refilling of tank is expected once in a year.

Tank is pile supported and soil in influence zone is clay CL-CI. Don't have any laboratory test data available.

Could anybody explain the reason of excess settlement even stress is at same level as before ?

How we can ensure that settlement in the next maintenance period (re-filling of tank) will be less than previous observed settlement (due to refilling) and after some time recompression settlement will not exceed its value ?

Please provide ant reference material if anybody have.

Regards

 

[SlideRuleEra]

Tank is pile supported...

If the piling rely on skin friction, instead of point bearing, the entire pile group could settle under load more or less indefinitely in deep, poor soils.

There is another possibility, if the piling were driven with a single-acting impact pile hammer that had a certain mechanical defect due to lack of maintenance. This defect will allow inappropriately cushioning of the ram's impact on the anvil, greatly reducing energy transferred to the pile. Correctly recorded blow count will overestimate the pile's true bearing value, the result can be settlement of piling that records indicate were properly driven. This does not happen often, but it does happen - I have seen the results (gross settlement).

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[GeoPaveTraffic]

How much did the tank rebound after it was emptied?

One possibility is that the tank partially rebounded putting the upper portion of the piles in tension. Then when the tank is reloaded the upper portion of the piles have a reduced capacity and the piles are being effectively jacked into the soil. I've seen this a little bit in pile load tests that are cycled, but have never seen it in a tank.

You might want to fill and empty the tank several times and see if the settlement stabilizes.

Mike Lambert

 

[BigHarvey]

Houston, we might have a problem !
There is something wrong in your project but with the information you give it's difficult to tell more. What is the amplitude of th settlement ?

 

[SuperSandman]

What is the accuracy of your survey information? Double check the settlements given by the surveyor and I suggest you try another surveyor to carry out an assessment to compare with your initial survey records.

 

[Dhoon]

Thanks you all ,

Piles are resting on hard strata with N > 40 . Soil type is SM . Thick of the hard strata is about 5-6m . hard strata is resting on a 3-4 m layer of CI-CL soil with N less than 10 (qu about 50- 70 kpa). rebound settlement was about 40% of total settlement.

Settlement of clay layer due to tank loading have already considered in analysis.

How to ensure from field load- settlement data that rate of settlement will decrease with next re-filling and ultimately NO more further settlement.

Although , settlement after first refilling is still less than the total settlement anticipated.

 

[BigH]

Why not give a graph / sketch of your settlements and strata - a lot easier than trying to describe in words

 

[aeoliantexan]

Perhaps the consolidation of the clay was not completed during the first loading period. when the load was re-applied, the consolidation resumed.

If settlement observations were taken periodically during the first loading, a time-settlement analysis might show this. Try running an Asaoka analysis on the data.