Loading & Unloading problems

[papuinuk2009]

The problem is consolidation of 21m thick fill under 3m high surchrage for 6 months.Settlement markers & Piezometers were installed in the fill material for monitoring in this period.The sketch showing the problem geometry and monitoring points is enclosed.

After 3 months it was found though there are lots of settlement (110-25mm)just below the points of surcharge. But points in the fill material has undergone very less movements.Monitoring the piezometer's it has been found that only 7%-10% consolidation has occured effectively.The enclosed file shows the movements & change in porewater pressure for the monitoring points with time.

Now it has been proposed to remove the surcharge.I am wondering if there be quite large rebounds in the ground.

If anyone has any ideas on this matter, please share.

Thanks and Regards
Debasis

 

[InDepth]

What was the intent of the clay surcharge? Usually I see large mounds like these when you want to consolidate the soil and then building a structure of equivalent weight (foundation). The real question is how long will it take to rebound & what is the initial rebound on removal.

Do you have more details?

 

[BigHarvey]

If I understand well your sketch, the vertical drains have been installed through made ground. Vertical drains will have very little effect. You will have more elastc settlement than consolidation.

As a first approach the modulus when reloading will be twice the original one as a minimum ( see pressuremeter's theory ), therefore the settlement will be have the settlement that would have happened without surcharge.

If you have settlement data you can run an Asaoka's approach to get an idea whether there was consolidation or not.

The best approach to treat the problem was not drain and surcharge but a displacement based ground improvement method ( stone columns, displacement piles or compaction grouting )