Fill surcharge on soft clay
Fill surcharge on soft clay
(OP)
A site consists of a soft clay deposit about 10 to 30 feet BGS. The ground water elevation is about 8 feet BGS. Dewatering is not an option for environmental reasons and therefore adequate removals can not be performed. An areawide fill 20 feet in height is proposed. Static settlements are anticipated to be about 8 to 10 inches.
The obvious option is to build up the fill and let it sit until primary settlement is complete. Because of poor drainage, the time frame is unacceptable.
My question is: if an additional surcharge equal to about 10 feet of fill is added to the ultimate design height of 20 feet, can that additional 10 feet of fill be taken off before primary consolidation is complete, but after the anticipated settlement for ultimate conditions is exceeded?
Thanks for all the help.
The obvious option is to build up the fill and let it sit until primary settlement is complete. Because of poor drainage, the time frame is unacceptable.
My question is: if an additional surcharge equal to about 10 feet of fill is added to the ultimate design height of 20 feet, can that additional 10 feet of fill be taken off before primary consolidation is complete, but after the anticipated settlement for ultimate conditions is exceeded?
Thanks for all the help.





RE: Fill surcharge on soft clay
Depending on the clay and the structures on the fill, secondary consol might be an issue.
RE: Fill surcharge on soft clay
Just to add to this excellent post - there is also elastic compression to consider. Just because you mitigate primary consolidation via a surcharge, you should also have some sense of the soil modulus to evaluate how much elastic compression may be realized by the new construction. I also agree with running the calculation for secondary compression.
f-d
¡papá gordo ain’t no madre flaca!
RE: Fill surcharge on soft clay
RE: Fill surcharge on soft clay
One of my concerns was: if the surcharge is removed prior to the end of primary, will the remaining excess pore pressure that developed from the surcharge load (which may still be significant) still produce settlement as it dissipates at the reduced ultimate load?
DGillete, how can degree of consolidation with depth be estimated at a given time?
RE: Fill surcharge on soft clay
One can also measure excess pore pressure. I don't know whether a properly sealed standpipe piezometer would work well, but in my younger days, dealing with soft, plastic mine and dredge tailings, we had some electronic probes we shoved in by hand. They would generally settle down within an hour even in very plastic material. Could also try that with CPTU if you can afford to tie up a rig for an hour, which probably costs more than a jr. engineer sitting in a row boat watching the readout box for an hour. (No guarantees are made.)
RE: Fill surcharge on soft clay
You can use the graph of consolidation v. Z as depicted in DM 7.1-233 (it's labled "Figure 14"). If you just have one-way drainage (i.e., at the top), you have the condition depicted in the upper half of the figure. You can then determine the excess pore pressure distribution for various times. Bear in mind when you remove the surcharge it has the effect of shifting the right axis to the left. If you are going for 90 percent consolidation you can then just use a graphical approach to see how much excess pore pressure remains after you shift the axis.
Not sure I helped, but this works.
f-d
¡papá gordo ain’t no madre flaca!
RE: Fill surcharge on soft clay
RE: Fill surcharge on soft clay
RE: Fill surcharge on soft clay
This worked GREAT!
f-d
¡papá gordo ain’t no madre flaca!