Fill surcharge on soft clay

[kcall2]

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.

 

[dgillette]

Yes, the surcharge can come off before the end of primary, but only if the effective stress throughout the profile has reached what it will be long-term. In other words, assuming there is no drainage at the bottom, the effective stress at the bottom of the layer must have reached a value greater than the original effective stress plus the weight of the fill excluding surcharge; otherwise, there will continue to be consolidation after the surcharge is removed. (If the bottom is a drainage boundary, substitute middle of clay layer in previous sentence.) If you don't wait that long, there can still be additional settlement even if the settlement has reached the ultimate value predicted without the surcharge. This is because the degree of consolidation is not uniform throughout the layer until the end of primary.

Depending on the clay and the structures on the fill, secondary consol might be an issue.

 

[fattdad]

Depending on the clay and the structures on the fill, secondary consol might be an issue.

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!

 

[civilperson]

Wick drains on a small grid spacing will accelerate the consolidation process. Try calculations with various spacings less than half the depth of the clay.

 

[kcall2]

Thank you for your responses. I have modeled a preconsolidation profile and secondary settlements (not significant over 50 yr design life) have been evaluated using laboratory consolidation data at several depths.

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?

 

[dgillette]

With emphasis on the word ESTIMATED, it can be ESTIMATED using the isochrone (equal time) curves that show the progress of consolidation at a given depth, at a given time. See, for example, NAVFAC DM-7. If you have the 1982 version, it's in volume 1, Figure 14, page 7.1-233. The dimensionless time factor T is equal to t*Cv/H^2, with t in appropriate units to go with Cv and H. For explanation, refer to Terzaghi and Peck. (In the 1967 edition of T&P, it's p. 173ff.)

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.)

 

[fattdad]

how can degree of consolidation with depth be estimated at a given time?

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!

 

[dgillette]

Great minds think alike. So do fattdad and I, which may have nothing at all to do with the previous sentence.

 

[BigHarvey]

to complete the very answer from dgilette from a practical point of view, use Asaoka's method to monitor the settlement. It will give you rapidly the real amplitude and time needed to achieve primary consolidation ( remember that the values you have now are just calculated values which can be significantly different from rality ).Checking pore pressure dissipation with specific piezometers would add serenity to your decision !

 

[fattdad]

On the matter of piezometers to gage the dissipation of pore pressure. I worked on a project that included wick drains and surcharge. Prior to placement of the surcharge (after the wicks were installed), I went to the site with some metal piping - 2-in diamter, I think - with a few slots at the end. I duct-taped some filter fabric to the end and just pushed the pipe into the ground (it was a soft site afterall). Got the end of the pipe to my target depth at several locations in the surcharge area. As the surcharge was placed, we were able to see the piezometer elevation and the corresponding dissipation over time. This allowed scheduling of the next lift of the surcharge. We were adding about 5 ft at a go.

This worked GREAT!

f-d

¡papá gordo ain’t no madre flaca!