Downdrag on soft clay 1

[longisland]

Hi,

I'm constructing a piled retaining wall over a soft ground. The borelog shows the first 30ft is very soft clay with plasticity index of 40 & average SPT of 2. The water table is 1 ft below the existing ground. I will raise the platform level by 4ft with compacted fill. I've referred to Bowles on downdrag. The method suggested is beta method. Since no triaxial test was conducted, is it sensible to use the chart in Bowles chapter 2 to obtain the phi value from plasticity index? I'm assuming undisturbed clay. The phi is roughly 24. The problem I have with chart is it does not indicate whether it's soft or stiff clay & hence a lot of guess work. The other issue I have is the density of the soil. The bulk density 1s only 80ib/cu.ft. If the density of water is deducted, that leaves 20lb/cu.ft.
Is there other better way to compute the downdrag?
Another way I tried is by using SPT N value but I don't think it's a correct approach since I'm dealing with peat.
I hope some of you folks can shade some light here

 

[fattdad]

You may also need to look at the undrained shear strength of the soft clay (i.e., phi=0 analysis). For the case of an undrained shear strength of 400 psf, you would just take the exposed surface area on each pile and add up what the total downdrag is likely to be, assuming that the interface shear fully transfers the undrained shear strength of the soil (this may be conservative).

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

From SlideRuleEra's site - he has a number of papers from an old publication on wood piles (most of which were in the New Orleans area). This gives a good practical talk about downdrag. Of course, you can always go to Tomlinson's piling book (better than Bowles) or Fellenius' web site.

 

[longisland]

Hi,

Thanks for the feedback. In my case, my problem is there is no shear strength test conducted. My next problem is how would you go about calculating the pile base capacity without any shear strength parameters? The soil at the pile base is a medium stiff clay with SPT N=25. I've read an article that suggested qf = 200N in metric unit, where N=SPT N value. Has anyone come across this equation? It's similar to the value suggested in Bowle's where qf = 400N for sand.

Thanks

 

[civilperson]

Bulk density is the weight of the soil divided by the volume. If you know the specific gravity of the saturated soil or the void ratio then you can calculate the amount of water in a unit volume.

 

[DRC1]

If you go to Fellenius's website he has several papers on downdrag, which occurs in all soils and only effects the structural capacity of the pile. It is the frictional capacity of the soil above the neutral plane.

Is the material soft clay or peat? If the material is soft clay, saturated total density should be around 120 a Peat could be 100 but 80 is low, may be a moist density. Even a little low for that.

SPT values and index values are not useful unless you have a lot of local knowledge about the material. Omn that note you may want to consult a local geotech as they may shed some light on the material. Tubes and strength testing is the only way to test soil strength. SPT's in soft clay can be unreliable. Beta method is a drained analysis. For a soft clay, an undrained (alpha) may be more approriate.
If the material is peat, all bets are off. Develop your capacity below the layer.

Although there are correlations and some "Rule of Thumb" equations out there, unless you have some previous knoledge of this material, these aids can get you an answer, but I don't know if it will be accurate.

 

[longisland]

Hi,

The soft layer is peat & hence the low density & high plasticity index. I'm worried the piles may buckle in the peat layer because the piles are supposed to resist the sliding force on the wall.

 

[Doc08]

Indeed you may calculate the pile capacity when embedded in Clay taking advantage of the SPT values. There is an FHWA maunal describing such a procedure which is based primarily on the works of Reese. You could also check this site

http://www.tsoftonline.com

as they have a software based on this procedure.

Basically for piles in clay, Cu = 10.N, Kpa ; and Qb=Cu.Nc.Ap, and for side friction, fs = Cu.Alpha, where Alpha varies. Naturally an appropriate factor of safety is required to determine the allowable.

I hope that helps.

 

[longisland]

Hi,

Thanks for the reply. Essentially, you are suggesting to substitute Cu with 10N & the equation would then be Qb = 10N x 9 x Ab for the pile base capacity. I hope I read you correctly

Thanks

 

[Doc08]

That is right.

 

[longisland]

Hi,

Thanks for the reply. Where can I find the literature of this approach?

Thanks in advance

 

[Doc08]

Sorry logisland, I am out the office and I do not have the reference on me. If memory serves me right it is FHWA manual HI-88-042 (Reese and O'Neil).

good luck