Consolidation Settlement Analysis on c-phi soils 4

[pelelo]

I am working on project in which the soil mass is composed of clays (CL), sandy silts (ML), silty sands (SM) and silty gravel (GM).

All these soils are below water table.

I am trying to analyse consolidation settlements up to a depth of 2B.

I understand granular soils (sands, gravel) will not experience consolidation settlements. But what about silty sands and Silty gravels?. Both have high silt content (20 - 40%), so I guess a very minimum consolidation settlement would occur on these soils.

Since I have never dealt with this, I was wondering how to correlate properties of Cc, Calpha and Cs for analyzing consolidation settlements on these soils?.

It is clear to me that sandy silts (ML) would be considered as clays.

Please let me know.

 

[BigH]

I doubt that you will see noticeable "consolidation" of the silty gravels or silty sands as it is most likely your dead loads will be applied slow enough to forgo the effect. To determine what effect if any, you may have to do a consolidation test (large diameter mould, etc.) - best to view some of the settlement procedures that have been put forth that do deal with silty sands/gravels. To be honest, I've never worried about it or taken it into account.

 

[FixedEarth]

For the granular soils, I have looked to the SPT values. If they were above 15 uncorrected in the upper 20 ft, static settlement was not a concern. If you are in a seismic zone, of course you will need to determine dynamic settlement and liquefaction.

NAVFAC design manual has a vertical subgrade modulus approach for the settlement of granular soils. There is one equation for continuous footing and another for a square foundation.

For the ML, if the Plasticity Index is say 8 or more, you can estimate the Cc from 0.009*(LL-10)or perform a consolidation test.

Try to get a hold of "Manual on estimationg soil properties for foundation design", 1990, by Mayne and Kulhawy.

Recall that the 2B depth is good estimate for square shaped footing & for wall footings, you need to be near 5B depth.

http://www.FoundEng.com

 

[fattdad]

use modulus values to evaluate the settlement (immediate) of the granular soils. There are SPT correlations to N-value.

f-d

¡papá gordo ain’t no madre flaca!

 

[pelelo]

Thanks fatdad,

Actually I was looking for some cc and cv values for sm and maybe gm soils (due to the high fine content, 25 - 40 percent). Under this high fine content some amount of consolidation settlement will take place.

I understand about elastic settlement, but due to the soil condition (shallow water table, compressible soils and granular soils with high fine content), consolidation settlement will be more critical than elastic seTtlements.

 

[BigH]

I doubt it . . .

Silts do have a relatively "high" permeability which is why they are the most troublesome for frost heave (with given high capillary potential).

As I said before, your loading rates will likely be such that the material can be considered as "elastic" without separately determining consolidation. Checking chart for use with Schmertman techniques, it is apparent that Schmertman accounts for settlements of silts using his technique (see:

http://140.194.76.129/publications/eng-manuals/em1110-1-1904/c-3.pdf)

(page 3-9/3-10)

 

[pelelo]

Ok,thanks for your reply.

I will keep that in mind.

One more question?,

What about if instead of silty sand and silty gravel, I have clayey sand (sc) and clayey gravel (gc)? (With high clay content, 30 - 40 percent), and of course below water table.

 

[fattdad]

Step 1: Evaluate whether your soils are normally consolidated (i.e., are they soft, is the liquid limit close to or at the natural moisture content, etc.)

Step 2: If the soils are not normally consolidated, assess whether the time frame for 1-D consolidation will affect your post-construction conditions (you can estimate Cv values or use any lab data that's available).

Step 3: Assess whether you want to use 1D consolidation theory, elastic theory or both to evaluate your settlements. This can be tricky if you have such clayey gravels and such.

If there is little liklihood that the soils are normally consolidated, I'd likely consider elastic compression for the whole works and I'd likely also consider elastic compression for the "coarse-grained" soils and 1D consolidation for the "fine-grained" soils. Using the "gravity turn on" method, you'd calculate settlement without any reference to time or construction schedule. Just see what sort of numbers you are dealing with from the onset.

I'm not sure there are any simple answers. There is just a thought process that we all have to go through to see whether a problem exists. After working on these matters, you can "tell" to what extent there may or may not be a problem that requires consideration of consolidation characterists for the "fine-" and "coarse-grained" soils.

f-d

¡papá gordo ain’t no madre flaca!

 

[grabens]

I have always used the rule of thumb if less than 50% fines, it will acts as a coarse grained material (i.e. elastic settlement). MOre than 50% fines, consolidation settlement. THe gray area is when you hover around the 50% mark. For your soils 20 to 40% fines, I would feel comfortable with elastic settlement method. However, this is because of my experience in the area I practice. The coarse grained material is going to drain fast under loading hence the immediate, elastic theory.