Bearing capacity of a high plasticity silt (MH) 4

[Atros]

Hi guys, what would you suggest to use for determing bearing capacity of shallow foundations on a high plasticity silt? The in situ permeability test were about 1E-04 cm/s. Water table is about 4 meters deep.

Would you use drained or undrained analysis? The results from the undrained triaxial tests where fricion angle=27 and cohesion=25 kPa. The undrained resistance is Su/r0=0,3

Thanks for your help.

 

[fattdad]

I'd check all that stuff, for sure. I'd likely determine the allowable bearing pressure on settlement; however. Did your run a consolidation test?

f-d

ípapß gordo ainÆt no madre flaca!

 

[dik]

any capillary rise with the WT? would have thought it would be saturated to near surface. With highly plastic material there can be a volumetric change with change in moisture content... about 20 years back I did a bunch of investigations into foundations that had 'settled' to find that it was high plastic and there had been several 'dry' years.

Dik

 

[Okiryu]

Based on the UU triaxial test results, appears that the layer where you took the sample is unsaturated. Perhaps you took the sample within 4 m below ground surface. Your footings will be installed in this layer. But, is there any chances to these soils get saturated as dik mentioned? I would check the allowable bearing pressures based on settlement considerations as fattdad suggested. If your footings are small so the depth of influence of stresses is shallow, you may be able to calculate elastic settlements using the approximate modulus from the stress-strain curves of the UU tests.

 

[fattdad]

(In my mind it seems impossible to attain saturation through capillary rise.)

f-d

ípapß gordo ainÆt no madre flaca!

 

[dik]

With fine grained highly plastic silt, I'm not sure... or maybe that is the top of the capillary rise. I assume the WT was determined by using tubes and left for a period of time.

Dik

 

[L0k]

Hi Atros
What is your footing width? If the fluctuation of ground water depth is outside of the failure criteria, the undrained parameters may be applicable. Since the soil is MH- a high plasticity silt, an undrained insitu test such as shear vane test may be considered as a design parameter. I believe that the insitu test is less disturbance than the laboratory test. Lok.

 

[Okiryu]

I found this for capillary rise:

http://scholarworks.wmich.edu/cgi/viewcontent.cgi?article=1713&context=masters_theses

I didn't read the entire thesis, but just looking at the conclusions, appears that the available equations to predict capillary rise for silts underestimate the actual values measured in the lab.

I will also check my notes from Prof. Stark's class. I recall he provided an equation for capillary rise as well.

 

[Atros]

Hi all, thanks for your help.

L0k, im considering a 1x1 m square foundation.

The unconfined compresion test of a not perturbated sample has this results:

-Undrained shear strength, Su: 0,32 kgf/cm2
-Undrained shear strength of remolded sample, Su: 0,89 kgf/cm2

Consolidation test (see attached image)

Tension (kgf/cm2) Void ratio
0 1,472
0 1,478
0,1 1,459
0,25 1,442
0,5 1,425
1 1,399
2 1,352
4 1,276
8 1,176
2 1,184
0,5 1,196
0,1 1,196
0 1,203
2 1,184
0,5 1,196
0,1 1,196
0 1,203

 

[L0k]

Hi Atros

The undisturbed sample will create the the undrained Su > the remoulded strength, it was not happening in your strength where Su=0.32kg/cm2 < Sr=0.89kg/cm2. This showed that your sample is having a certain disturbance, that's why I prefer the in-situ strength. I cannot calculate your ultimate bearing capacity.
Your consolidation test gave the Pc value is about 1.2kgf/cm2. I guess the Po+dp value will be in the elastic condition. So the Cr value will determine your settlement.

 

[Okiryu]

The residual clayey soils in my area also show higher remolded unconfined compressive strengths than the undisturbed samples strengths. My soils are unsaturated overconsolidated clays. This is interesting. I am not really sure about why this is happening and am still researching on this.

Anyways,for your footings which are 1x1 m and assuming that the Df is about 1m, your footings may not be greatly impacting the saturated silts that are at 4 meters below grade (depth of stresses influence for square footings for 10% of stresses at approximately 2B: 2m in your case). So I would use the elastic modulus from your unconfined compressive test and calculate elastic settlements of the silt layer. Again, consolidation settlements may be very small since the stresses for the saturated layer are low.

 

[LRJ]

I've only ever encountered soil sensitivity (s[sub]u,undisturbed[/sub]/s[sub]u,remoulded[/sub]) less than unity when the soil had notable structure. The fabric/structure of the intact soil can result in weaknesses to loading in certain directions (and also extra strength when loading in some other directions). In such cases the process of remoulding can remove these weaknesses associated with the soil fabric/structure.

If there is no notable structure or fabric to the intact soil, then it may be the case that the testing had some issues. Was the remoulded test performed on the same sample with the same density and water content?

 

[BigH]

You cannot really apply the concepts of sedimentary soils to that of residual soils.

 

[BigH]

 

[Atros]

Thanks for your answer. I think the material is from volcanic ash (alofanic) and thats why the remould resistance is bigger than the indisturbed.

The in situ density test gave this results:

Wet density 1,65 t/m3
w% 48,20
Dry density 1,11 t/m3.

The structure is a house with no big loads. What do you think? Remove 2 meters of material and use a structural fill?

Thanks.

 

[Okiryu]

Attached is some notes about capillary rise for reference.

pelelo, 2m of overexcavation and structural fill may work but looks expensive for a house. I think that you may be able to work with the soils you have but you need to check settlements and so on. If settlements are too high, try with 1 m of overexcavation and structural fill and see how it goes.