Soil properties 1

[mitsiou]

Hi everyone

Could you please help me out with definitions of the following:

Submerged Density;

Drain and Undrained condition;

and in practice what state of the soil do the describe

Thanks

 

[lovethecold]

Submerged density and saturated density are the same thing, when the soil is at 100% moisture content (ie: below the water table).

A drained condition would be a soil above the water table with no appreciable pore water pressure. Undrained would be below the water table, this is the condition where soils are at their weakest. These are the two extremes in soil mechanics, in reality most soils will fall between the two. Most of the time we consider a soil to be in the undrained condition (in my area the soils at 5 feet are typically saturated anyway). In my experience, sand with a very low water table would be the only potential exception.

Hope this helps.


- EIT Geotechnical Engineer. I am here hoping to help, and learn a little along the way.

 

[moe333]

There really is no strict definition, but submerged density may also be referring to what I describe as buoyant unit weight which is the saturated unit weight minus the unit weight of water.

In terms of drained and undrained condition, I believe you may be referring to shear strength of the soil. The drained strength condition can occur with soils below groundwater provided the strength is developed over a sufficiently slow period of time where excess pore pressures do not develop. So a clay soil below the groundwater can have a drained strength. Generally an undrained condition is the critical condition for soft clays, and the drained condition is critical for stiff clays.

Shear strength of sandy soils are typically treated as drained even below the groundwater since their permeabilities are relatively high so that excess pore pressures do not develop. The exception is if there is quick loading such as an earthquake. In this case, the loading is so quick that excess pore pressures are developed in some loose sandy soils and these would be considered undrained.

 

[lovethecold]

moe333
"...and the drained condition is critical for stiff clays."

- Would you explain this one please.

"The drained strength condition can occur with soils below groundwater provided the strength is developed over a sufficiently slow period of time where excess pore pressures do not develop."

- How can you tell if a soil below the water table is in a 'drained' condition?

The rest of what you say makes perfect sense. Thanks.

- EIT Geotechnical Engineer. I am here hoping to help, and learn a little along the way.

 

[moe333]

Stiff clays have high undrained shear strength, as tested with TX, or unconfined compression test, therefore the factor of safety when using this strength will be high, which corresponds to this material being strong for shoprt term loading such as end of construction. The soil is dilative under shear and develops negative pore pressure which increases the shear strength.

In the long term, you will loose negative pore pressures and have a weaker soil. Effective stress tests can be obtained with direct shear, ring shear, or even with TX but these are difficult, expensive, and not often done.

Soil below groundwater is not in a "drained condition" The condition of being "drained" or "undrained" is specified by the type of testing you perform, and the anticipated loading and type of analysis you are performing.

 

[lovethecold]

"Soil below groundwater is not in a "drained condition" The condition of being "drained" or "undrained" is specified by the type of testing you perform, and the anticipated loading and type of analysis you are performing."

- Would you expand on this. Where I am at, we deal largely with saturated fat clays. And, we assume the saturated condition (phi = 0) for all clays. Regardless of type of depth or location of water table.

I am trying to imagine a soil with a negative pore water pressure. I guess I figured that it would either be zero or positive. A negative implies a vacuum.

Thanks

- EIT Geotechnical Engineer. I am here hoping to help, and learn a little along the way.

 

[moe333]

When a very stiff, saturated, clay is sheared quickly, it wants to dilate, but cannot because there is no volume change. No volume change because porewater is taking the load and porewater is incompressible. The result is the pore water pressure goes negative, increasing the effective stress and thereby the soil shear strength.

Given enough time (slow loading), the pore water will drain and you lose negative pore pressure, and reduce shear strength. This is suitable for analysis of a long term condition...years maybe, depending on the permeability of the clay.

A soft clay contracts under shear and causes pore pressure to increase, thereby decreasing effective stress and shear strength of the soil. This analysis is suitable to analyze a short term condition...months maybe, depending on the permeability of the clay.

A phi=0 analysis is fine for soft, contractive clay soils, but not for very stiff clay, particularly high plasticity clay. You need to look at an effective stress analysis for very stiff clay.

You can find this discussion in any good soil mechanics book-Holtz & Kovacs, Terzaghi&Peck, Lam and Whitman, etc. Also, look for papers by Stark and Mesri who have done a lot of work with shear strength of very stiff clay.

The analysis you choose to perform (drained or undrained) should be determined by what the critical condition will be as discussed above. The drained analysis uses effective stress strengths, and the undrained annalysis uses total stress strength. Saturation should be at least 85-90% for an undrained analysis.

I think I'm done.

 

[fattdad]

Drained and undrained soil strength has nothing to do with the position of the water table or the saturation of the soil. There are many clays that are below the water table that are unsaturated and many clays above the water table that, under various loading conditions, are critical in the undrained state.

Submerged density is typically the bouyant unit weight, which negates the unit weight of water.

Drained strength is when the soil is sheared at a rate that allows the dissipation of excess pore pressure as the shearing progresses.

Undrained strength is when the soil is sheared at a rate that does not allow the dissipation of excess pore pressure as the shearing progresses. For many loading conditons, the critical strength for clays is the undrained strength (as referenced already above). For dynamic loading (i.e., earthquakes), sands can liquify as the shearing occurs so rapidly that the pore pressures cannot disipate. This is critical when the sand is above the "critical void ratio" (i.e., wants to densify). If the tendency to densify occurs to quicky for the water to get out of the way, there's a problem.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[mitsiou]

I am still confused with waht values of density to choose for a piling platform design.
I have 2m of granular material which is compacted and water table is at 1m below gound surface. The subgrade is cohesive material.
What weight do i choose fo the granular material; is it the unit weight ;saturated ;

 

[fattdad]

To calculate stress at depth, you take the unit weight of soil and subtract hydrostatic pressure. Let's say you want to know the vertical stress at a depth of 10 ft, the moist unit weight is 115 pcf and the saturated unit weight is about the same. Let's also say the water is at the depth of 3 ft.

10*115=1150 psf
7*62.4=436.8 psf

Vertical stess is 1,150-436.8=713.2 psf.

Some folks bypass this complication by using a "bouyant" unit weight below the water table. That would be the unit weight of the saturated soil minus the unit weight of water (i.e., 62.4 pcf).

That calculation would look like this:

3*115=345 psf
7*(115-62.4)=7*52.6=368.2 psf
345+368.2=713.2 psf

Hope this helps, that is after the confusion wears off - ha.

f-d

¡papá gordo ain’t no madre flaca!