Soil properties
Soil properties
(OP)
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
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





RE: Soil properties
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.
RE: Soil properties
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.
RE: Soil properties
"...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.
RE: Soil properties
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.
RE: Soil properties
- 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.
RE: Soil properties
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.
RE: Soil properties
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!
RE: Soil properties
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 ;
RE: Soil properties
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!