Cohesion and overburden stress

[ac4u2nv]

hi all
How do i take into account the cohesion of clay when calculating overburden stress.
the overburden stress is just the wet density x the height of the material -( the density of water x the height of the water)
how do i take into account the cohesion of the clay?

thanks

 

[BigH]

You don't . . .

 

[fattdad]

cohesion is an effective, drained strength parameter that is unrelated to overburden pressure. Cohesion is the strength that remains in the soil when there is no overburden pressure!

Now you may be really asking about undrained shear strength. Undrained shear strength does very with confinement. In normally-consolidated soils taht variation may be at 20% of the overburden pressure. Don't rely on that value though. As you may already know, geotechnical laboratories are a much better resource.

f-d

¡papá gordo ain’t no madre flaca!

 

[ac4u2nv]

Thanks, in the absence of lab data how do i take into account the undrained shear strength on my overburden calcs?

thank you

 

[fattdad]

We need the whole story. You doing slope stability? You doing deep foundation design? You looking at spread footings? Is this for short-term loading only?

What information do you have? Do you only have boring logs? Is there any Atterberg limits to contrast to natural water contents?

All we know is somewhere there's a site and an engineer is concerned about soil strength. Your original post was focused on effective (drained, long-term) strength. Your subsequent post seems to be now focused on short-term, undrained strength. To me they're both relavent.

f-d

¡papá gordo ain’t no madre flaca!

 

[ac4u2nv]

Sure
Im trying to calculate the overburden pressure at point 12m bgl where a horizontal borehole for installation of an electrical conduit will be drilled.I have logs and some PI values.
Ive taken standard density values for the strata and calculated my effective overburden stress at that point. There are clay layers and i was told that i need to take into account undrained shear strength.

...?

 

[Mccoy]

Like BigH hinted, you don't need Su to calculate the overburden pressure. Density of soil layers, density of fluid, layers thicknesses are the variables involved, which you already have measured.
Unless there is some other aspect involved in the design of the electrical conduit and Su is a necessary variable which does not govern overburden stress though.

 

[Ron]

Like BigH hinted

Hinted like a 2x4 to the forehead!

 

[Mccoy]

Lol!

 

[ac4u2nv]

apparently the Su is needed to taken into account passive pressure?

 

[fattdad]

what about friction angle to determine passive resistance?

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Where is this thread going? OP - don't get mixed up between overburden stress and strength effects on things like shear stability, lateral loads on walls, etc. Are you a student?

 

[ac4u2nv]

Big H
yes - getting a bit confused as im told when calculating the overburden stress i have to take into account the undrained strength, and that means multiplying by a passive pressure resistance coefficient.

 

[BigH]

ac4yu2nv - who told you? Either he is "off his game" or you didn't quite understand what he wanted you to do. The Su value (or friction angle for cohesionless soils) will give you the passive earth pressure coefficient which you end up multiplying by the overburden pressure and a few other factors. The strength component DOES NOT COME INTO PLAY IN DETERMINING THE OVERBURDEN PRESSURE (sorry for shouting, but I thought it necessary) but does come into play in determining lateral presures/loadings.