Hi This is an elemental question I

[killswitchengage]

Hi
This is an elemental question I know but can somebody please clarify how do you compute the normal forces used in a direct shear test? Is it the effective vertical stress at depth z plus the surcharge?
Thanks

 

[EireChch]

normal, means normal (or perpendicular) to the plane of shear. The plane of shear in a direct shear test is horizontal so normal force is essentially the vertical stress.

Thats right.....eh, doubting myself now

 

[killswitchengage]

Thank you kind sir

 

[morecoffeeplz]

Normal load = stress * area. Watch your units.

If you want to apply a normal stress of 1000 psf over an area of 0.0203 ft2 (dia = 1.93"), the load would be 20.316 lbs. Our equipment has a beam ratio of 10, so we would apply 2.0316 lbs.

 

[BigH]

morecoffeeplz: Do you really apply the load to an accuracy of 0.0454 gms?? or 0.0016 ounces??? That is impressive

 

[morecoffeeplz]

BigH, I was showing off. If I'm honest, we adjust the mass of the dead loads to plus or minus 1 gram.

 

[BigH]

morecoffeplz - I figured but for us old guys like SlideRuleEra, having used slide rules gives a better idea of accuracy. Too many today - and you see this in technical papers all the time - do not know "where to cut off the decimals" . . . I saw a paper of the flooding years ago in the Mississippi where the levees failed; the engineering report reported densities to a hundredth of a pound! Tell me you can tell max dry density to that accuracy. Take care morecoffeeplz

 

[killswitchengage]

Thanks again but my question was on how on choose the pressures to be used

 

[morecoffeeplz]

Base it upon the expected overburden?

 

[killswitchengage]

Yea effective overburden but what about the surcharges? Should you apply Burmister/Boussinesq type of analysis to compute their effects at designated depths or what

 

[EireChch]

It would be more conservative to reduce the surcharge based on boussinesq.

But please explain what the application is? Are you trying to calculate the friction angle for use in a bearing capacity question and whether or not the applied surcharge will fail the soil?

 

[killswitchengage]

More like for slope stability analysis

 

[EireChch]

Actually my point above is incorrect, it shouldnt make a difference to phy if you include the surcharge or not.

If you add more overburden stress or confinement the shear stress will increase. However phi will still be the same as all you will do is add a higher point to your failure surface

 

[killswitchengage]

But didn't they teach us that depending on your consolidation state the shear parameters varies?

 

[EireChch]

Are you doing this test on a cohesive sample? There is no consolidation stage in a shear box test?