Passive soil bearing notations

[Ron247]

When specifying an allowed passive soil bearing, I generally see it cited as pounds per square foot per foot of depth or psf/ft. I occasionally see it presented as pounds per cubic foot (pcf) but this seems incorrect even thought the units will simplify to this. Doe this matter?

Also, when using a friction coefficient, would the value be the same regardless of whether you are using ultimate or working loads?

 

[le99]

I think it is just the "subgrade modulus" or "modulus of subgrade reactions", which indicates the pressure required to produce a unit settlement in a soil. I consider it is an index number through testing. Both forms of the unit have seen wild uses.

 

[EireChch]

passive soil pressure is the better term. It is also different to subgrade modulus, which is in kN/m[sup]3[/sup]

I have only specified passive pressure in units of kPa (or psf for you). Psf/ft is also correct too as we are looking at a plane strain condition so assuming the wall is infinitely long. We are looking at a 1m (or 1ft) section of the wall, or looking 1m or 1ft "into the page".

When discussing lateral force, I write it as XXkN/m. But I dont really do this for lateral pressure.. I suppose for force its a reminder that the width of the wall into the page is important of course for prop loading etc.

I have never seen nor think its correct to use m[sup]3[/sup] or ft[sup]3[/sup]

 

[Ron247]

Thanks everyone.

 

[CarlB]

Passive pressure, just like active & at-rest pressures, are expressed in psf/ft as they are considered to increase linearly with depth. The pressures can be treated as that caused by a fluid, thus they are also expressed as "equivalent fluid pressures", in lbs/cf. So if you have a passive pressure of 300 psf/ft, the pressure at 10 feet is 3000 psf. And the total force on the wall for the upper 10 feet is 300*10*10/2, or 15,000 lbs/ft of wall length.

 

[EireChch]

Carl - i dont agree that its psf/ft because they increase linearly with depth. I would argue its due to the fact that we are looking 1ft into the page. We dont use EFP much this side of the pond and pressures are often in psf/ft (well, kPa/m)

Soil pressure increases linearly with depth in the same layer but not in mulit layerd soils.

 

[Mccoy]

I don't know if you guys agree but I believe that the concept of passive pressure can also be modeled by Winkler-like springs (for example, the p-y model in piles) so in such models a horizontal modulus of subgrade reaction with relevant units (pressure/length) may be envisaged. In this case, the length would be the measure of the horizontal settlement behind the structure.

This is a pertinent illustration in a sheetpile structure