Sliding resistance of gravity walls

[pietro82]

Hi all,

I'm in doubt with the calculation of the sliding resistance (Ps) of gravity walls where the base is under the ground leve.. In few documents, it is stated the "Sliding of a retaining wall is resisted by the friction between the soil and the base" (From Peck and Terzaghi) so, that means Ps=Pv*tan(δ), where: δ is the soil-wall friction angle, Pv is the sum of vertical forces acting on the base. In other documents, it is considered the soil shear strenght as resistence force, so Ps=c'B+Pv*tan(φ'), where c' is soil cohesion and φ' is the effective friction angle and B is the base length.
I'm wondering which approach is the right one. I always used the first formula, but it should be considered both and used the highest value because the base might slide but if the soil in front of the base doesn't fail, the base would be firm. Is it right?

Thanks

Any suggestion is appreciated.

 

[fattdad]

It's all about the free-body diagram! I wouldn't rely on cohesion, so I'd use the first equation. The reason I write? Don't forget to show the water table at the elevation of the weep holes.

f-d

ípapß gordo ainÆt no madre flaca!

 

[pietro82]

hi Fattdad,

thanks for your reply. About the cohesion, how do you deal with tension cracks which may occur in the active side which arise from the following formula σx=ka*σz-2c*sqrt(ka)?


cheers

 

[oldestguy]

Don't forget the passive resistance of the earth in front.

 

[aeoliantexan]

Different design guides will give you different opinions. Here is mine:
if it is a flat-bottomed footing with no key, I will use your first equation. Alpha could come from a table in DM-7 (Page 7.2-63 in my edition); tan alpha for clay against concrete is about 0.35. I do not include cohesion because the well-meaning contractor may spread a 2-inch layer of "cushion sand" on the subgrade before placing concrete.

I do not use the passive resistance against the toe unless the footing is quite deep or there is some protection, like a concrete slab for erosion protection at the toe. You will understand the first time you see the telephone crew cut a trench along the toe to bury a line. Also, in cold country, that top two feet can be saturated and weak during spring frost breakup.

I like to put a key at the heel and assume that will force the failure to pass through the soil below the footing, then use C and Phi.

You will probably get some other opinions now.

The lateral pressure against the wall is not predictable; you can bracket it. The strength of the foundation soil may change depending on what the contractor does to it. The trick is to be conservative enough that the wall will survive the conceivable conditions.

 

[fattdad]

. . . and in slope stability (or global failure evaluation), I'd include a tension crack to 4 or 5 ft and likely add water to the crack.

I agree with aeoliantexan too. Somebody should be expected to dig along the toe and passive resistance of the upper 2 or 3 ft should be ignored. If needed, you'd need a shear key.

f-d

ípapß gordo ainÆt no madre flaca!

 

[pietro82]

Thanks!
I heard that I should not rely on cohesion, so I should do calculate the bearing capacity without the cohesion contribution?
It happened a couple of times that the critical depth (I mean the depth where cracks likely occur on the active side) is much higher than the soil layer thickness. Would that mean there is no active force in that layer? What should be done in that condition?