From IBC 2018 1806.3.1 "Combined Resistance"..."The total resistance to lateral loads shall be permitted to be determined by combining the values derived from the lateral bearing pressure and the lateral sliding resistance specified in Table 1806.2."
Doesn't get much more clear than that in the code.
The argument for not using both simultaneously is that it takes a non-trivial amount of movement to mobilize passive lateral pressure. And once you've got that non-trivial movement, by definition you no longer have static friction. That said, it's a nuanced thing:
1) Maybe you use kinetic friction in conjunction with passive earth pressure resistance.
2) Maybe you use active earth pressure resistance which implies less movement (and less resistance).
While I don't recall where off of the time of my head, I've certainly seen it in print that one should consider the temporal simultaneity of the various mechanisms of lateral soil resistance. It does you little good if one mechanism is exhausted before the others are mobilized.
There may be times when it is worthwhile and reasonable to use passive resistance for retaining wall design, but it certainly hasn't been standard practice anywhere I have worked. Apart from anything else, there is a good chance someone will dig a trench in front of your wall and remove almost all the resistance.
It's a little like using the combined resistance of skin friction and end bearing on drilled concrete piles. It is common practice in my neck of the woods to use one or the other but not both because by the time end bearing has developed, skin friction is no longer available.
I see piles as a little different. Once you break the "bond" between the piles and the soil, it's gone. But once a footing slides on soil the friction is still there, potentially lower is it's kinetic vs static.
As with the skin friction on a drilled shaft, it seems it would depend on the soil type, with rock losing the most friction once it moves, cohesive soils losing some, and granular soils having nearly the same friction after it moves, as it did before.
Rod Smith, P.E., The artist formerly known as HotRod10
I don't know why the discussion is getting into drilled shafts and piles. The OP was clearly asking about footings.
Standard practice everywhere I've worked for spread/wall footings has been to ignore the upper 1 ft for passive similar to MIStructE_IRE (or more if soils are really bad at the surface) but otherwise combine friction and passive pressure for sliding checks. As noted in my previous post, the IBC seems to explicitly support this approach.
Ideally, we don't want to utilize both, but sometimes we just have to. It is correct, if shear friction alone is adequate, the reactive passive earth pressure won't develop. The shear friction will always come to play first though, because it is on the active/pushing side.
This argument is not ALWAYS true..
It depends on the type of foundation and how the structure ' tolerable' to the horizontal displacement of the soil..
The picture depicts the Active, At rest , and passive pressure coefficients and relative movement necessary to mobilize the type of pressure. ( Taken from Foundation analysis by BOWLES ); In order to develop active pressure, a small translation is enough , how ever, for 'full' passive thrust development, in the range of 50 mm translation is necessary..
I would like to remind also, the minimum earth pressure is active pressure...
Some cases ;
- A thrust block for a pipeline; the total horizontal resistance of the thrust block is the sum of base friction and passive thrust..
- A retaining wall ; total resistance against sliding is base friction and 50 % of passive thrust ( some codes allow the use of 50 % of passive thrust ), personally i prefer only base friction,
- A portal frame foundation ; do not depend on friction and passive thrust, use tension ties between footings,
- Multistorey bldg with a basement; the horizontal forces due to wind resisted by base friction and some horizontal thrust around at rest pressure of the soil; and for EQ, resisted by base friction and some passive thrust..
If you tell the type of structure and foundation, we may tell you what you should thrust ..
Woke up to a pleasantly advanced conversation. Thanks for your thoughts, everyone.
The element I'm considering at the moment happens to be a pipeline thrust anchor, and I'm inclined to agree with those that allow combining friction and passive pressure effects. The amount of movement required for this particular application to engage the passive earth pressure is not detrimental to the system, and once the passive pressure is engaged and the movement stops, I don't see how the friction should be ignored.
The more I thought about it the more nuanced the topic appeared... as I suppose tends to be the case.
The friction always exists !!! and the amount depends on friction between soil and concrete or coating applied.. If you post a sketch showing the thrust anchor together with pipeline material, cover deppth ( if buried ), test pressure etc. we may discuss ..
My understanding is essentially the same as what KootK said.
I have been on projects that where the project criteria (not sure if we got this from the soils report or not) said that we would use 100% of one value and only 30% of the other.
That, seems to me to be a way to account for the fact that both will contribute to the resistance, but to acknowledge that you can't enable fully passive pressure without movement. And, that if you do have full passive pressure, you should use a lower value for friction.
I am with HTURKAK on large displacement/rotation is required to develop the "FULL" passive resistance as calculated using the formula. It is similar to the wall movement required for active pressure to develop, otherwise, without movement, the soil is in at rest state.
