shear resisting mechanisms for piles caps

huangyhg

shear resisting mechanisms for piles & caps
the canadian concrete manual has a supplement where they discuss (very briefly) the mechanisms that can be used to resist shear delivered to a pile cap assemblage.  i've encountered this in the context of working on a shear wall foundation system.
anyhow, they suggest four possible mechanisms:
1) friction between the pile cap and soil below.
2) passive bearing of the pile cap / shear key against the adjacent soil.
3) passive bearing of the piles themselves against the soil.
4) bearing of the pile cap / shear walls against the adjacent slab on grade (where available).
after listing these mechanisms, they also state that "all four mechanisms may not act concurrently".  so my question is this: which do act concurrently and in which order do they engage?  i'd appreciate any insights that others may have.  additionally, does anyone know of any good references that deal with this topic?
some of my thoughts on the issue are:
a) my guess as to the order of mechanism engagement would be as follows: 1) friction 2) friction gives and sog bearing takes over 3) sog bearing gives and passive soil bearing (piles and pile cap concurrently) takes over.  following this logic, the piles and pile cap bearing against the soil would be the only mechanisms to act concurrently. sound reasonable?
b) if you decide to use passive bearing of the pile cap against the soil, is it wise to use a lean concrete backfill between the pile cap and the extents of the excavation to ensure proper bearing against undisturbed soil?
c) is it really appropriate to use friction between the pile cap and the soil?  with piles absorbing the bulk of the gravity load via end bearing, wouldn't the amount of friction developed between the pile cap and the soil be quite limited (and difficult to predict).  my thinking is that the soil under the pile cap would tend to settle away from the underside of the cap over time.  thoughts?
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when you need to use piles usually the soil will be weak and friction in general an unreliable term. hence, in general way only passive push and -if available- bracing from a sog against say a rigid supporting soil or mountain wall at the other end of the building. so we would be in a kind of p-delta situation against the piles, the bracing action of the sog and passive resistance of the soil equilibrating the lateral displacement of piles.
soil settles away from pile cap, thereby negating any frictional contribution of the pile cap/soil interface.
slab on grade may be removed for some other expansion in the future, thereby negating any contribution there.
they may regrade around the pile cap, cut and fill around the pile cap, or any number of other variations which would affect the passive soil conditions around the cap.  therefore, passive soil pressure can not be relied on and should not be accounted for.
lateral pile resistance is the only value which remains and is the one that should be used.
i design mostly in heavy industrial, so others may see things differently.

in bridge design, we typically use the pile lateral capacity only, and add in passive resistance on the pile cap when needed. however, this is typically only for seismic design. for service, we use the service capacity of the piles only. you probably won't move enough to mobilize the soil to the point where you can count on passive resistance on the pile cap. i'm not sure whether it would be worth it to backfill using lean concrete in terms of cost vs. benefit.
i believe that using passive resistance of soil is not permitted in seismic design, correct me if i am wrong.
i think to re  
slickdeals - i'm not sure about buildings, but passive pressure is definitely counted in seismic design for bridges, at least in california. ca has its own seismic code for bridges, though, and does not follow aashto.
ishvaaag - piles act counter to seismic forces, so they'll resist the forces from the superstructure/wall.
hmm...something for you guys to ponder:
wind forces travel down the structure to the foundation.
seismic forces travel from the foundation up the structure.
can't soil liquefy in a seismic event?  how much passive resistance does it give in this liquified state?  don't we have to design retaining walls in seismic areas against this liquified soil?
thanks for all the helpful responses so far.
i think that ishvaag's point is simply that, since the eq forces are induced through the soil (as chip points out as well), it is rational to assume that the same soil is available for passive resistance.
chip: i think that you're right for certain types of soils. if the geotech says that liquefaction is a serious problem, you probably need to start thinking raft foundation.
ishvaag's mention of inclined piles brought to mind a related question as well.  in many textbook examples, you see shear being taken by inclined piles often referred to as "batter" piles.  mechanically, the concept is very appealing.  are these still used in practice?  more common for bridges maybe?

battered piles are used for lateral service loads. the piles don't take the load in shear, but in axial load (components). battered piles tend to behave poorly in seismic events for many reasons, but particularly because battered piles only behave well in compression. battered piles are used to design seat-type bridge abutments and retaining walls (earth-retaining structures), but with these structures seismic forces are different and the battered piles will not see tension.
generally we consider seismic loads to come from the structure, not the soil. it may seem counterintuitive, but the mass of the structure is what causes the force.
liquefaction may be an issue, but that's usually only an issue for the pile itself since it tends to happen in deeper layers of soil, not under 2-3 feet of fill. scour conditions, however, would impact passive resistance against the pile cap.
battered piles are not recommended for high seismic zones. check the nehrp commentary section for more information.