Shear Resisting Mechanisms for Piles & Caps
Shear Resisting Mechanisms for Piles & Caps
(OP)
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?
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?






RE: Shear Resisting Mechanisms for Piles & Caps
RE: Shear Resisting Mechanisms for Piles & Caps
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.
RE: Shear Resisting Mechanisms for Piles & Caps
RE: Shear Resisting Mechanisms for Piles & Caps
RE: Shear Resisting Mechanisms for Piles & Caps
Thinking freely for embedded piles, it is through them that the seismic shear is passed to the building. So we have the whole piles+superestructure thrown to one side, then by vibration action back. At the moment of this back the soil will be enacting its passive action, so not counting on it would be irrealistic; other thing is that it need be evaluated at a lower value on statistical reasons and perturbed state of the soils.
Furthermore, the alternative inclined piles have in some cases buckled (I assume it will be mainly for end supported piles) and failed the pile caps, so there's no really general alternative to take somewhat into account passive strength as something that may be equilibrating a lateral force at the pilecaps under Earthquake.
RE: Shear Resisting Mechanisms for Piles & Caps
ishvaaag - Piles act counter to seismic forces, so they'll resist the forces from the superstructure/wall.
RE: Shear Resisting Mechanisms for Piles & Caps
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?
RE: Shear Resisting Mechanisms for Piles & Caps
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?
RE: Shear Resisting Mechanisms for Piles & Caps
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.
RE: Shear Resisting Mechanisms for Piles & Caps
RE: Shear Resisting Mechanisms for Piles & Caps
Thanks for the info on the battered piles. That's helpful.
If one had battered piles on each side of an element, and only considered the contribution of the battered piles in compression, might it then be acceptable count on them for strength load cases?
For seismic:
The body forces in the structure are generated as a result of the acceleration of the masses in the structure. That acceleration is, in turn, imparted to the structure by the soil. Really, the only external force on the structure is applied by the earth against the foundation. The rest is just the internal forces developed as the structure attempts to resist relative translation between its masses and the ground.
So, if there's earth there to toss the structure around in the first place, then there should also be some earth around to count on for passive resistance. I think that's what Ishvaag was getting at. Although I should probably let Ishvaag speak for himself.
RE: Shear Resisting Mechanisms for Piles & Caps
And of course one may have piles in soils with risk of liquefaction. In fact I checked it even for the structures of the recently named lane extension job, here that we have only .04g basic surface acceleration for normative earthquake (this maybe 8% of what coming at worst places in California), but seismic action was mandatory out of the highway being a fundamental infrastructure of the road's network. There was no problem, even in the silty soils there.
So when risk of liquefaction is a problem we have the alternative of what codes say, experience AND making an autostable foundation+superstructure outfit. Experience is what the paper on the wood piles behaving well under Hugo hurricane scouring the soils said: the wood piles behaving well if deeply embedded in foundations. This is hurricane action, and earthquake is other beast, but records of behaviour there must be. And the ship or alike outfit is available for everyone.
RE: Shear Resisting Mechanisms for Piles & Caps
RE: Shear Resisting Mechanisms for Piles & Caps
If you're not sure, I'd use vertical piles. You can count passive resistance on the pile cap if you mobilize the soil - but for service loads and low-level seismic loads this won't happen.
RE: Shear Resisting Mechanisms for Piles & Caps
- The battered pile produces a vectored load on the pile cap. The lateral resistance and the vertical resistance should not be considered separately.
- The cautions regarding battered piles in high seismic conditions is due to the production of a higher shears in the pile cap from the piles. This is never good and is to be eliminated from happening by use of the seismic overdesign factor Omega. I remember seeing a case study where a battered pile punched through a pile cap due to seismic loads.
To answer your question regarding concurrent resistance to lateral loads:
1. Axial load from battered piles
2. Bending resistance from deep piles. Use L-Pile analysis or sim (be careful with shadowing effects of piles in a row)
3. Passive resistance from pile caps (I have always included this)
4. Passive resistance from the SOG (I have always tried to not use this - susceptible to shrinkage)
5. Friction between the pile cap and ground below - NEVER use this.
RE: Shear Resisting Mechanisms for Piles & Caps
Take an example of pushing a table sideways. The resistance is in the connection of legs to the top.
RE: Shear Resisting Mechanisms for Piles & Caps
Based on the reason, we don't tconside the friction between the pile cap and soil below.
If the adjacent soil is stiff or the backfill around the cap is satisfying the requirement of the dry density, we shall conside the the pile and its front soil may together resist the horizontal action (seismic or wind).