Footing coefficient of sliding friction in conjunction with a structure supported on piles? 1

[Wandering Wallaby]

Hello all, I'm designing a single family residential structure with a basement on potentially liquefiable soils. The geotech has recommended 4" diameter driven micro piles, and the use of battered piles for lateral resistance, but has also given a coefficient of friction to use for sliding resistance. I'm designing the walls as cantilevered retaining walls with a grade beam/footing spanning between piles (vertical piles under the wall and battered piles at the toe of the footing). I end up with about 3.3 kips/ft of sliding force at the soil interface, which I'm finding out is too much for a battered micro pile to resist, even at a 30 degree batter. The geotech recommends neglecting the upper 24" of soils for the passive resistance so that doesn't help much unless I use a very deep footing key. This is a daylight (walk-out) basement so relying solely on the slab resistance isn't enough.

Is it typical to use the coefficient of friction at the bottom of the footing in conjunction with a structure supported on piles? If so, the numbers work out for a stable design. However, I imagine that would not be recommended due to the grade beam spanning between piles and not necessarily exerting a normal force on the soil. Any advice or references would be greatly appreciated. Thanks in advance.

 

[EireChch]

If you are confident that there will always be contact between the footing and soil then I dont see an issue with considering friction along the base.

I can see the argument that the portion of soil/footing contact directly in front of the grade beam is likely "moving" as one as the beam is pushing into the soil so there is very little frictional resistance, however this portion of reduced frictional resistance is likely only to extend the same distance as height of grade beam.

Have you considered using larger diameter piles? like a 300mm reinforced bored pile? would that offer the required additional lateral capacity?

 

[SlideRuleEra]

The geotech has recommended... the use of battered piles for lateral resistance, but has also given a coefficient of friction to use for sliding resistance.

Is it typical to use the coefficient of friction at the bottom of the footing in conjunction with a structure supported on piles?

Not when using batter piles. As the slab tries to slide, batter pile resist the load (horizontally) but the vertical component causes uplift on the slab. Uplift reduces the normal force that creates the friction force... as batter piles pickup load, frictional resistance to sliding goes down:

 

[Wandering Wallaby]

Yes, I see that now. Thanks for the sketch. If the piles were vertical, then I should be able to use the frictional resistance but wouldn't have any lateral resistance from the piles. Perhaps I can design the basement slab to resist the sliding forces and dowel into the sloping sides of the structure (where the battered piles are perpendicular to the direction of sliding resistance). Or maybe just use vertical piles only and increase the footing size to whatever is needed to resist all of the sliding forces (that would also eliminate the worry some have with battered piles in high seismic areas) but relying solely on the slab and not any battered piles doesn't feel right.

As far as larger diameter bored piles, it is common around this area (PNW) to use 3", 4", or 6" driven steel pipe piles on residences near bodies of water...I'm sure I'll get pushback if I stray from the norm; but what I've gathered by reviewing a few other designs is that this issue gets overlooked often.

Anyways, thanks again for the responses. I think I'll explore a combination of 6" battered piles and slab dowels for now.

 

[EireChch]

Good point SRE, I forgot about the battered piles.

Purple star for your sketch

 

[SlideRuleEra]

EC - Thank you.

WW - IMHO, don't try to combine lateral resistance from any combination of approaches. Use just one method; larger batter piles, per EC's suggestion, is probably the best. I've been involved in more than one project where splitting load between two independent restraint / support techniques gave unsatisfactory results. Problems occur for two reasons:

Design based on erroneous assumptions about load sharing.

Construction results in one or both load carrying methods being flawed. Often the result of site conditions, not necessary shoddy work.

 

[Wandering Wallaby]

SRE - Good to know. Thank you for sharing. I greatly appreciate your feedback.

 

[BridgeSmith]

Even where the piles are vertical, I don't consider sliding friction for a footing on piles, due to the possibility of settlement of the soil under the footing (and that the piles typically don't). If there aren't piles under the back of the footing, I'd consider using the part of it that can be counted on to still be bearing on the soil, after the expected settlement.

 

[EireChch]

BS- many pile foundations are often designed with some load sharing between piles and slab bearing. If its designed to consider some support for slab then frictional support can also be considered IMO.

But I do accept that most of the time is no load sharing considered. I would be looking at larger diameter piles, or deepening grade beams maybe.

 

[BigH]

Follow up to SRE, under seismic conditions, battered piles have been known to decimate pile caps . . . the 1964 Japanese Earthquake study - there was a paper along these lines in the ASCE Geo Journal back in the early 70s I believe or late 60s.

 

[SlideRuleEra]

BigH - Risk of batter piles has been studied quite a bit in South Carolina because of our high seismic risk. We had to consider this risk when designing our generating stations, since some are located in the highest risk zone (see below).

Takes a good size earthquake to shatter a pile cap... a residential structure (like this thread) would probably be "trash" from other damage before then. Here is a 2012 paper by my alma mater for the SCDOT: "Behavior of Pile to Bent Cap Connections Subjected to Seismic Forces".

 

[BridgeSmith]

If more resistance to sliding is required than what the battered piles can provide, I'd consider a shear key on the bottom of the footing, or a row of vertical piles. On a small job, once the pile driving hammer is on site, that's most of the expense for of piles, anyway.