Footing coefficient of sliding friction in conjunction with a structure supported on piles?
Footing coefficient of sliding friction in conjunction with a structure supported on piles?
(OP)
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.
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.
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
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?
RE: Footing coefficient of sliding friction 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:
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
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.
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
Purple star for your sketch
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
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.
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
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.
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?
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".
RE: Footing coefficient of sliding friction in conjunction with a structure supported on piles?