Knowing the characteristics of the ground is what will permit you analyze the interaction between your structural arrangement, say in the foundation some piles, and it. If you can adscribe a modulus of Young to the soil or some lateral springs along the shaft, you have reduced your problem to one of mere analysis.

Again for rocks, it will depend on the material what depth unto it is required to gain fixity. Yet in no case I would think an enbedment less than twice the depth give in any case fixity. Better use at least thrice the depth.

Of course all piles, vertical ones included, take shear, as long their capacity is not exceeded. Inclined piles may even give fragile roture in an earthquake event, so in seismic areas, use vertical piles.

But if you think of preventing relative slippage you are thinking of anchors to something, that will be more efficient than piles for that, putting the thing in tension.

By "depth to fixity", do you mean depth to maximum moment (which could be an idealized fixed support in a structural model) or depth to point of zero deflection and zero rotation (basically the pile tip elevation)?  What is normally meant by "depth to fixity" in geotechnical engineering?

As I understand it, with programs such as lpile, determination of these points is made in iteration amongst applied loads to a fixed or free head pile (determined by structural analysis & boundary conditions), allowable deflection criteria, and bracketing of soil response.  This would need to be done in the same manner for both friction piles and end-bearing piles either founded in rock or not.  Is it always reasonable to assume, for the sake of structural modeling, to take the "depth to fixity" as three pile diameters below the level of competent lateral soil resistance?  Does this give a close enough approximation to give accurate results in a structural model?  Using this depth as "depth to fixity", to me, would indicate the depth to maximum moment in the pile.  Is this what you intended ishvaaag?

~dison

No, heavens, when I was referring to 3 times the diameter I was referring to socketing not in "competent soil" but in rock. A soil even competent normally won't be able to mimick at such depth zero rotation and lateral displacement.

So you see I generally agree with you on how to ascertain or bracket the behaviour of buried piles.