Battered Pile Lateral Capacity 1

[JStructsteel]

Can someone help me explain how a battered pile gets it capacity. I understand it as a function of the batter....i.e. the component of the vertical load. So a 100K axial load with 4:1 batter would give you 25K lateral resistance.

The engineer I am discussing this with is saying the pile axial capacity is 200K, and thus at 4:1 it would be 50K lateral. I agree, if I had 200K vertical load on top.

The best analogy I can come up with is that if you dont have the vertical load, and you put the 50K lateral load, then what is stopping the pile from just being pushed over back to vertical any beyond.

The piles are end bearing, geotech says soil offers no lateral resistance.

Whats my best argument to convince the engineer that lateral load in a battered pile is the function of the vertical load?

 

[r13]

Note the difference of axial load and vertical load. in the battered pile, the axial load is the vector sum of the horizontal and vertical loads. In another words, the vertical and horizontal loads are components of the axial load. Your geotech is correct, battered pile is considered end bearing only.

 

[JStructsteel]

I guess I can point that out, if you dont have the vertical component, then you cannot get a lateral component.

 

[r13]

Correct. The sum of forces at the pile head must equal to zero, and the sum of moments about the pile tip must be zero too. So the system is in equilibrium.

 

[SlideRuleEra]

1) I guess I can point that out, if you don't have the vertical component, then you cannot get a lateral component.

2) The piles are end bearing, geotech says soil offers no lateral resistance.

JStructsteel - You have the right answer for the wrong reason. You are mixing two very different ways to resist lateral loading:

1) Yes, a batter pile can resist a lateral load without having a vertical component... but the way the pile resists the lateral load has nothing to do with the pile being battered. If the soil is "good" the soil surrounding the pile provides the passive pressure needed to resist lateral loading. Think about it... a plumb piles can and do resist lateral loads on a regular basis. There is no reason this mechanism would not work for a batter pile.

The confusion you are seeing is because:

2) The geotech is telling you that the soils surrounding the pile are "crap", and are not capable of resisting lateral loading with passive pressure. With "poor" soil, of course the pile is point bearing. Then, without the vertical component there is no horizontal component for a batter pile, as you have concluded.

BTW, unlike has been stated, a batter pile does not have to be point bearing. Instead, the batter pile has to resist vertical & horizontal loads only with axial loading. The axial loads can be resisted by point bearing, skin friction, or a combination of both.

http://www.SlideRuleEra.net

 

[phamENG]

SRE nailed it as always. Here's another way to think about it, which I hope won't muddy the waters:

If you remove all the soil from around the piles, what do you have? Columns. Now how can a battered "column" support a lateral load? You either need a fixed connection at the bottom, or all pinned connections with a vertical column connecting to it to create a simple, 2 member truss.

Let's bring the dirt back in. How do we achieve fixity in our "column"? The soil has to restrain it laterally. According the geotech, that can't happen (which is the norm where I am if you're near the water). So...you can't rely on option one. That means you need your 2 member truss arrangement.

 

[BAretired]

But in the second sketch of phamENG, the vertical pile must be capable of resisting a vertical force. If the soil has no lateral resistance, it may have negligible friction resistance as well, so we get back to the OP's assessment of the situation.

BA

 

[r13]

Jstructsteel,

The linked paper contains many design examples. Please review, if interested. Link

 

[phamENG]

BA - or it can be counteracted with dead load. I've used this with concrete caps on piers/dolphins in river bottoms with soft clays for the first 80+ feet below the mudline.

Of course this depends on the nature of the superstructure - not sure what that is in this case.

 

[r13]

The conclusion from a paper that introduce the applications of the battered pile:

"Batter piles can provide driven pile foundations a significant advantage over drilled piers and other vertical elements for deep foundations subject to lateral loads. Batter piles are particularly advantageous when there is a large unsupported pile length or in weak soils where there is little lateral support, as vertical foundation elements typically carry lateral loads by bending over their top 10 diameters of length below the pile cap. Over the range of batters typically employed in practice the bending capacity of a batter pile is essentially the same as that of a vertical pile, thus lateral load carried as axial load by the batter pile is essentially entirely “additional” lateral capacity for the pile. Batter piles are not suitable in all situations. Batter piles are particularly problematic when the soil through which the pile is driven may settle, as the settlement will impose lateral loads and bending moments along the entire length of the pile. Furthermore, careful attention is required to the design of the pile / structure connection to provide either sufficient strength to resist applied moment and shear loads or sufficient rotational ductility to limit loads at the pile head to the pile capacity. However, with proper attention to these details, batter piles remain the most economical way to carry lateral loads in many common situations, providing a true driven pile."

 

[SlideRuleEra]

Unlike implied above, batter piling are not always economical for lateral load. In high seismic areas, such as coastal South Carolina, excessive lateral force from an earthquake applied to well driven batter piles can force the axial pile loading high enough to shatter a typical pile cap.

http://www.SlideRuleEra.net

 

[r13]

IMO, design the batter pile as axial member (avoid fixed end pile as shown), and ensure there is adequate bearing to prevent settlement of the tip.

 

[PEinc]

JStructsteel, you said the "axial" pile load is 100 kips. The geotech said 200 kips. Someone may be wrong or else the geotech wants the pile to have a safety factor of 2 = 200 / 100. I suggest that you tell us the actual vertical and lateral loads (coming from the structure) that the pile, battered at 4V:1H, will need to resist.

http://www.PeirceEngineering.com

 

[BigH]

SRE - what I was going to say - if I remember correcly, this became apparent in the 1964 Alaska eearthquake

 

[canwesteng]

My preference is usually to have a pair of piles at least per pile cap if battered - then either you do like Phameng's sketch, or if you have lateral both ways make them both battered in opposite directions.