Behavior and Analysis of Batter Piles

[MrFurleyEIT]

I have got a question about the behavior and analysis of batter piles supporting a pile cap. Hope all the foundation guys, esp. batter piles experts, will have a go at it. This post might be a bit lengthy, but not extremely challenging, so please don't be put off, ok?

Imagine, if you will, an isolated concrete pile cap, say 20 feet squared, with batter piles at the north and south ends of the pile cap. The batter piles at the north end (left side of page) are, say 75 degrees from the horizontal, while the ones at the south end (right side) are, say 105 degrees from the horizontal, forming what looks like the letter "A" without the horizontal bar. Assume that lateral loads are only in the north to south, and south to north directions, i.e. loads are only left (north) and right (south) of the page. The soil below the footing is peaty and highly organic and will not provide any skin friction with the pile. Good bedrock is found 50 feet below the surface. There is a lateral load of 600 kips in the north to south (left to right) direction.

1. If my pile capacity is 50 tons (100 kips), what is the lateral resisting capacity of the batter pile? Is it just a horizontal component of the pile capacity, i.e. 100*COS 75 = 25.88 kips?

2. Will both the batter piles provide lateral resistance to the load? I am assuming no. Only the south pile (the right one) can provide any resistance through end bearing and this pile is compression. The north pile (left one) cannot provide any lateral resistance as it is in tension and the bottom of the bearing cannot take advantage of the end bearing due to the direction of the north load. Skin friction and moment due to bending of the north batter pile is neglected. Is this true?

Now, imagine that I have a very substantail axial load (from equipment, self weight, etc.) acting on my isolated pile cap.

3. Due to the heavy axial load from above, are the two batter piles now in compression?

4. Now, will the north (left) batter pile be able to provide lateral resistance to the lateral load? This is the question that has got me stumped and I find hardest to
comprehend. This batter pile was assumed as not being able to provide any lateral resistance before as it was in tenstion. Now, is it in compression, and thus can provide lateral resistance? Please elaborate.

5. Further to question 4, can the north (left) pile take advantage of end bearing to resist the lateral load?

6. Does the south (right) pile lose some of its lateral resisting capacity due to some lateral load from the axial load?

Question about batter piles, in general.

7. Do batter piles generally work together, in tandem, i.e. both resisting the same lateral loads coming from the same direction? Or is it that a batter pile will resist load from one direction, while another batter pile (in a mirrored position from the former) will resist loads from the opposite direction. I am thinking the latter. Am I right?

Thanks in advance.

 

[civilperson]

Is the heavy axial load from above of a magnitude of 2240 kips, (or less)? This force resultant would match or be flatter than the axis of the South piles. To develop tension in the end bearing piles, they must be socketed into the bedrock with sufficient bond area to develop the tensile strength of the pile.

 

[MrFurleyEIT]

Thanks for your response, civil person. I would say that the magnitude of the axial load can range from about 80 kips to about 700 kips, max. Much less than the 2240 kips you mentioned. How did you get 2240?

The batter piles are not "socketed" into the bedrock? What must be done to achieve this? I believe the piles are bearing on the bedrock, relying on end bearing.

 

[DRC1]

Batter ples have traditonal been analyzed as 2 force members. Most of the thrust is resisted by the lateral component of the compression batter piles. The tension pies can devlop resistance from friction, although in your case I see why you would negelct it. The pile weight of the tensioon piles somtimes is added in, although it is generally not a very large number. To answer your last question first, assuming no tension, which even in good soils is often done, then no, the piles do not work in tandem. Best to assume the compression batter pile resist the lateral load. If both piles are in compression, and there is a lateral load applied, one pile will resist the lateral load. Note that there will be 2 components to the lateral pile load, the lateral componet due to supporting the weight on a batter and the component due to lateral load, so you must have sufficent pile capacity to accomadate both. Note that although both piles are in compression, the lateral components opose each other, thus if there were no lateral load at the cap, the lateral load in the piles would cancel out. Finally, the pile resistance to lateral load is a function actual pile load, not capacity.
This method has been around forever and as such stands the test of time. Analyze your pier cap as a being suppoted by pin connnections, then appling those reactions to 2 force members. Thats it.
Methods such as p-y curves are more realistic, but are more complex to solve. Realize that piles are not that simple and be conservative in your design. One final note: If you have soils that settle, your batter piles wil settle also. Say you anticipate 2 inches of settlement. the pile wil settle at its midpoint 2 inches also. Thus the moment in pile is equal to the moment that is causes a deflection of 2 inches.
You should find a dicussion of batter pile anaysis in many foundation text books.
Hope this helps