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RE: Battered piers/piles

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Thank you for this response Jason - you have confirmed what I was afraid
is true. I have 3 structures - a deep retaining wall with undrained fill
(adjacent to a flood control levee); a deep pump station built into the
side of a flood control levee; and a gate control structure that is
loaded by unbalanced water loads (no significant soil loads). They are
on soft clays with bedrock down about 20 to 30-ft and are subject to
uplift pressures and high lateral effective fluid pressures. For my
preliminary design, I designed battered piles to resist all of the
lateral load; I used mirrored battered piles to provide balanced support
conditions when only vertical load is present (but have neglected the
"opposite hand" piles in tension when lateral loads are present); and I
"assumed" the excess vertical pile capacity was just "gravy" in my
design. But I have come to realize that my vertical forces will not
balance using this approach. 

It now appears that I must consider some tensile resistance in some
piles, where the structure/soil weight is not large relative to the
applied lateral load. Alternately, I could analyze the piles to "include
the lateral soil forces on the side of the piles and the bending moments
induced by those forces" - but I'm using the piles due to the low shear
strength of the clays. It is strange to me that texts that discuss
battered piles do not seem to clearly address this problem. 

With regard to my third item, I am considering either the case of the
gate structure with balanced water loads or the pump station during
construction - there are no applied lateral loads, so it appears that
mirrored piles would be the best way to balance the "induced" lateral
loads for a net vertical resultant. 

With regard to battered drilled shafts, I have found that the AASHTO
bridge specs recommend against such practices "due to their difficulty
of construction and high cost". 


William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 
> -----Original Message-----
> From: jwknospam [mailto:jwknospam(--nospam--at)comcast.net] 
> Sent: Wednesday, January 05, 2005 4:23 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Battered piers/piles 
> 
> > 1. Can drilled concrete shafts be installed with a batter? (I would 
> > think not, but I've heard rumor that it has been done before.)
> 
> In the right soil with a competent crew a drilled shaft can 
> be drilled at a slight batter.  Actually, with an incompetent 
> crew it's very likely every shaft is drilled at a batter.
> 
> 
> > 2. If I have a deep soil retaining structure with 
> > significant lateral load, I can come up with battered steel H-piles
driven to 
> > refusal in rock to resist the lateral load (2V:1H batter). But if I 
> > assume that the pile resists the load axially, there is a
significant vertical 
> > component that exceeds the total vertical load of the 
> > structure - what happens to the excess vertical reaction from the
pile? (Most 
> > references I've seen say to design the battered pile for the total 
> > lateral load but do not address what to do about unbalanced 
> > vertical loads. But it seems that the free-body diagram does not 
> > balance. How do bridge abutments using battered piles handle this?)
> 
> It looks like you're assuming a pure axial reaction in your 
> piles, which is how the classic pile design works.  Because 
> of the shallow angle of the batter, the vertical component of 
> the force in the battered pile sometimes does exceed the 
> gravity load.  This is handled by the vertical piles in tension.
> 
> If you have *only* battered piles, then the classic design 
> assumptions won't work unless your pile batter is less than 
> the angle of the lateral + gravity applied force resultant.  
> To satisfy statics you have to include the lateral soil 
> forces on the side of the piles and the bending moments 
> induced by those forces.
> 
> Also, if the lateral loads are high, gravity loads are light, 
> and the lateral capacity of the soil is low (soft clay or 
> loose sand), then your analysis might be exactly right - the 
> structure will fail by riding the piles sideways and upwards 
> in an arc.
> 
> > 3. Similarly, if I remove the lateral load but still want to resist 
> > vertical load (such as for dead weight without applied 
> > lateral load), the vertical reaction in the battered pile creates an
unbalanced 
> > lateral load (if it acts axially). What happens to the 
> > unbalanced lateral load? My proposed solution is to install equal
and opposite 
> > battered piles to exactly offset the lateral load - and neglect
these 
> > battered piles when in tension for the laterally loaded case.)
> 
> The extra lateral load caused by the pile can be resisted 
> either by soil pressure on the side of the piles, mirrored 
> battered piles, or by passive pressure back against whatever 
> is causing the lateral load.
> 
> Jason

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