Laterally Loaded Battered Micropiles

[palmahouse]

I am working on a project where micropiles are proposed to support a building with design seismic lateral loads. The literature indicates that battered micropiles should be considered because of the low lateral capacity of vertical micropiles.

I calculated allowable lateral loads for reasonably long piles by taking lateral components of skin-friction capacity. Then, I moved to a laterally loaded pile analysis using L-Pile. What I found, for my project, if you reduce deflection to a tolerable level (one inch), that your allowable lateral load capacities are very low (about one-half of the horizontal component of skin friction capacity), even for battered piles (battered 20 degrees from vertical in this case).

I am calculating only about 5-kip allowable lateral capacity per pile for 8-inch diameter micropiles and a 1-inch lateral deflection - battered or vertical. It seams the low stiffness of small-diameter pile governs whether they are battered or not.

So, this got me thinking - what is the advantage of battered micropiles for a building or other cases if your tolerable deflections are small?

 

[Gimbli]

You are right when concluding that the low stiffness of the element is what governs. Micropiles, even battered, will no add any significant lateral capacity. Have tried to considered passive forces on footings or pilecap? Use a factor of safety if so...

 

[BigHarvey]

We do a lot of seismic reistant deep foundations in the West Indies and we never use micropiles : they are just not adapted to sustain lateral loads.

Concrete piles will need a lot of horizontal reinforcement which doubles the total steel quantity ( 130 kg of steel per m3 of concrete instead of 60 just to give an order of magnitude )

We generally go for steel driven piles ( tubular or H ) with the necessary thickness, which is the most economical solution for that type of design.

 

[palmahouse]

What through me off: something I read in a design manual implied (at least the way I read it) that you can add the component of lateral load capacity of a pile (from L-pile) and the FULL horizontal componenent of allowable skin friction to estimate your allowable lateral capacity. Not true - because the mobilized skin friction load capacity and mobilized lateral load capacity are tied by vector relationships - the vectors must close.

So, you need to calculate allowable horizontal component of skin friction and lateral load capacity using L-pile, draw vectors (that must close), and evalutate what governs. In my case, and I expect in most cases, the lateral load capacity of the pile in bending was the limiting factor and governs the design.

BigHarvey - I wish we could drive or drill large diameter piles. Micropiles are the way to go here because we have weak and thick colluvium over huge hard rock blocks that you can't drive piles through and that you can't easily drill through using rotary equipment. So, air-track and precusion is the way to go (micropiles) This is a very challenging and steep site that we can't get large equipment to. A huge cut was made to notch the building into the hill, and =a thin fill was pushed out on the downslope side. If we tried to remove and recompact the colluvium, we would be taking down half the hillside or we would need a very tall shoring wall - so earthwork remediation options are cost-prohibitive.

Gimbli said it - micropiles just can't handle much lateral load unless you batter them really flat, and then they are too difficult to drill and install. So, we are stuck with assigning a low capacity to them and using lots of them.

I appreciate both of your insights, and will appreciate any further comments.

 

[BigHarvey]

For the kind of soil profile you are describing you can use construction methods derived from the overbrden methods used in small diameter drilling ( ODEX ).

With the Symmetrix method you can drill piles up to 1 m diameter ( you might not have to wait for the construction of the components ). For diameters like 600 mm, the equipment is off the shelf.

 

[InDepth]

Out of curiosity, why can't you bundle the micropiles to increase your stiffness. Or can you use drilled and cast in place piles to also take the lateral load.

You should look at "Micropiles for Earth Retention and Slope Stabilization" by Tom Armour. I believe you'll find helpful information.

Have you also looked into ground improvement to increase passive resistance and bearing pressure?

 

[palmahouse]

InDepth,
The structural engineer is currently laying out the micropiles - he will likely lay them out fairly frequently and "in trees". Is that what you mean by bundling them or are you referring to reinforcement? We are going to end up with pipe reinforcement instead of bundled bars - but even that want gives us impressive capacities.

We could use larger diameter drilled piles, but that ground would be very difficult to drill using rotary drilling methods (see my previous discussion in this thread about this) Also, we have an airtrack drilling contractor on-board who built a soil nail wall for the project.

I do not have that book by Tom but I do have the FHWA micropile design manual that he co-authored. I know Tom but did not consult with him on this one (I ate up a lot of his time consulting on a recent and previous project).

We introduced the possibility of overexcavation and replacement and ground improvement (grouting) on this project but ruled them out as being not feasible and cost-prohibitive based on our experience. Drilling is difficlt there - and you need to drill a lot for grouting. The site is small and steep and the small micropile drilling equipment is a good fit.

I think we are on the right path. I would appreciate any more ideas you or others have.

 

[InDepth]

palmahouse, can you provide details on the structural foundation system currently proposed, size of building, estimate DL,LL, EQ loads, and what site / seismic design category the project is in. I suspect that maybe a change in the foundation system concept may help.....and save cost

Also, be careful on battered piles; certain codes and standards don't allow them....(maybe this doesn't apply to micropiles because of their low flexural rigidity). But the issue in the past has been that thee design assumed a pin supported structure for analysis, when in reality the battered pile had fixity at its end. i.e. Behavior did not equal assumption.

 

[palmahouse]

Not sure about DL and LL, but the design lateral EQ loads are about 6 kips per column and 3 kips/foot on the grade beams. It is a one-story (high story) metal-framed building with a concrete floor designed as structurally supported, and with heavy equpment on it (tanks for a winery). We are in Zone 4 from the 2001 California Building Code (CB), but, I am not sure if these zones are used any more in the 2007 CBC.

At any rate, we have a well qualified Structural Engineer (a California SE) on board, and he is well versed in the code.

We already went through a whole value-engineering session with engineers and contractors, but, if you could offer yet another opinion, we would listen. The problem is, you would need to be more intimate with the ground condtions, site conditions, and building design to offer an adequate asessment. I can tell you about the ground. We have 4 feet of colluvium that creeps and compressible and is being ignored for both lateral and skin friction resistance because it is weak, we are on steeply inclined ground, and it is susceptible to creep. So, the top of the piles will be unsupported columns. Beneath that is less compressible colluvium, distended andesite bedrock - hard but severely fractured (like tallus), and then fractured andesite.

As I said, we cannot remove and replace - nowhere to put the excavated material and too difficult to dig anyway.

We are well aware of the failures of battered piles that occured at the Port of Oakland during the Loma Prieta earthquake, and will design battered piles, if used, assuming fixed head conditions (and design our connections as such). Looks like we may go with only vertical piles anyway - just lots of them. Battered piles are very difficult to drill - collapsing ground, and have only slightly higher lateral load capacity.