Micropiles Supporting Thick Reinforced Slab

[marinaman]

I've got a client who wants an elevator added to the exterior of his building. The soils report states that we need micropiles to support the masonry elevator enclosure.

I can tie the masonry elevator enclosure to the building for lateral stability, so I'm not worried about overturning of the added elevator enclosure. Also, the elevator will only service the ground floor and the first elevated floor.

The question is about the micropiles and how they should be embedded/attached to the thickened slab. To begin my design, I was thinking of using a 24" thick reinforced slab to support my enclosure. The plan size would only be about 12' x 12' square. I was thinking of embedding my micropiles 6" into the reinforced slab. I was going to locate my micropiles, in plan, beneath the center of the reinforced CMU wall above...probably one in each corner and one at the midspan of each wall, and maybe a couple out in the slab (away from the hydraulic hole).

I am going to check my 24" slab for punching of the micropiles thru the slab when loaded (including the distance from the centerline of the micropile to the slab edge in my clacs). That being said, do you guys make any attachment of the pile to the slab, other than embedment? Are there any design guides available for micropile to thickened r/f slab detailing?

 

[EngineeringAdam]

I haven't used micropiles extensively, although others in our office have.

Just to make sure we are talking about the same thing, micropiles are small diameter piles, typically 6 inches in diameter, with a single center all-tread rod (spliced with couplers as needed) and are grouted using a tremie tub. After that, the piles can be post-grouted via a pre-installed post grout tube for added strength.

Typically, I have seen the top of the all-thread have a 1 1/2" x 10" x 10" plate embedded 18 inches into the mat slab above.

Maybe your application isn't as demanding as what we deal with, but if you have a 24" thick mat I would suggest you embed more that 6 inches.

 

[garpe]

I've used a sleeve with a cap plate welded to it. Check punching shear

 

[EngineeringAdam]

If you really aren't worried about any overturning then I imagine you only need to check punching shear as you say. Then the embedment of the micropile into the mat isn't as critical.

 

[dik]

Are you sure you want to attract any lateral load to the new enclosure? Might pick up a bunch not intended.

Dik

 

[msquared48]

How is the rest of the building supported? If not on pile too, I would be somewhat concerned about vertical settlement if the structure is five years old or less, not if it is over 20 years. Does the Geotech report address this?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[marinaman]

Good Question msquared - Yes, the existing buiding is founded upon piles and is about 40 yrs old. I relayed that information to the geotech prior to the geotech doing his work.