Question about a micropile

[geo321]

Hello this is my first time I use a micropile. It has a diameter of 250mm. It will contain reinforcement consisting of 4 high deformed bars with some spiral reinforcement. My question is:
1. Is it mandatory to use grout and not concrete? Is it because I cant go with the tremie up to the bottom of the micropile due to its small diameter?
2. When reinforcement is mixed with grout, how can I check the section capacity of the micropile ?

Thank you

 

[oldestguy]

Save samples of grout and have the samples tested for compression strength converted to shear strength )(usually half) at different curing times, such as 5 days 10, 20 30, while curing in damp conditions. Apply a safety factor to that value and add it to the working strngth of the steel bars. Then of course what is the shear strength of the material in which the piles are driven? Don't forget to consider group action.

 

[geo321]

Thank you oldest guy. Any examples you can share ?

 

[geo321]

Oldest guy I wasn't clear in my original post. When I asked about the section capacity I meant the structural capacity and not the Geotechnical capacity.

 

[oldestguy]

Assume in question 2 you mean the structural load the pile can take and question about which part carries what? Right? Thus, your grout will carry part and the steel another part. Grout with high water content (very liquid) will be weaker than grout that is not highly liquid (has less water). Also assuming this is a steel pipe, that also takes some load. Finally, you can't dismiss how the load from the external soil gets in the pile. If it is all end bearing, that's one thing, and different from all as side friction.

In answer to your question 1, try to get a concrete mix in there and the answer is an easy NO!

Along with this exercise, laboratory testing of the materials for strength of the parts helps a lot.

One also might go so far as to determine the strengths as related to deformation. Grout and steel likely quite different.

 

[geo321]

Thank you oldestguy

 

[PEinc]

geo321, I don't think that you can tremie pump concrete with aggregate from the bottom of a 250mm micropile casing or rock socket if there are 4 deformed bars, spiral reinforcement, and spacers or centralizers. All of the micropiles I have designed and seen have had one threaded bar as the core steel. The bars have centralizers. There are no spirals. The micropile is tremie filled with grout before or after the core steel is set. The grout has always been a mix of water and cement, possibly with some admixture(s). In the United States both The International Building Code (IBC) and FHWA have equations for calculating the structural axial compressive capacity of the micropile based on the cross-sectional areas of the grout, the core steel, and the permanent steel casing pipe, and on the strength of the grout and steel.

http://www.PeirceEngineering.com

 

[emmgjld]

Hmmm. "a diameter of 250mm" ... diameter of 10", same size as much of the driven piling in my area. It is also the same as the drilled piers I have designed over the years, with same reinforcement. The issue may become, how long are the micropiles? Is the micropile cased or not. These are big issues which need to be addressed before recommendations are made.

 

[Obregon]

DESIGN FOR REQUIRED STRUCTURAL AND GEOTECHNICAL CAPACITIES (ONE SHALL PREVAIL OVER THE OTHER) BY FHWA AND ALWAYS USE GROUT.

"Methods used in micropile installation allow for high grout/ground bond values
along the grout/ground interface. The grout transfers the load through friction from the
reinforcement to the ground in the micropile bond zone in a manner similar to that of ground
anchors. Due to the small pile diameter, any end-bearing contribution in micropiles is
generally neglected" FHWA NHI-05-039.

Regards.