Micropile ends in rock

[FilipVz]

Hi Everyone,

I am designing bored, reinforced concrete micropile that ends in marl(rock). Above the marl is a 3 meter thick layer of compressible clay.

Can somebody recommend the formula for calculate miropile bearing capacity?

 

[PEinc]

Micropiles are almost always designed for side friction (grout bond) with soil or rock. Micropiles usually are not designed for end bearing because the end area is usually insignificant or there may be concern about loose drill cuttings at the tip of the pile.

http://www.PeirceEngineering.com

 

[FilipVz]

I know, but i have to design underpinning of strip foundations, and there is not enough place for big piles machinery.

My idea is to underpin the foundation by a set of reinforced concrete (micro)piles (diameter - 25cm).

Can i calculate them in the same way as piles?

 

[jayrod12]

you're going to reinforce 1" diameter piles? how do you plan on doing that and still get clear cover. and how many are you planning on installing, 80? that just sounds like a bad idea all around. Where we are the underpinning piles are standard diameters 12", 14",16" etc and they can get their rigs into some fairly tight places to make things work.

 

[jayrod12]

My bad. I read it as 25mm not 25cm. Ok then I would think yes you would design it the same.

 

[PEinc]

25cm is not a big micropile diameter. The equipment does not need to be very large equipment. With such a small diameter, you may not have enough bearing area for supporting the full design load without considering grout to rock bond in the rock socket length. If your load is not very high, you will not need a 25cm diameter. It would probably be cheaper to use a smaller diameter micropile and design for bond in the rock socket instead of end bearing.

http://www.PeirceEngineering.com

 

[AndreBC]

How about using Franki piles (expanded base pile)? That way you can have bigger end bearing than micropiles.

 

[FilipVz]

I am very limited in space. There is not enough space for Franki piles machinery.

PEinc, by bond in the rock socked, you mean that the grout should be injected under pressure?

 

[BigHarvey]

In France we consider 4 types of micropiles :
type 1 : just concrete or grout
type 2 : same reinforced with a steel tube or re-bar
type 3 : reinforced with a steel tube allowing IGU pressure grouting(unique global grouting from the top pf the steel tube
type 4 : reinforced with a steel tube allowing IRS pressure grouting (repetitive and selective grouting)

They get a higher capacity going from type 1 to type 4.

 

[PEinc]

No. The grout in a rock socket does not usually need to be pressure grouted. The grout is placed by tremie method from the bottom of the rock socket using a grout tube.

Franki piles would probably be too expensive. There is a limited number of contractors who install Franki piles. Many more contractors install micropiles.

http://www.PeirceEngineering.com