minipiles, bored micropiles 3

[blrmdcr]

I have been asked by a designer to provide recommendations for drilled micropiles to resist both compressive and uplift loading. The piles will be drilled into bedrock. I am looking for a resource (manual, etc.) that addresses grout/bedrock bond strenths, etc. Can anyone provide assistance? Thank you.

 

[Erdbau]

FHWA Micropile Design and Construction Guidelines is a good place to start.

http://isddc.dot.gov/OLPFiles/FHWA/009966.pdf

 

[ishvaaag]

http://isddc.dot.gov/OLPFiles/FHWA/009966.pdf

http://www.gnpgeo.com.my/download/publication/L_03.pdf

http://www.structuremag.org/OldArchives/2005/December 2005/Micropiles-by-Andrew-Brengola.pdf

For a start ... there are many beautiful documents out there.

 

[blrmdcr]

Thank you Erdbau and ishvaaag. Interesting note...In the Micropile Design and Construction Guidelines, they recommend that a factor of safety of 2.5 be applied to grout/bedrock bonding strengths. By contrast, the Drilled Shaft Manual published by the US DOT for caissons indicates that for a caisson drilled into a moderate bedrock, an allowable bonding strength of 3 to 5 percent of the concrete compressive strength can be considered. On face value, this would lead you to believe that allowable bonding strengths are greater for the caisson than for the bored micropile.

To add fat to the fire, a rock anchor may be designed for higher bonding strengths, but rock anchors are routinely tested. In bored micropile applications, this is not always possible.

Any thoughts?

 

[ishvaaag]

The only thing that comes to my mind is that being quite thin elements, and also the cover of the steel core they may be being wary of corruption of the grout and diminished adhesion both to the steel and the ground.

 

[jdonville]

blrmdcr,

Drilled shafts behave differently than bored micropiles and rock anchors due to large relative diameter of the drilled shafts and the fact that drilled shafts permit relaxation of the surrounding soil/rock during construction.

Ground anchors and micropiles typically have grout injected under pressure (even if it is just gravity fluid head) as the casing is withdrawn, so less opportunity for relaxation of the bearing soil/rock to occur. Ground anchors can also be regrouted under pressure to increase normal stress on the bond zone.

As you mention, ground anchors have testing performed on each element, therefore you can design closer to the bone in terms of FS/reliability. You could probably do this with micropiles if your test program sampled a large percentage of the total number installed and the geotechnical setting did not vary greatly.

J

 

[blrmdcr]

jdonville

To follow what you are saying, there is more opportunity for bedrock relaxation in the larger diameter caissons. By virtue of this, there is a tendency for this relaxation and inward movement of the bedrock to increase normal pressures against the concrete and therefore promote a better bond. Is this correct?

Couldn't one argue that by grouting the smaller diameter micropiles and anchors under pressure, the action of the pressure grouting promotes at least as good a bond as that of the caisson?

 

[jdonville]

blrmdcr,

Relaxation is less of an issue for drilled shafts in rock.

For rock-socketed drilled shafts, the area of the contact with the rock is much larger than for micropiles/anchors. Thus the overall rock mass properties tend to dominate, which may not be as strong as intact rock or even the concrete. You will still max out skin friction in the rock socket before either skin friction in the overburdern soils or end bearing on the rock due to strain compatibility.

See AASHTO for discussion of side resistance in rock sockets for drilled shafts.

See also O'Neill and Reese's / FHWA drilled shaft design guidance.

J