Maximum Reinforcement Ratio for Micro-pile Design?

[Stillerfan]

I was wondering if anyone knew of a code specifying the maximum reinforcement ratio for a Micro-pile socketed into bedrock? I did not find anything in FHWA-SA-97-070 or ACI 318. Currently I have one (1) #18 Grade 75 full length in a 6-inch rock socket (7-inch cased section).

 

[civilperson]

No such code exists. I would assure at least 3" cover to the center steel where not cased.

 

[Teguci]

A project I worked on recently had a 7" diameter pile with 1/2" steel shell and a #22 (d-dag) bar down the center. It was good for 200 Kips (except at the sinkhole).

As noted in FHWA, a section with 50% steel is not uncommon.

Did you mean a 6 ft rock socket?

 

[DRC1]

Strain compatibility has been an issue for mini piles where load is intended to be shared between the grout and the steel core. Massachuesetts building code had limits on reinforcement to prevent stain incompatibility. THis can be a significant issues as it can result in brittle(sudden) fracture of the grout near the top of the pile.

 

[Doc08]

Back to the top of this page, I would like to advise against using a single bar and use an equivalent 3 to 4 bars placed in a cage. From experience this provides a better performance than a single bar especially where bending / torsion are of concern.

 

[DRC1]

Minipiles have a single bar for reinforcement. Recently larger diamter variations called mini cassions -12 inches and larger have been used in NYC which have a small cage of 3-4 smaller bars. However, these are fairly new and significantly more expensive than traditional mini piles.

 

[Stillerfan]

Thank you for your responses. I will consult a structural engineer about the strain incompatibility issue.

 

[Doc08]

Indeed, it is a known fact that a single bar may be used in a minipile and this would be no problem for piles in compression. However, I have to disagree with DRC1 especially that I have particularly addressed the case where lateral or torsional loads are involved and especially if part of the pile is exposed or the upper portion is in bad soil. I have witnessed at least two such incidents where the single rod more than satisfied the required percentage for steel and the grout had 40MPa strength, yet the pile (20 cm dia.) still failed. on one case, the load applied was no more than 2m of clayey sand with no surcharge to speak of.

I cannot see how using 4 bars instead of an equivalent single bar would be "significantly" more expensive. Perhaps there would be some extra labor cost for the assembly of the cage, but I cannot call it significant.

cheers

 

[Teguci]

Doc08
Please provide additional information. I would like to know what loading was on the pile (you mention clayey sand) and what the failure mechanism was. I think we would all like to avoid a similar situation.

Thanks

 

[Doc08]

Teguci

The micropiles were spaced 50cm c-c, held at the top by a cap beam, and thin shotcrete screen was sprayed just to guard against the possibility of sloughing between the iles. The cut was about 2m high and given the soil at hand (C ~ 10Kpa, ? ~ 30°) the pile was not expected to deflect more than 1cm and experience a moment of 300 kg-m. Thus a single ?25 bar would have satisfied the steel requirement for the section.

The incident took place when a buldozer started excavating the ramp leading to the foundation level. I suspect that the vibration created by the buldozer may have caused loss of cohesion in clayey sand. If we assume that and reduce the cohesion to zero and the friction angle to 28°, the moment rises to about 800 Kg-m but the expected deflection jumps to 2.5cm at the top of the pile. However, strictly from a structural point of view, the ?25 bar would have still been OK for such an moment as it provide 4.9cm2 of steel area.

I'll try to decribe the situation on the ground as best as I can as I do not have a picture of the site. The piles were placed in a single row covering about 40m distance. The ramp starts at the entrance of the site and ends at about mid distance. The first pile to fail was that located at the end of the 40m distance, and folded in a clock wise rotation (the break gave an impression of a twisting mode) along with 6 more piles (because there were all tied at the top by a cap beam).

I hope that helps.

 

[PEinc]

Doc08's micropiles sound more like soldier beams than piles. If the micropiles are supporting lateral earth pressures, then these composite members (casing, bar, grout) or at least the casing pipes should be strong enough in bending to resist the earth pressures. Stillerfan's original post sounded like it was about micropiles with vertical load only.

 

[DRC1]

I agree with PEinc; Micro piles do not perform well under signifcant laeral load. Their bending capacity can be enhanced by leaving the top 20 feet or so of casing. Otherwise the bending is resisted by the bar alone which does not have great lateral stiffness. The casing will provide both bending capacity and confinement for the grout.
The failure of the minipile described in the earlier post may have been due to strain incompatibility, although not neccicarily.
Smaller bars require larger cassions to provide enough clearance for clearance, regrout tubes, spacers etc. It just becomes too congested for your typical 6-8 inch minipile.
Properly designed and installed minipiles are very efficent for vertical loads

 

[PEinc]

With respect to strain campatability, if the threadbar is grouted into rock or if there is a steel casing around the grouted threadbar, strain compatability should not be an issue. Therefore, if cased or in a rock socket, you could take advantage of a higher strength steel threadbar, such as 75 ksi, 80 ksi, or 150 ksi steel (unless you building code limits the allowable steel stress).

 

[Teguci]

It sounds like we are talking about two different types of piles.

Mini-piles, as I refer to them, are small diameter (8" or smaller) steel pipe piles with a reinforcing bar down the middle and filled with grout. They have a grout bulb at the bottom which causes the mini pile to act in end bearing as opposed to skin friction which is typical of pipe piles. The grout bulb is especially useful is Karst areas since the grout can fill in some of the underground voids.

Doc - it sounds like you are describing something that I'd call an augercast pile except I have never seen an augercast pile smaller than 12" in diameter. For an augercast pile that experiences shear or moment, the top cage should be 4 bars tied into a cage. This actually gets shoved into the concrete after pouring. There is also the single bar that runs the full depth. This is easily placed through the auger port during the pouring.

 

[PEinc]

Teguci,

According to FHWA's Micropile Design and Construction Guidelines, Publication No. FHWA-SA-97-070, "A micropile is a small-diameter (typically less than 300 mm), drilled and grouted replacement pile that is typically reinforced. A micropile is constructed by drilling a borehole, placing reinforcement, and grouting the hole... Micropiles can withstand axial and/or lateral loads, and may be considered a substitute for conventional piles or as one component in a composite soil/pile mass, depending on the design concept employed.... 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."

Generally, there is no bulb. High-pressure grouting may cause a very small increase in the bond zone diameter but no "bulb." Micropiles are usually designed as straight shaft, frictional, compression members similar to tieback anchors, but with compression loads instead of tension. The micropile may have a permanent casing pipe. The core steel can be a partial length threadbar in the bond zone and with a casing pipe above trhe bond zone or the core stel can be a full length deformed bar, threadbar, pile, or bars with or without a casing pipe. There are a number of combinations for making up the components for a micropile.

In karst areas, usually a permanent casing pipe is used to PREVENT filling of a void or sinkhole with grout. The casing pipe assures that the grout will remain around the core steel and that the micropile will have sufficient lateral rigidity through a void area, if encountered.

 

[Doc08]

Indeed our piles are drilled using an auger or DTH, depending on the ground conditions and they vary in diameter from 10cm to 30cm.

As I had described earlier, we have used a single dywidag bar or a steel pipe with either a concrete or a cement grout mix and these were generally used for vertical load. Micropiles with steel cages consisting of 4 to 6 bars dia 14 to 16mm have been used successfully to retain excavations in either soil or weak rock (siltstone, marlstone,...) with or without tensioned anchors, or to underpin exiting structures with or without the possibility of exposing these piles such in top-down type construction. Typically a 20cm dia micropiles may be designed to withstand up to 2.5 T-m moment especially when utilized with a cement grout mix.

With this approach we were able to get away without using a permanent a steel shell to increase the bending capacity of the pile or using an inner high strength steel pipe which usually turns out to be more expensive especially in confined areas (headroom wise) or in deeper piles such as the pipes will have to be threaded at shorter distance thus adding the cost of threading and the coupling.

 

[kslee1000]

Strickly to your original question, the max reinf ratio should be 1% of x-section area (treated as column). The amount/number of steel will also limited by the requirement to maintain clear cover, and to allow for the max aggregates to pass. All covered by ACI-318.

 

[civilperson]

kslee1000 has it exactly wrong, the minimum is 1% for columns. No maximum for piles even when modeled as columns.

 

[PEinc]

Micropiles also are grouted without coarse aggregate and usually without fine aggregate.

 

[kslee1000]

Yes, slipped over this one. The max is 8%.