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Pile / pier design review fundamentals?

Pile / pier design review fundamentals?

Pile / pier design review fundamentals?

I am looking at structural design calcs for solar panel arrays to be constructed at our facility.  The panel assemblies will be mounted on JIS piles vibrated into fresh pour 16" diameter reinforced high strength concrete piers.

Because of huge boulders on-site, the pile locations were first excavated and filled with a low strength concrete slurry (CLSM), then the CLSM was auger bored for the piers and piles.

The CLSM was batched with on-site soil, cement and water with a 200-400 psi design spec.

1. The structural engineer used LPILE and on-site soil properties from the original geo-tech report to design the pier depths and diameters, Lateral Bearing pressure = 325 pcf and allowable skin friction 285 psf/f.  Shouldn't the design values be from testing on the CLSM itself, rather than the soil it replaced?

2. Some of the 28-day strengths of the CLSM were only 100-150 psi.  The contractor will try to back into this value as adequate for the design. What should I look out for as to how this might affect the design and the pier / pile performance.

3. The structural provided computer print outs of the pier size design for horizontal and vertical loads. Should I also want to see uplift analysis?

4. Other questions will surely follow if you get trapped into this thread....

Denice, P.E. water industry California  

RE: Pile / pier design review fundamentals?

#1.  I find it hard to believe that the lateral bearing strength of the pile is less than that of the soil.

#2.  Even at 100 psi for the fill, I still think the soil will control.

#3.  Yes.

#4.  OK...  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Pile / pier design review fundamentals?

1. Do you mean lateral bearing strength of the CLSM?

2.  Low temps @ construction, then subsequent freeze/thaw cycles, in-hole CLSM at 38 degrees at 6 inches deep after 24 hours--apparently cement had not gone off.  Also, there is cracking about 2" deep visible at the surface. For some CLSM, the geotech cannot take cylinders in upper foot because they fall apart...What would this mean to you?

4. The design appears to be skin friction only between CLSM and concrete pier.  Reinforcement cage goes to 6" above bottom of bore. Some piles could not be vibrated to bottom of cage. What are some questions re H-pile embedment?        

RE: Pile / pier design review fundamentals?

1.  Yes

2.  I see your point if the CLSM is that badly cracked.  Still, if the earth is stronger laterally than the CLSM, where would the CLSM deflect to laterally?  If the CLSM is stronger, then it could crack and separate further as the soil yields.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Pile / pier design review fundamentals?

Provided that there is shear friction between the CLSM and bored/filled piers, the system acting against the soil is still applicable.

If the adhesion of the new concrete pier with the CLSM is good then a bit of cracking should harm anything.  In this manner the system (new pier plus the CLSM) is confined by the soil and interlocked with it.  

If anything I would be concerned if the CLSM broke apart during drilling and created a void and exposed soil for the concrete of the pier.  Where possible soil could have breached the new concrete pier.

Vibration of the concrete deep in the hole is likely unnecessary.  Drilled Shafts are routinely constructed at 80+ feet with no mechanical consolidation.  

If in doubt perhaps a test pile tension load test should be performed...

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