Steel pipe piles hitting an early refusal. Any recommendations? 1

[hughits]

I am a structural engineer working on driving steel pipe piles for an oil sands treatment facilities. Based on the soils report and design drawings, the design embedment of most of the piles range from 20 m to 26 m. The pile top cut-off elev. is about 1.5 m above the final grade, so the total pile length would be 1.5 m longer than the design embedment. For additional info, each pile will support a steel column (no skid or concrete pile cap).

Most of the piles we have been driving met the design embedment, but there were several piles that hit early refusal. Some of them needed to be driven about 26 m deep but were actually driven 20-21 m. The rest of them are quite or way off of the design depth. They needed to be driven about 24 m but were actually driven only 7-8 m. Since we had more than 100 blow counts at the refusals, I wouldn't worry about the pile axial compression capacity, which was also confirmed by our geotechnical consultant. The concern is the piles tensile capacity. The design tensile capacity is way more than the actual capacities we will get from those piles.

Now we are trying to come up with ideas to fix this up. One solution we are discussing is to take them out, drill the holes down to 20 m or something, and put them back in the holes, and backfill it. I would like to perform additional borings around the area, but the client wouldn't accept it.

I am wondering if there are any better ideas out there. Any idea, opinion, and/or recommendation on this issue would be highly appreciated.

Thanks.

 

[Ron]

I would pre-drill through the probable hard lense, then drive to refusal. I would not remove the piles already driven...just extra expense. Another option is to use Augercast piles adjacent to the bad piles.

 

[hughits]

Thanks for your valuable recommendations!
Well, if the pattern of the probable hard lense was predictable, I would pre-drill down to wherever I am comfortable with, but it is not. Hence, the second option sounds better. One question in this option would be once Augercast piles are driven around the bad piles to compensate the loss in tensile capacity, do all of the piles need to be capped all together?

 

[Ron]

The Augercast piles are not driven...they are cast in place. I would not tie a short pile on a hard lense to a longer pile adjacent to it. Just cut it off and leave it.

 

[hughits]

Ron, thank you again for your input.
Yeah, that was my mistake. So basically what I will get from the Augercast piles is additional frictional resistance in either tension or compression due to the fact that the piles densify the soil around the main pile. Hope this is correct.

 

[PEinc]

If the short pipe piles are open-ended, if the pile tips are not crushed, if the pipe pile diameter is sufficient, and if the economics work out; you could drill down through the short piles and install micropiles which could work for both tension and compression.

http://www.PeirceEngineering.com

 

[Ron]

hughits...that depends on whether your steel piles were displacement piles or not. If open steel pipe or shoeless H piles, there's not much densification of surrounding soil, but if closed pipe or H-pile with a displacement shoe, then densification will occur...but that's not necessary for the Augercast pile. If you have a reasonable soil profile, they will work either way.

PEinc's idea is good as well...it would be worth considering for open ended pipe. You can also blow the end out of closed end pipe using a mandrel if you want to try his method.

 

[hughits]

Thank both of you! I am using open steel pipe piles, so I don't have to blow the end as Ron suggested. Micropile idea sounds good to me. I am not sure if the pile tips are crushed or not. The pile diameter is 508 mm, which I think large enough.

Thanks again!

 

[Ron]

hughits...yep, 508mm is large enough!

 

[VoyageofDiscovery]

I know you stated that you have a geotech but just to play devil's advocate, refusal does not necessarily mean design capacity. You may have an unusual situation where the driving energy is being dissipated into the surrounding soil. Driving formulae are actually quite complicated depending on soil.

VoD

 

[hughits]

That's true. Nobody knows what's in there and what's preventing us from reaching the design depth, which is the reason why I am seeking for solutions for this issue. Currently what we rely on are two things; more than 100 blow counts/25 cm at the refusals (we concluded that any further attempt to drive the piles could cause damage to them) and the confirmation of the geotechnical consultant that is working for us. He seems to be concerned about tensile capacity only as he is sure that the piles are getting enough compression capacity (actually end-bearing capacity).
Thanks for your opinions!

 

[DRC1]

Another possibibilty is to load test the shorter piles to find the actual ultimate resistanc and drive some additional piles to give you sufficent tensile capacity. Usually compression is much higher than required tensile and thus drives the bus. Shorter piles may be okay. If you did any PDA testing during pile driving, you may be able to estimate tensile capacity from that.

 

[hughits]

Thanks for your answer! Actually I made a decision to do PDA on all of the piles in question. I didn't want to do anything further without knowing what capacities I need to get and I have currently. Then I will go from there. Thanks for your opinion that supports my decision. This is good!

 

[PEinc]

Uplift resistance needs to consider both the frictional bond of the pile to the soil and the mass cone stability. A short pile may test OK for friction but there may not be enough of a cone to prevent the pile from pulling up the soil with it. Often, the required size of the cone mass dictates a longer pile that does the frictional bond.

http://www.PeirceEngineering.com

 

[hughits]

I got your point. I understand that it's similar to deep-seated failure of a retaining wall conceptwise. I will have to have the geotechnical consultant check this failure cone, too. Thanks again for your valuable opinions.

 

[PEinc]

If the cone of soil extends below the water table, you have a lighter, buoyant, soil weight which will increase the required size and depth of the cone.

http://www.PeirceEngineering.com

 

[Ron]

PEinc...isn't that only valid in cohesive soils?

 

[PEinc]

Why? Any submerged soil is still buoyant.

http://www.PeirceEngineering.com

 

[Ron]

No... to develop a tension cone requires cohesion...otherwise just friction on the pile wall.

 

[PEinc]

Per FHWA's GEOTECHNICAL ENGINEERING CIRCULAR NO. 4
Ground Anchors and Anchored Systems, Report No. FHWA-IF-99-015,

http://www.fhwa.dot.gov/engineering/geotech/pubs/if99015.pdf

5.9 TIEDOWN DESIGN
5.9.1 Introduction
Tiedowns refer to vertical or downward inclined ground anchors subjected to uplift forces.
Examples of tiedowns include foundation elements for structures subject to overturning or uplift
such as transmission towers and vertical anchors used to resist hydrostatic uplift forces in gravity
dams and underwater slabs. Tiedowns are designed to resist two possible failure mechanisms: (1)
individual anchor capacity to resist uplift pressures; and (2) overall stability of the ground mass
wherein the tiedown group geometry is sufficient to envelope a mass of ground to resist uplift forces.

5.9.3 Uplift Capacity of Soil Tiedown Anchors
For tiedown anchors installed in soils, the failure mechanisms of cone breakout and interface shear
along the soil/grout interface are analyzed. Like rock anchors, the cone breakout mechanism
dominates for shallow anchors whereas interface shear dominates for relatively deep anchors. A
grouted soil anchor subjected to uplift behaves similarly to a small diameter drilled shaft subjected to
uplift.

5.9.4 Design of Tiedown Anchors to Resist Hydrostatic Uplift
Tiedowns may be used to provide resistance to uplift forces caused by hydrostatic pressures. A
notable use of tiedown anchors in the U.S. was to resist hydrostatic uplift of a depressed roadway
section as part of the Boston Central Artery project (see Druss, 1994). The primary issues related to
the use of anchors for such tiedown applications are: (1) general stability of the enclosed ground
mass; (2) changes in anchor loads resulting from movement (i.e., surface heave, consolidation
settlements, creep deformations) in the enclosed ground mass; and (3) corrosion protection and
watertightness of the ground anchor. Corrosion protection and water tightness are discussed in
chapter 6.
General stability of a structure subjected to uplift is shown in figure 54. The system is in equilibrium
when U=W1+W2, where W1 and W2 are total weights of the structure and the enclosed ground,
respectively, and U is the total uplift resulting from the uplift pressure ?wh. The geometry of the soil
mass assumed to be mobilized at failure may be evaluated as shown in figure 54. Frictional
resistance that may develop between the ground and the sidewalls of the structure may be
conservatively neglected.

Although driven piles are not normally considered to be tiedown anchors, when called upon to be such, the same mechanisms apply. Since the original poster's piles are short, mass stability should be a design cansideration. It will probably control the length of the piles.

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