Pile Driving-tip elevation, bedrock or cobble? 4

[Silty]

Hi All,
I am involved in a project that piles needs to be driven to bedrock. There is a 6 ft thick layer of cobbles and boulders just above the bedrock. Piles are H-piles with shoes and the QC include recording number of blows per 10". The drawings saying using Hiley formula for ultimate loads, and there is no PDA testing.
Question is:
How we know we reached bedrock?
I should mention that the contractor stopped 1 m above the geotech report bedrock level saying they hit refusal.
The second question is what should be criteria for refusal to make sure we reached bedrock and at the same time do not cause damage due to the over driving.

Thank you very much,

 

[jayrod12]

Normally the geotechnical engineer would specify what he considers refusal. In these parts (central Canadian prairies), refusal for H-Piles on bedrock is 15 blows for 1".

 

[SlideRuleEra]

1) How we know we reached bedrock?

2) I should mention that the contractor stopped 1 m above the geotech report bedrock level saying they hit refusal.

1) Assuming the hammer is properly sized for conditions, an experienced pile driver will know with the first hammer when bedrock is reached.... the pile will ring like a bell and, of course, blow count will approach infinity.

With a correctly sized hammer, a few more blows to confirm contact are probably acceptable for HPs with shoes.

You are correct to be concerned about overdriving... more often a problem than underdriving.

2) Not surprising. Drive other piles, you can return to this one if results from the others bring this pile into question.

http://www.SlideRuleEra.net

 

[Silty]

Thank you Jayrod12, do you know what could be the criteria for shale bedrock? the project is located in SW Ontario, Canada.

 

[jayrod12]

I'd recommend asking the geotechnical engineer. I imagine if you're driving H-Piles that you would have a project geotech on board.

 

[oldestguy]

This spec is pretty darn loose.

QC include recording number of blows per 10"

What about the blows per last inch?

 

[Silty]

Thank you oldestguy, I agree. The refusal criteria is not mentioned and now the contractor is defining that.
What would be good blow count for last 1"? Bedrock is Southwestern Ontario shale.
and how may I make sure the tip is not on top of cobbles and boulders?

 

[jayrod12]

An H-Pile with a driving shoe would not refuse on cobbles and boulders with an adequate refusal criteria. They break through them before the refusal criteria is met.

The contractor should not be defining it, the geotech should. It's his design, he should be indicating the refusal criteria and teh hammer energy being delivered to the pile. If he's not, request it.

Where I practice full time inspection of all deep foundations is required. And we have gone so far to amend the NBCC from saying "by a suitably qualified person" to a "qualified person responsible to the designer". Which we have interpreted to mean either the geotechnical engineer of record directly, or someone they approve of.

 

[Silty]

Thanks Jayrod12, good to know the state of practice in other areas.

 

[MTNClimber]

An H-Pile with a driving shoe would not refuse on cobbles and boulders with an adequate refusal criteria.

You kind of say this in the next line but....the pile could "refuse" early if the hammer is undersized or the energy setting is set low.

 

[jayrod12]

the pile could "refuse" early if the hammer is undersized or the energy setting is set low.

Good points, I guess I could've expanded my answer however figured it was implied with the "adequate refusal criteria", and the following line after that. But yes, you could have premature refusal if you aren't hitting it hard enough.

 

[Silty]

assuming the hammer is or right size, what would define "the adequate refusal criteria"? can you please elaborate.

 

[r13]

The criteria is based on the driving method and equipment selected. Here is an example. Link

 

[MTNClimber]

So in your case where you have a very dense layer that you must drive through to get to a desired bearing stratum, you might have to deliver hard hitting blows to a point where you are getting pile stresses approaching 0.9Fy (which requires a PDA to determine). If the pile stops moving then you have hit true refusal. You tell you have hit refusal because the pile isn't moving anymore.

Specifications should provide some guidance as when to stop the pile driving in order to protect the pile from bad inspectors and contractors. Most of what I have seen is 10 blows for 1/2" or less or 20" for 1", but you can tell its on hard bedrock before you get to 10 blows... and in some cases the inspector may want to call it before its reaches that point. The ram has a tendency to bounce higher and higher after each blow when the pile is sitting on something truely hard. The higher the ram goes, the harder the pile gets hit, the closer you get to exceeding the yield strength of the steel pile. It is very important that the inspector is on top of their game when the pile reaches bedrock.

As a side note, piles driven to a softer bedrock may gradually "fetch up" to get to the 10blows/0.5" so there is less concern of overstressing the pile unless your inspector is terrible. In my field days, I did see piles refuse on very large boulders nested in glacial till... so keep that in mind.

 

[Ron]

As others have noted, the Geotech should have established the driving and termination criteria. To not do that is irresponsible and, in my opinion, deviates from his/her standard of care.

Absent that, SlideRuleEra (SRE) has provided a good salvaging approach and should be followed. His experience beats the hell out of anything else we can offer.

 

[Okiryu]

I assume you have boring logs within the area you are installed the piles. Have you installed piles close or at the locations where the borings where conducted? How the pile length correlate with the information shown in the boring logs? I would expect that blow counts/penetration is erratic in cobbles and boulders. You should have more consistent blow counts/penetration in bedrock.

 

[BridgeSmith]

Seems I might be alone on this one, but refusal is refusal, is it not? If you have refusal, based on a adequate number of blows/in, does it matter whether the pile reached bedrock or hit a big boulder on top of the bedrock and stopped?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MTNClimber]

In my book there's two types of "refusal":
(1) Refusal at a high hammer energy/upper limit of allowable pile stress
(2) "Refusal" at a lower energy/low pile stress

On a site like the OP's, if you need a geotechnical capacity close to the structural capacity of the pile then you probably want a hammer big enough that can drive it to 0.9Fy. When dealing with high capacity piles, most geotechnical engineers will feel better if that pile made its way through the boulders and down to bedrock. Doesn't mean you can't get capacity in the boulders. Our team has allowed piles to bear on large boulders in glacial till when they were supposed go to bedrock because we obtained the required blow count while hitting the pile at the correct energy level. Theoretically we obtained the required axial capacity.

I should also note that we also had the contractor re-strike some piles afterwards when we noticed minor amount of movement while driving adjacent piles in the same pile cap... some of the piles ended up punching through the boulder(s) on re-strike. Max driving stress recorded on the PDA for that pile cap was around 34 ksi with an allowable 45 ksi. Which brings up another point. Make sure a surveyor is recording the pile tip elevations at the end-of-driving of each pile to help check if you're breaking up the boulders and losing capacity.

 

[geotechguy1]

Is it absolutely critical that bedrock be reached? Would be curious to know what capacity you got out of a pile embedded in cobbles and boulders sitting on bedrock versus one sitting on bedrock.

 

[PEinc]

Unless you make educated guesses or assumptions from the soil borings and rock cores, how will you know if the pile is refusing on bedrock or big boulders and cobbles? You can't. Refusal is refusal but may depend on hammer energy and how hard you want or need to hit the pile without damaging the hammer or pile.

http://www.PeirceEngineering.com