Problem with Dirty Concrete - Bored Piles

[cgseso]

Currently we are having issues regarding the integrity of our bored piles. Our project is a residential building with 20 floors. We are using dry method when we construct the bored pile since bedrock is very shallow (less than 6 meters from Natural Grade Line)

After subjecting in cross hole sonic logging test, the third party contractor of the tes advise us that our bored piles have issues in the upper layer (2-4 meters). They are thinking that is due to dirty concrete that was not expelled in the construction process. In this light, would like to create a discussion 1) What is the best method of expelling dirty concrete from your experience 2) What is the chance that the cross hole sonic logging test may be receiving bad signals and may not be entirely reliable?

 

[oldestguy]

Have you considered using a test boring machine and coring down the center f a few for correlation to sonic and other non destructive means.? Even a small diameter rock drilling bit and a log kept of the rate of progress might tell a lot.

 

[dik]

Some questions:
How large a diameter are your end bearing piles?. Unless you are on granite or something of that ilk, I would think your shafts would be in the order of 900-1000mm dia. How are you inspecting the bearing surface (for a 20 storey building, I'd do that) How heavy are your loads? What is your bearing value? I assume bedrock is 'flat'. How highly stressed is the concrete? How is the contractor cleaning the holes? at bedrock? What is dirty concrete? What is the condition at the pile/bedrock interface?

Dik

 

[cgseso]

Hi oldestguy, actually that is our course of action at first. But we are having a hard time doing concrete coring due to tight rebar spacing. And if we core rock samples, will it be easy to identify soil inclusions?

 

[cgseso]

Hi dik, the diameter is around 2000 - 2500 mm. Length is around 20 meters. We cannot inspect the bottom that much, so we just rely on the tremmie pipe's pressure to expel the dirty concrete. Dirty concrete meaning, concrete with mixed of soil from the boring. The bedrock is severely weathered and poor in strength.

 

[dik]

With that size of shaft, the bearing pressure would be quite low... Does a little 'mud' in your concrete cause any problems? unless chemical issues, I would think it wouldn't matter. I thought the shafts were only 6m long, not 20m. Are the shafts sleeved prior to casting concrete?

Dik

 

[cgseso]

Hi Dik Yes, but not all through out. It is sleeved from NGL to 5 m of depth. I am wondering are there any methods/ countermeasures that could secure the release/ flow of dirty concrete?

 

[dik]

How deep is the shaft? if 6m then increase the 5m sleeve and hand clean...

Dik

 

[BigHarvey]

If you have been concreting in a dry hole, it's most mikely the sonic tests that can be questioned. Locate a shallow identified defect and excavate around the pile to expose the defect. If no defect is found you will have your answer.
Check if the sonic tube are mada of steel. PVC should not be used because lack of adherence between concrete and PVC can alter the signal.

 

[cgseso]

Hi BigHarvey, why concreting in a dry hole can affect the results of the sonic test? We are currently using B.I. pipes for access tubes.

 

[BigHarvey]

Hi cgseso
You should not have problems concreting dry. I believe the sonic tests can be questioned.
However if the slump value of the concrete is not maintained above 16 cm during the complete concreting section you might have dirty concrete inclusions in the sound concrete. This is because the concrete of the tremie tube goes up alongside the tremie tube instead of being well distributed. At one stage the concrete level is much higher close to the tremie pipe than on the outside of the pile and then can overflow the dirty concrete from the bottom of the excavation.
One way to know is to ask the concrete foreman if he encoutered difficulties with the flow of concrete in the tremie pipe in the last meters of concreting.
Anyway since depth of defects is not that big, you should try to expose alleged defects. If they don't show up, it means that the sonic test contractor proficiency should be questioned. Alternatively you could bring another company (provided you are sure they are working well) to do the tests again.