40 storey tower footing on PCC backfilled excavation.

[Addy78]

Hi everyone,
I have encountered problem which falls between structural and geotechnical discipline.
The contractor has over excavated the site with rock material having BC of 2000 kN/m2 up to 10m. Now he wants to elevate the site to reach the required level with mass concrete. He made two proposal for it, for up to 3m depth only PCC of 20 MPa will be used. For depth greater and up to 10m plum concrete with wire mesh at centre will be used.
My question is can the concrete compressive strength can be equated with soil BC and will the concrete behave in the same way as rock would be if it is subjected to same load.
My other question is can PCC up to 3m can be poured without any reinforcement to counter thermal stresses as the contractor said that he will lay concrete in layers up to 750mm and curing it properly so that no thermal stresses will be induced and hence no thermal cracking will occur.
I hope I have explain the problem clearly enough to understand.
Regards,
Addy

 

[HTURKAK]

Over excavating the rock up to 10 m ??? What was the role of surveyors and inspectors ???

I would prefer to construct extra two basement storeys rather than filling with PCC ..










Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[Addy78]

Yeah, I know it's unusual, but the site was handed over to us like that. It is not everywhere like that but at places it is 1m to 3m and sometimes goes up to 10m. So can you suggest how to go about filling the area with proposed methodology.

 

[SlideRuleEra]

I have encountered problem which falls between structural and geotechnical discipline.

1) My question is can the concrete compressive strength can be equated with soil BC and will the concrete behave in the same way as rock would be if it is subjected to same load.

2) My other question is can PCC up to 3m can be poured without any reinforcement to counter thermal stresses...

1) I believe you are asking the wrong question. I would want to know if concrete is a suitable replacement for rock, considering the load applied by the building.

The answer will depend on the building's applied load, not bearing capacity of the rock (2000 kN/m[sup]2[/sup]). Design of the foundation may have to be altered, say, changing from (small) spread footings to a (large) mat... there is no one, specific answer, the details are the key.

2) This question is a lot easier... "yes"... if due care is taken when placing/curing the unreinforced concrete. "Due care" may go beyond the Contractor's general statement to you... but, IMHO, he is off to a good start.
It is not unusual, with proper design, for heavily loaded bridge piers to be constructed on top of thick (3m+) unreinforced concrete used for a cofferdam seal.

Find a consultant who is qualified to practice in combined structural/geotechnical... they exist.

 

[Addy78]

@SideRuleEra
Actually the building was already design with proposed footings keeping in view rocks BC. But since contractor did over excavation which is being replaced by lump of PCC that’s why I asked if same configuration of footings can be used or not for PCC back filled area.

 

[LittleInch]

I think you need to ask the contractor / designer that question formally and obtain a formal response with calculations that show what he has assumed as new BC for what is undoubtedly a weaker material than "rock", which is also a rather variable description...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Addy78]

@LittleInch
The contractor will use 10MPa concrete as PCC which has compressive strength of 10000 kN/m2 compared to rock BC of 2000 kN/m2. I was bit skeptical in taking these two variables same and the behavior of soil as compared to concrete.

 

[EireChch]

to be honest, I dont see the issue here. If the concrete has a higher compressive strength than the rock then it should be fine.

Also, I am not a structural so take this with a pinch of salt, but is thermal cracking even something to be worried about? The rock that was over excavated could have included natural fractures and discontinuities, which are likely more open and filled with crap. we would have still considered this fractured rock as an excellent founding material....

 

[Addy78]

Thanks everyone for their valuable input. Good to see people with different disciplines sharing their point of view.

 

[dik]

Can you pressure grout the fissures and brink the concrete up to a proper level and just build on that?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Just Some Nerd]

Not particularly concerned about the bearing capacity of the concrete as it is well above the rock, or whether it cracks...it's meaningless if water infiltrates through some cracks in the concrete - the footings themselves don't know where the water is coming from if it is sitting on rock or more concrete. As long as your footings have correct cover for the soil conditions, I don't see much of an issue.

You'd also get far less settlement on the deeper concrete areas I would guess...concrete is running around in the neighbourhood of 20-40 GPa for elastic modulus (not sure for 10 MPa, but 20 MPa gets you 24 GPa, 32 MPa gets 30 GPa, etc) while I often see rock only in the 100s of MPa to maybe a few thousand MPa on the high end.

Footings if anything could likely be reduced, but I personally would not be spending any time redesigning things. Can't think of anything about the concrete that would make it weaker than the original rock.

Thermals are an interesting question...10m depth of concrete could take a very, very long time to cool, and I'm reminded of things like dam projects that had to actively pump water to cool the concrete, though a dam obviously has much greater concern about leaks from thermal cracking.



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Why yes, I do in fact have no idea what I'm talking about

 

[cyphos168]

Just as a reference, the KL twin towers in Malaysia were built on cofferdams infilled with mass concrete down to quite a significant depth. You can Google the info. It was concreted down to competent strata. I believe it's limestone which is very common in KL.

 

[FacEngrPE]

Adding ice to the mix may help keep the temperature under control during the curing process. The case I remember was 5 foot thick (1.5m) not 10m, that thick likely needs more thought, and an expert plan to keep the thermal cracking under control.

 

[EireChch]

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