Residential strip footing with 300mm diameter mass bored piers at 2m max spacing ? (AS2870)

[Huong83]

I currently review some structural drawings. Footing to be found on natural clay of 400KPa.
No soil class here. The slab on grade is 120 thick SL82 mesh. It look ok.
Strip footing typical is 300 wide x 600 deep (internal) and 450 wide x 600 deep (external double brick wall).
Residential strip footing are used with 300mm diameter mass bored piers at 2m max spacing. Bored piers are extended to hard clay foundation of minimum 450KPa foundation bearing stratum.

This 2 storey residential house (1st flor 200 thick reinforced concrete slab) is 11m wide 22m long.

Strip footing I think is enough to carry load ? so why use bored pier here and no reinforcement.
And the pier of 300mm diameter seem to be small to have enough soil bearing capacity ?

 

[rowingengineer]

AS2870 needs a soils classification to be relevant so I think you need the soils class.
p
Natural clay with a allowable bearing capacity of 400 kpa sounds wrong, whom came up withe the 400 kPa? are these in reference to end bearing kpa for the piles and have no relevance to the strip footings? If so this would make more sense.

Do you have a sewer near the building?

There is too many unknowns to give any advice.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Huong83]

Rowing,
No sewer near the building.

It look like no geotechnical report.
With 400Kpa bearing capacity, you have weather shale not clay.
Bored piers sitting on hard clay 450KPa ? It doesn't look right ?
Nothing say about socket length so 300mm diameter sitting on bearing of 450KPa will give low compression loading capacity. Therefore, foundation just rely on strip footing. I mean it is not necessary to have these 300mm diameter mass concrete pier at 2m spacing.

I am not clear about the way using the foundation system of strip footing and bored piers. Maybe piers are used to reduce the span and movement of the strip footing ? Of course, these pier carry some compression load.

 

[civeng80]

If the edge beam is on a boundary it may be because there could be an existing building on the boundary. So they use bored piles not to undermine it.

Maybe there will be a proposed building on the boundary.

On boundary footings I think its always a good idea to go deeper.

 

[Huong83]

Civ,
it can be a good reason. But piers are used for internal strip footing too.

 

[hokie66]

Whose drawings are you reviewing? Have you had this conversation with the engineer who designed the footing system before coming here for advice?

 

[Huong83]

The drawing designed by another engineer. The client asked me to review it to the suspended slab only. But I have a look the ground floor slab by the way.
But, I see some other drawings, some engineer like to use 300mm diameter mass concrete piers (for one or two storey building).

With allowable bearing capacity, the width of a strip footing can be designed to carry the load of 2 storey building. If using strip footing and bored pier system, then the strip footing will span between pier and pier. If the end bearing capacity (+shaft adhesion capacity) of the piers is not enough to carry the load, then the strip footing transfer some (or full) the load onto ground and rely on the allowable soil bearing capacity under the strip footing.

 

[hokie66]

Again, you should be having this conversation with the other engineer. The design of footing systems is highly dependent on location, local soil conditions, and local experience. I agree with rowingengineer that 400 kPa clay is a rare thing.

 

[civeng80]

Are you sure 400kPa is the working pressure and not the ultimate pressure?
As Hokie and rowing says it sounds very high, normally between 100 to 200kPa.
Maybe designerwant to bear on firmer more stable soil and not relying on the unstable clay.
Do you know how reactive the clay is ?
Agree with Hokie you should consult with the designing/geotech for further advise.

 

[Huong83]

I think it is the design engineer giving wrong statement (400 KPa should be weather shale).
You can use deeper beam for high reactive soil or using bored pier, it will limit soil movement.

I think using 300 diameter mass concrete pier is a waste.
Strip footing of 300 wide can give 0.3x400 = 120KN/m working load should be enough carry the upper floor level and brick wall.

 

[hokie66]

Please give us an update after you confer with the design engineer.

 

[Huong83]

He said footing is based on bored pier of 450KPa bearing capacity, not on strip footing (so the statement of clay bearing =400Kpa is wrong).
But I doubt about those pier of 300 diameter without socket.

 

[oldrunner]

Although I am from California, I am very much aware of the reactive problems in Australia. This sounds like a pier and grade beam system where the engineer is trying to make the whole system stiff enough to prevent or limit differential movement. Do the grade beams have three bars top and bottom? Most of the American homes have continuous footings with one bar top and bottom and we wonder why our houses crack. There has been a lot of very good research down under. There is some codes there focusing on foundations in reactive (expansive) soils that you should be able to download,published by your national code committees.