Using short piles for foundation of tall structure. Any shortcoming?

[cyphos168]

Hi,

What's the general rule about using short piles for the foundation of tall building? The thing is the ground is neither good nor bad. 250mm square concrete piles only penetrated around 7m into the ground. With pile cap in place we are only looking at around 4.5m pile remaining in the ground supporting the structure. This is no longer so much a friction pile but rather end bearing pile.

Is it ok to use short piles or would I be better off switching to bored pile instead? Assuming over excavation of the ground does not happen and the piles remain in place. How about bearing capacity failure and heaving of the ground? Thanks.

Alex

 

[SlideRuleEra]

If the piles do not have to resist uplift (such as overturning moment from wind load), the length of the piles (within reason) should not matter. IMHO, 4.5 meter embedment for a 250mm pile is ok.

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[aeoliantexan]

You need to be sure that the soil below the piles can support the building weight without excessive settlement or even bearing capacity failure.

 

[ATSE]

Alternatively, did you consider using a mat foundation with ground improvement or over-ex / replace with engineered fill?

 

[Mccoy]

In the above case a critical issue to consider mandatorily might be the stability of the whole pile group behaving as a block foundation, if there are substantial lateral loadings.

http://www.mccoy.it

 

[cyphos168]

Hi All,

The stricture is 7 stories high but the base is quite large. So lateral load issue would be minor in this case. My only concern with short piles as aeoliantexan pointed is the strength of the soil underneath to prevent bearing capacity failure. The piles were driven to set so they are founded on very good soil. We will be doing a static pile load test to confirm the capacity and settlement.

We have also decided to set a minimum depth of penetration of 9m for driven piles (before pile caps) and anything shorter than that we will opt for bored piles instead.

The soil we have here in Brunei is highly heterogeneous (if there is such a word) and hence to over excavate and replace with engineered fill for a raft foundation may not be a good way to go. Plus we have not used raft foundation before, experience wise we are kinda short on.

Anyway, thank you for all your responses and will update here when the load tests have been conducted with good results.

Alex

 

[bdbd]

There are many dangerous comment in this thread.

You should make a settlement analysis. (Imaginary footing method, for example.) Then, you should decide on the length of your piles. If settlement is ok and if your piles are safe against the building load (if not increase pile length or diameter) you can go on.

 

[pileNizo]

i would expect you have a soil investigation done that way you can know what material underlays where your piles are terminated.

 

[Okiryu]

Appears that you will be installed piles to bedrock (it may be soft or hard rock). If you use short driven piles I would be careful on selecting the correct hammer size to avoid structural damage in the piles. Geotechnical capacity may be higher than the structural capacity if you install piles to bedrock so monitor the installation to avoid structural damages. Along with your static test, you can also specify dynamic testing for checking capacities and pile structural integrity.