grading specification requirement for soil to be used as fill below a pile supporting slab 1

[Bobbyvishal]

Hello,
I like to know what type of soil is required as fill below a pile supported slab. Is there any grading requirement for such.
Does each layer 150mm thick need to be compacted to 95% of standard proctor or the uppermost layer.

Rony

 

[SlideRuleEra]

There are no requirements for grading or compaction since a proper design has the piling supporting 100% of all loads. There may be reasons to have good soil with some compaction to aid construction - that depends on temporary project requirements. See this thread for a discussion of the subject:

http://www.eng-tips.com/viewthread.cfm?qid=327925

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BUGGAR]

It's costly but if warranted you could backfill with a sand-cement slurry, sometimes called flowable fill. If you keep the cement content low, it will be digable should you have later regrets. It can be pumped below pile supported structures just like concrete.

 

[BigH]

Basically you want a soil (i like granular fills for compaction purposes) that will not settle appreciably during the casting and initial curing. SRE stated that groups such as this has the load taken 100% by the piles - but this isn't necessarily so in that piled-raft foundations are quite common now where the bearing on the soil by the pile cap/raft and the piles both contribute. (see Poulos' paper on the high building in Dubai).

 

[SlideRuleEra]

Thanks, BigH - I learned something today. Googled on "piled raft foundation" and got many relevant links.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[msucog]

Agreed. No particulars to have it as structural fill but do consider constructibility issues. Also, getting at least a target compaction of say 92% standard Proctor is not that difficult and can save lots of other headaches down the road. Many things play into this such as settlement of the subgrade over time, how much water may end up under the slab, constructibility is a huge deal, underslab utilities, grades and materials surrounding the building, etc. I have seen such a scenario where the pad was "slopped" in then outside surface water came straight in through granular fill...made a swimming pool under the slab. Or where a similar case had utilities within this slopped fill which weren't suspended...so subgrade settled then utilities moved with it causing maintanence issues a couple years later.

 

[BigH]

All - might find this article by Poulos interesting:

http://www.masterbuilder.co.in/challenges-design-tall-building-foundations/

 

[Ron]

In addition to the other excellent advice, for purposes of a capillary barrier, you might want to specify granular soil having a fineness modulus of 2.5 or above.