RAFT ON PILES 3

[Robert-32]

WHEN DESIGNING RAFT ON PILES SHOULD WE NEGLECT THE SOIL PRESSURE AND DEPEND BARELY ON THE PILE SPRINGS? OR WE CAN CONSIDER BOTH FOR ECONOMICAL DESIGN?

 

[KootK]

WHEN DESIGNING FART ON PILES..

1) I don't recommend farting on piles. Your proctologist would frown upon that.

2) Yeah, it usually doesn't make sense to consider the soil and the piles as resisting vertical load concurrently. That, just because most piles will be very stiff relative to the soil. In some situations, the soil may even pull away from the raft as a result of moisture fluctuations etc.

 

[slickdeals]

When designing raft foundations, depending on the soil conditions, it is not uncommon to count on the soil to resist a portion of the loads. The geotech will usually provide a spring constant for the soil and the pile separately based on a detailed study of the soil layers.

 

[Greenalleycat]

My normal raft-on-pile foundations will be designed for ultimate limit state loads spanning between piles
This is the dependable load path and this must be the one utilised in design
In practice, there's usually a good chunk of hardfill immediately under the slab, so I expect that there will be at least some support in the early days
However, it's much more likely (to me anyway) that the soil sinks relative to the piles rather than the piles sink relative to the soil, so I would expect any soil support to diminish over the years

 

[TLHS]

It depends on the relative stiffness of the various items.

If you're building in a soft upper strata with large overall loads and settlements in the range that you'd generate loads on a shallow footing, then you're definitely getting load sharing. This is a floating pile system.
Your piles aren't going to a bearing strata, they're spreading load along the length and working to spread bearing area out deeper and wider. I've done rafted pile caps in soft clays under large tanks before and that sort of system really doesn't make sense looked at as individual piles, especially since overall group settlement is going to govern design. You also see various composite pile/soil systems that work in similar ways.

In more conventional systems where you're running piles into a stiffer strata, end bearing, or otherwise going to a generally stronger depth this isn't really the case. Your induced loads on the bottom of the raft will be much smaller. In these conditions, the upper soils are also likely to settle or consolidate under load more than the lower strata and the loads the raft takes through bearing may further reduce as part of that process.