Reducing pile risk in liquefaction zone

[skeletron]

I'm designing a foundation for a monopole structure out in a farmer's field. There is a big layer of sand that the soil report identifies as a potential liquefaction zone. The zone starts around 10 ft below grade and continues to 35 ft below grade.

I've designed a single pile extended to competent bearing material (about 45 ft below grade) as recommended by the soil report and DCPT tests. Checked the design for lateral pressures and deflection at-grade. However, I would like to present the client with an option to mitigate the seismic induced liquefaction risk.

Is there a way to make a foundation like this more "seismic proof"? It's too heavy to have a shallow footing supported by batter piles (for the lateral). The footing footprint ends up blowing up. A coworker was talking about vertical drains to dissipate the pore pressure, which I thought was an interesting possibility. But I'm wondering how possible (constructable) that would be?

I'm posting this to see if anyone has suggestions to steer toward or away from.

 

[MTNClimber]

If it's sands then you can use ground improvement to increase the shear strength. DDC, VPC, rammed aggregate piers, ect. Probably not economically worth it though unless you have lot of structures to mitigate and they're relatively close to each other. I would assume just installing the pile deeper would be more cost efficient.

 

[EireChch]

You seem to have given the best option with monopile to a non liq layer.

An alternative is ground improvement with shallow foundations, is your estimation of footprint size based on existing ground or improved ground? Cause that will obvs be less.

 

[fattdad]

is it a reasonable claim that the sand layer would liquefy? Is the area prone to some typical earthquake? Is this for code?

The only soils that will liquefy are saturated granular soils with native void ratios greater than the critical void ratio. Then there's the anticipated design earthquake.

if you have blow counts and soil information you can look at the nomographs of Seed and Idriss to look at it for yourself. Then again, there may be codes?

f-d

ípapß gordo ainÆt no madre flaca!

 

[MTNClimber]

Looks like the soils report recommended designing for liquefaction. For liability reasons, I wouldn't go against their recommendations to save the client a buck even if there was a low chance of happening.

 

[skeletron]

Yeah. The soils report provided the liquefaction insight. Loose to compact sand. Site Class F (NBCC).

Thanks for the insight.

 

[ATSE]

Do not use vertical drains alone for liquefaction mitigation.

You may consider ground improvement in lieu of piles (not piles plus ground improvement).
If you don't need uplift (net tension) for the foundation, then aggregate-based piers like vibro-stone columns or impact piers (previously proprietary via Geopier) are good alternates that provide both liquefaction mitigation and also increased bearing strength / reduced settlement potential.
Or deep dynamic compaction. Or RIC. Lots of ground improvement to choose from that does not involve concrete.