Foundation option for liquefiable soils 1

[Silty]

Hi Everyone,
The site consists of 5 to 6 m of very loose to loose liquefiable sand over bedrock with high groundwater.
Will you consider drilled shafts or H piles for this site? The loadings of the building are relatively high and it’s somewhat sensitive to settlement

And what should be considered in design if the soil around the pile liquefy?
And for H-piles, will driving trigger liquefaction in your opinion?
Thanks in advance for sharing your thoughts

 

[JoshPlumSE]

I'm not an expert in liquefaction. So, I'm only going off of a few case studies that I've read about. But, my thoughts are the following:

a) If there is underlying bedrock and stable soils, then the obvious suggestion is to have piles or piers that go all the way down to bedrock or far enough into the stable soils to be secure.

b) I read a case study where the geotech solution was drilled shafts of gravel spaced around the site. The idea was that the pore pressure (I think that's the correct term) in the liquefiable soil couldn't build up enough to cause liquefaction. Instead, you'd get geysers shooting water up these gravel piles. My belief is that this location was a parking lot rather than a heavy structure. So, I don't know how feasible this procedure would be for a larger building.

 

[TigerGuy]

Much depends on the structure you intend to build. A light structure can probably function on a reinforced aggregate "mattress" fill, of some thickness. Moderate loads might be well suited to a stone column (GeoPier(R)) system, probably using a drain rock instead of regular base. Depending on the structure footprint, it may be more cost effective to remove and replace or rework the existing material. If time permits, surcharging the site could be an option to reduce the liquefaction potential.

Whatever you choose to do, it will cost more than "standard" construction on shallow foundations, but most sites can still be developed.

 

[SlideRuleEra]

The site consists of 5 to 6 m of very loose to loose liquefiable sand over bedrock with high groundwater.
The loadings of the building are relatively high and it’s somewhat sensitive to settlement

1) Will you consider drilled shafts... for this site?

2) Will you consider... H piles for this site?

3) And what should be considered in design if the soil around the pile liquefy?

4) And for H-piles (or prestressed concrete), will driving trigger liquefaction in your opinion?

1) Probably not, because of the high water table. If site-wide dewatering will be performed anyway (say, for foundation construction) drill shafts may be an option.

2) Yes, and also prestressed concrete piles.

3) The structure is designed so that no lateral support from (liquefied) soil is needed.

4) No. Pile driving delivers local energy pulses (to one pile at a time), not sustained energy over a large area needed to liquefy soil.

 

[EireChch]

1) Drilled shafts (which i think is just a bored pile eh?) will work fine. You just need to use a casing and tremmie concrete from bottom of hole to displace groundwater.

2) H piles could also work. I would prefer drilled shafts as rock socketing piles into rock will be better than driving H piles to top of rock or penetrating weak rock. I work in an area of rock at approximately 10m depth. 99% of piles are bored piles. Very rarely driven is used. An extract from Tomlinson Pile Design and Construction 6th Edition page 186 on the matter. What strength is your rock?

Steel tubes driven with open ends or H-section piles are helpful in achieving the penetration
of layers of weak or broken rock to reach virtual refusal on a hard unweathered stratum.
However, the penetration of such piles causes shattering and disruption of the weak
layers to the extent that the shaft friction may be seriously reduced or virtually eliminated.
This causes a high concentration of load on the relatively small area of rock beneath the steel
cross section.

3) As SRE said, you should also consider negative skin friction. Boulanger (2003) - piles founded in liquefiable and laterally spreading ground. This is your best reference on the matter. The method to consider NSF is a modified Fellenius neutral plane method.

4) Agree with SRE.

Part b) of what JoshPlumSE described is just stone columns. They work in two ways, one to dissipate excess porewater pressure and two, they increase the stiffness of the ground due to the lateral displacement of sand as the column is expanded in diameter during installation.

In my experience drain rock (which I assume is a uniform graded material to promote drainage?) is not used with stone columns. The primary purpose of stone columns is to densify the ground. A well graded sandy gravel is the best material for this as it will reach higher peak stress and can be compacted into a denser state which will in turn increase lateral stresses in the sand.

 

[geotechguy1]

If you have bedrock at 5m and a settlement sensitive structure, you could also consider a super old fashioned ground improvement: Dig it out and backfill it with something non-liquefiable.

Alternatively seconding the others - piles to rock designed for downdrag (and also lateral deflection if that's a risk)

 

[MTNClimber]

Digging below the groundwater table will require a large dewatering system which will probably be expensive when compared to newer ground improvement techniques. Other ground improvement techniques such as DDC, aggregate piers, rigid inclusions, vibrocompaction, ect. could work though. Aggregate piers, DDC, and vibrocompaction can also mitigate liquefaction and lateral spreading.

 

[Mccoy]

During the early years of my career, I used to work with a foundation company, specialized in drilled piles.

There would be no problem whatsoever to drill a large diameter bored pile thru the saturated ground, by use of steel casing or light 'to waste' casing or drilling mud. And further down to achieve a suitable socket if the rock is not very hard.

In hard rock, a DTH hammer would be necessary, it would just be a messy business with plenty of cuttings around the drilled hole.

I would use large diameter bored piles since they would work as a concrete column, no containment from the liquified ground is provided, it would be like a building suspended on columns surrounded by water.