Foundation on recent volcanic soils 1

[Rakra]

Hi Everyone,

I'm seeking recommendations for foundation types on very loose to loose volcanic soils (Silty SANDS/Sandy SILTS) with low bearing pressures - <50kPa (as deduced from dynamic cone penetrometer), where the water table/sea level (site is about 1km from sea) is around 3.0m depth. Foundations are for an open shed structure so I don't expect loads to be more than 100kPa. No other geotech data is known and it is a remote site. Any thoughts?

 

[oldestguy]

I was once in a similar situation, having traveled to a site by air and not having any heavy equipment along to examine a dumped fill matrial forsupport.. In your case settlement, rather than shear strength, probably governs. I'd think about what I could do to measure the "settlement" properties with a small scale load test, and extrapolate it to building footing size. For example, I'd take along a spring scale to weigh a bucket. I'd fabricate some sort of "footing", say 30 x30 cm. I'd load that material up with increments, in the bucket, each wwighed, with local soil for loading the "footing". I'd fabricate a referennce support across the "footing" for a long throw dial gauge, as well as ability to rough measure settlement with a ruler. This way I'd come up with a load vs settlement chart, even though rough and extrapolate that up (down for settlement) with the expected column loads to size the structure footing for a tolerable settlement.

 

[fattdad]

In some instances, I've also just made a 3 or 4 ft undercut below the entire footing. If you backfill and compact that undercut subbase material, then you have a more known element receiving the foundation load. For lightly-loaded structures, the stress increases are often just in proximity to the bearing surface. Having the stronger soil in that region will stave off settlement and bearing concerns.

then again, the meaning of recent volcanic soils is a bit confusing to me? Is it really volcanic ash? Is it soil that resulted from the weathering of much older volcanic rocks? to what extent do these, "Soils" contain odd mineralogy that could lead to another peril?

No clue.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

It sounds like your "recent volcanic soils" must be ash deposits- if they were weathering product of older volcanics I would expect a more plastic soil, since most of our "recent" volcanics are basaltic.

I would assume then that your soils are normally consolidated- I would look at improving the footing conditions just as recommended by fattdad...even if you are restricted to using the on-site volcanic materials as backfill to the undercut, it would put your mind at ease regarding bearing capacity and settlements will be significantly less / more evenly distributed than founding directly upon the in-situ. Keep your bearing pressure low- use big footings if possible together with adjustable anchor bolts at the column base.

All the best,
Mike

 

[EireChch]

I would concur with the above. Excavate and replace. I would also recommend using some geogrid. It won’t reduce total settlement but reduce differential settlement

 

[fattdad]

In response to Mad Mike: I'd also agree that recent ash deposits are normally consolidated, but a granular soil unlikely to generate excess pore pressures. Also, we don't know if the ash is above or below the water table.

Let's say it's above the water table. How likely is it to realize consolidation - i.e., delayed settlement owing to relief of excess pore pressure?

Not saying it won't settle. Just wondering to what extent there'd be much of a time factor.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Rakra]

Thanks everyone for the responses.
@oldestguy - I was also thinking along those lines, excavate out insitu and backfill with suitable material. The volcanic soils are recent unconsolidated ash deposits from nearby active volcanoes, with depth of these volcanics estimated persist to around 20m depth. Again water levels are at around 3.0m but bound to fluctuate with tidal effects. There is available river gravel/cobbles/sand mix that can be utilised as backfill. I would think excavation down to say 1-1.5m and backfilling with the river gravel mix with a potential to use a geotextile between the fill and insitu soils so as to "contain' the backfill in case of fluctuating water levels that has potential to 'wash out' fines in the backfill. Settlement is most likely the governing factor however, its magnitude is unable to be asserted with the data available. Most can be done is as commented, to have larger footings and thicker backfill.

Any comments on this approach anyone?

 

[Mad Mike]

Fattdad,

You're probably right- settlements likely to be elastic and possibly fine for the nature of the structure...that is until the contractor hits an unforeseen wet clay stratum at the same level!

But then again we're never privileged to the full model of the site- what initially seems simple too often turns out otherwise. I think a minor undercut/replace improvement beneath footings is cheap enough to be justified wherever there is a hint of doubt. I've never been shy to recommend it...sure it might suffice to plate compact the blinding level, but I like to think of the undercut and replace as my factor of safety!

All the best Rakra,
Mike

 

[Okiryu]

Rakra, your approach looks fine, however you may be hitting groundwater during excavations and dewatering will be needed which complicates things. Also, you mentioned that this is an open shed structure, is uplift not an issue? If so, you will need more embedment for your footings. It will also be helpful to know what are the size of your footings so we can have an idea of the depth of excavations/soil replacement that you need.

Lastly, are you in a seismic area? If so, you should check for liquefaction as well.