Footings on dilated material 1

[ruleofthumb]

I am a junior geotechnical engineer and currently working on a proposed two storey school. The drilling has been finished for the school and the general soil profile is essentially a silt-dominated soil with some clay and sand from the existing grade to about 4m below. The 'N' values range from 15 to 25 revealing a compact material. This material has minor dilation. The soil gets considerably worse as we go deeper. Below 4m it is heavily dilated in comparison with the upper layer. N values range from 7 to 12. On-site when I was supervising the drilling activity it was my understanding that helical piers would be a viable solution as the first 4m would be sufficient despite it being dilated and I was told by my superior to terminate the first borehole at 12m as it was initially only supposed to go to 8m. So here comes the issue, there was a preliminary geotechnical investigation done four years prior and they recommended conventional spread/strip footings for the school, and the soil we had determined to have minor dilation in our borehole logs was not dilated in their reports. Another issue is that after speaking with the helical pier contractors, they may extra boreholes that go beyond 12m as there is a possibility that helical piers would have to go that deep.

Are there any other solutions to dealing with dilated material? My boss won't ever put a footing on material that had minor dilation as it would make the base unstable and prone to excessive settlement. Is there a possibility that the material that we have as minor dilation was not dilated four years ago? and it is a practice to never put footings on dilated material even if it minor or is subject to the experience of the engineer. It's been tough trying to find material on this so any input would be much appreciated.

 

[geotechguy1]

Can you describe what you mean by 'dilated' i.e. what behaviour are you seeing in the soil that causes you to describe it this way?

 

[EireChch]

Why are you looking at helical piles. Shallow foundations will be able to work for a 2 story school. 7-12 SPT is in the loose to low end of medium dense but still not bad. No need for piles.

And yes, I do not understand what you mean when you say it becomes "more dilated" the deeper it goes. How are you determining dilation. For silt we use to do a simple test of rolling it into a ball and shaking it in your hand. If it shows moisture on the surface there it MAY be a soil that can dilate, but that didnt really change our assessment of parameters etc. Which are still primarily based on atterberg, PSD, odeometer testing etc.

Describing a soil as dilatant does not mean it is bad. A very dense SAND or highly over consolidated CLAY dilates when sheared sufficiently, regardless of this I would still consider those as pretty good founding materials.

 

[ruleofthumb]

From my understanding the soil is dilated either when you shake it the moisture would be seen on the surface and there’s a “wiggly” action when the soil sample is slightly twisted. It a little tough to describe but it’s the best I’ve got. So essentially the soil is prone to oscillate even with most minimal vibrations and therefore it is unsuitable for bearing purposes. In my boss’s experience any soil that experience this effect is unsuitable for bearing purposes. So I was just curious as there was a preliminary geotechnical investigation done with completely different recommendations if this was a standard practice or it’s based entirely on the engineers experience.

 

[geomane]

Are you working in the US?

 

[ruleofthumb]

No. Southern Ontario, Canada

 

[dik]

With high amounts of silt, you could be looking at major frost heave if the area freezes. Strip foundations may not be a good solution and piles may be the way to go.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[geotechguy1]

How I understand it is you've done something along the lines of a 'shake test' (

https://www.transportation.alberta.ca/Content/docType233/Production/ATT29.pdf

3.39 and 3.1.10 in this link). The water on the surface means you've got either a non plastic or slightly plastic soil with lots of silt or fine sand, this could be a problem soil which might be why your supervisor is concerned about shallow foundations. At face value those SPT values and the thickness of the denser layer at the surface means shallows would be OK, but the silt could warrant some concern.

I have heard some people refer to this test as a 'dilatant' soil test (and for example the New Zealand Guidelines still refer to it that way). In the modern lexicon dilatancy and contraction are terms used to describe whether a soil contracts or expands in volume under shear.

Alternatively it could be that your supervisor is in tight with the screw pile contractor

 

[ruleofthumb]

It is more of an observation than an actual test. I was just curious as my boss is adamant that conventional spread footings must not be on material that we observed to be 'dilated'. I realize that this may be a very specific practice that's done within the southern Ontario region.

Haha, we typically only recommend a helical pier contractor when asked. But my boss is in general a very conservative engineer.

If I may ask what area @geotechguy1 do you work?

 

[geotechguy1]

Currently NZ but before that educated and practiced in Alberta

 

[dik]

I'm glad you've moved up in the world...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[geotechguy1]

Down in salary, away from oil related projects, up in vacation time and flexibility. Better climate than Edmonton or Calgary.

 

[EireChch]

I dont see how you can design foundations based on an observation, I know engineering judgement is a big part of what we do, but basing whether a foundation should be shallow or piles on a shake test is just wrong in my opinion....

Take a sample, run a odeometer, design accordingly. if no budget for odeometers then derive consolidation parameters based on correlations between atterbergs.

Now, some people may say SILT is just an elastic material but thats another debate.

 

[theCorkster]

Dilation - a measure of the ability of water to move in and out of a soil, usually performed as part of a visual classification in the field to provide guidance as to whether a soil would be classified as silty versus clayey. For visual classification, a pure silt would dilate (water moving quickly in and out) versus a clay that does not dilate (water does not move in and out) of a soil pat. This test is performed on a sample of remolded soil and does not provide any guidance regarding the in-situ characteristics of the soil's strength, compressibility or permeability.

 

[EireChch]

Very well put corks

 

[BigH]

It would be interesting to know the area of Southern Ontario. Typically, Ontario surface is underlain by silt till (PI of about 10) - good material to build on. However, there area some areas that have poor soils (in TOronto) -along the Go Train line - we ran, once, into a silt that was underlain by sand of artesian pressure. Elsewhere things are pretty good - Sarnia area is underlain by clayey silt with a desiccated crust; Some soft soils in the Hamilton area . . . just reminiscing about my time there back in the mid to late 70s and early to mid-90s. If you are worried about frost - about 1.2 m deep, say, you could protect footings by insulation.

 

[geotechguy1]

I always found that definition of dilation to be bizarre. Isn't 'a measure of the ability of water to move in and out of a soil' just permeability