settlement/induced stress below embankment Fill

[Shakta]

Here is the situation:

5' fill will be placed on an existing 25' wide road embankment. SPT N values are 10-12 in upper 5' which is basically sand and existing embankment. Below that N value is like 2 to 3 for next 40' sand and then very dense 50+ blow sand. Groundwater is at 5' from the existing ground which is just at the bottom of existing embankment.

Will it be too conservative if whole 40' loose sand layer is considered for immediate settlement? Will there really be immediate settlement in that 40' loose sand? Thanks!

 

[geotechguy1]

To know whether or not the settlement is immediate you need to know something about it's grain size distribution / permeability. The USCS allows a 'sand' classification for many soils which have low permeability and are likely to experience consolidation.

For a 25' wide road embankment of presumably great length, you'd expect something like 30-50% of the stress increase at the surface to occur at 40'-50' depth, with a load of say 30 kPa at the surface that could be 15 kPa at the depth. So you could have some settlement at depth.

How much and whether or not it's 'immediate' is a different story.

 

[EireChch]

Geotechguy1 - where do you get the 30-50% from? Boussinessq?

Zone of influence of large fill areas is something I've never really got to the bottom of. Ive done some research and still never got anything conclusive.

If you consider your filling as a flexible foundation, then 25m wide x an infinite length, if its a road, is akin to a strip footing (l/b of 10)

A strip footing has a zone of influence down to 4 x b, so theoretically 100m. In your instance I would just consider 2B maybe so down to 50m or even shallower and stop a settlement calc at 50+ blows. 5' of fill is nothing really so you are getting very little from the layers at depth.

I have nothing to back the above up with, only gut feel.

A question for forum? How deep is the zone of influence of a 200 x 200m filled area?....surely not 400m....

 

[geotechguy1]

I got it from the Boussinesq chart for a strip footing - very approximately 50' = 2B = 30% and 25' = 1B = 50%

What is meant by zone of influence, and how should stiffness be modelled? We talk about a square footing as having a zone of influence of 2B and a strip footing as having a zone of influence of 4B, which is the depth when the stress increase is roughly 10% of what it is at the surface. But then there is the second question which is about how we model stiffness.

If the zone of influence is defined as the depth at which the stress increase calculated by Boussinesq or similar is 10%, then yes, 400m is the zone of influence of a 200mx200m filled area. However, if the zone of influence is defined a different way - say, the depth at which the applied stress at depth translates into meaningful settlement at the surface, then very likely it is much less than 400m, and the reason for that as best as I can understand it is that stiffness is nonlinear and as the applied stress decreases the stiffness increases and so if we apply a linear modulus to a 'deep' problem like a 200mx200m filled area or a 25' wide infinitely long embankment we are apt to end up with embarrassingly large calculations of settlement ( But then, our estimation of the stress increase is imprecise and our estimation of soil properties even more so). For 'smaller' problems like say, a 2mx2m footing, because the depth we are integrating over to calculate settlement is smaller using a linear modulus and ignoring nonlinearity isn't a big deal and works out to a neat little built-in factor of safety.

In CPETIT this problem is "Solved" by defining the zone of influence as the depth at which the applied stress is less than 20% of the effective stress. But not certain I'd apply that to a large embankment.

So in short, I believe Bousinesq (or Westergaard, or the multi-layer stress distribution in settle3d) are reasonable approximations or at least the best we'll get for the stress distribution in soil, but nonlinearity cannot be ignored for 'big' zones of influence like a 25' wide embankment or a 200mx200m fill. I think the 'old guy in the corner office' way of dealing with is to just make the zone of influence shallower and keep using the linear modulus.

 

[Shakta]

The existing road is there for like 60 years, now if new 5' fill would be placed on it, is it realistic that the loose sand layer will experience such increased stress? Is there any thumb rule any of you use to determine the influence zone for embankment ? Embankment footprint width could be very large.

 

[EireChch]

Good points GG.

For foundation assessment, zone of influence is typically defined as the depth at which the increase in stress (q) is 10% of the stress applied (qo),which is approx 30kPa for the OP. I dont think there are any rules of thumb than can be applied to large fill areas. Noting that a 25m wide embankment is that large in the scheme of engineering.

What you are referring to re non linear stiffness, is essentially the hardening soil small strain constitutive model in plaxis. Below a certain strain level the program uses G[sub]0[/sub] and subsequently E[sub]0[/sub] to calculate settlement. After the strain level is reached it uses E that we are more accustomed too.

The only issue is that we cant really consider this in our hand/excel calculations. Our only option, as far as I can see, is to calculate settlement based on elastic (and consolidation, where appropriate) theory and essentially ignore settlement below a certain depth. It appears that me and the old guy in the corner are similar.

RE- CPe-IT, I have heard and occasionally applied similar rules on thumb, basing zone of influence on the ration of increase in stress as a proportion of overburden stress. I did a quick guestimate for the OPs problem.

At approximately, 0.5B (12.5m depth), 1B (25m depth) and 1.5B (37.5m depth), the increase in stress is 10, 3 and 1.5%. I think limiting a settlement calculation to 10% (i.e. 12.5m) is way too shallow, but you could convince me that 1.5% (37.5m) is appropriate. Surely, a soil that has been there for a million years, which sees an increase in stress of 1.5% can not settle too much....

Very interested to hear others opinion

 

[geotechguy1]

Agree r.e. hardening soil and hs small.

>At approximately, 0.5B (12.5m depth), 1B (25m depth) and 1.5B (37.5m depth), the increase in stress is 10, 3 and 1.5%. I think limiting a settlement calculation to 10% (i.e. 12.5m) is way too shallow, but you could convince me that 1.5% (37.5m) is appropriate. Surely, a soil that has been there for a million years, which sees an increase in stress of 1.5% can not settle too much....

Our 1.5% increase could be argued to come from groundwater fluctuations - surely in a million years an increase in effective stress due to a long drought or other factors changing the groundwater table would have occured, and that stress increase could be large compared to the embankment load. A similar thing happens in urban areas with leaky stormwater and wastewater pipes.

Regarding the guy in the corner office - I agree that the conventional answer provided is correct (that is, cutting off the calculation at a shallower depth than implied by various stress distribution theories) but I think many engineers have forgotten or do not understand the reason for doing so and so we get caught up in arguments about the stress distribution and don't consider the whole picture of stiffness and stress distribution.

 

[BigH]

Personally, I would not be too worried about settlement. Place the 5 ft fill . . . and then wait for some time - a couple of months if possible before placing the pavement layers - you can then grade off the new fill to level; it is a road embankment. If the embankment has been there for so long, there will be creep settlement has happened which should reduce any settlement due to overcoming "overconsolidation".

The big issue which could be a problem depending on where you live is what happens should there be a seismic event??

 

[geotechguy1]

I still think OP needs to elaborate on what is meant by 'sand'. I can share some photos of what visually any laymen or even expert would say looks like a big lump of clay that classifies as sand according out our glorious USCS derived soil classification systems.

 

[Shakta]

Thanks everyone. It's a good discussion as always.

 

[JHeisenberg]

Late to the discussion but regarding whether to expect immediate vs. consolidation settlement...

In my area we would roughly base it on the following: Less than 20% fines then we would assume immediate settlement. More than 20% fines with a PI less than 10, immediate settlement. More than 20% fines, PI greater than 10, start looking at consolidation. Obviously this is somewhat location specific and requires to use your best judgement but this is ballpark what I would look for.