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Foundation over very soft ground

Foundation over very soft ground

Hi everyone.

Requesting your expert guidance on what we normally call "standard engineering practice".

I have a soft ground with about 40kPa bearing capacity. I will have an 2 metre high engineer's fill above this soft soil where my 2mx2m footing will be seated at. The footing was design with 200kpa bearing pressure.

I initially thought I have an issue with shear failure due to insufficient bearing capacity but I was told that assuming the engineer's fill is able to achieve the 200kpa bearing capacity, the soft ground below it will have significantly less bearing pressure. This is so because the original 4 sq.m. footprint will become 36 sq.m. once we project a 45degree angle below the footing (I would assume this is the angle of internal friction). As a result, the bearing pressure at the soft ground will become 200kPa x 4m2 / 36m2 = 22kPa which is less that 40kPa.

Is this an acceptable assumption? Thank you all in advance.

RE: Foundation over very soft ground

This is an interesting problem.

RE: Foundation over very soft ground

I would be more concerned about settlement of the "soft soil" by the 6m of fill being placed on it than I would be about the bearing capacity.

The confinement of the fill will increase the bearing capacity of the "soft soil".

In your analysis, you have neglected the fill.  Need to include.

RE: Foundation over very soft ground

you really need to worry about settlement.

40 kpa is the same as 840 psf.

2 meters of fill is the same as about 825 psf and that pressure will influence to a much greater depth than a footing, 'cause the fill pad has a greater areal extent.

a foundation that's 4 or 5 ft removed from the soft clay will impart additional load (i.e., and greater than the geotechnical recommendation).  So, you'll be on your own 'cause the geotechnical engineer has a sob story to tell if there's a problem.

You need the geotechnical engineer to properly address the nature of this overall problem.d


¡papá gordo ain't no madre flaca!

RE: Foundation over very soft ground

Thanks guys for the very prompt reply.

I can identify from your replies that I need to account for settlement and the influence of the 2metre fill into the underlying soft or weak soil. With this taken into account, I would assume there would be very little left of the weak soil bearing capacity for my footing.

For arguments sake, lets ignore all other loads and just be concerned on the impact of the footing load, how much bearing pressure am I expecting to impact at the underlying weak soil 2 metres below the fill? I have mentioned a design approach above which is simply project an angle (this case 45deg) which gives the impression that the 200kpa was distributed over a wider area, hence lower bearing pressure. Is this the right approach or there is a proper way to analyse multi layer soil to determine the final design bearing capacity?


RE: Foundation over very soft ground

Don't be concerned about the bearing capacity of a soil that is 6m below your footing.  Bearing capacity is a greater concern for the soil in contact with your footing and the direct load on the soil by the footing.  You are taking care of that by using select fill.

As both F-D and I have concerned about the settlement!

RE: Foundation over very soft ground

Thanks Ron for your patience. I got your point from your first response.

Ok, let me exagerate a little bit. Say I have a 5m x 5m footing. What Im after is how to account for the effect of the footing load to the weak soil 2 metres (not 6 metres) below the engineered fill. Is my proposed approached described above correct?


RE: Foundation over very soft ground

O.K. I'll talk a bit about "bearing capacity."  Foundation performance is dependent on the depth of burial as the surrounding soil acts as a gravity resistance.  If you are preparing a geotechnical study (i.e., in order to conclude such 40 KPa "bearing capacity" you should have some sense of the typical line or column loads so you can sorta of forecast a typical footing width for your "bearing capacity" calculation.  Let's say you do this for a footing that's 2 sq. meters and based on the soil strength conclude a net allowable bearing pressure of 40 KPa.  If you were to then run the numbers for a larger (or smaller) footing size, your "net allowable bearing pressure" would be different - maybe not by too much, but different.  As such the "bearing capacity" for thick areal fill is much different than the bearing pressure for a 2 sq.. meter footing.

Let's say you now have 2 meters of fill and a "footing" that's at the depth of 0.7 meters.  Let's say the fill is real strong (heck put some geogrid in it and really increase the bearing capacity).  You may calculate a net allowable bearing capacity of 200 KPa.  That foundation load will affect the stresses in the underlying soft clay and lead to settlement.

This is the question that we're asking:  "What's the likely settlement associated with the earth fill and what's the likely settlement related to the foundation fill after the column loads are in place (or footing loads).

There are a lot of ways to increase the bearing capacity in the fill that have little to do with stresses in the clay.

Just a few thought.


¡papá gordo ain't no madre flaca!

RE: Foundation over very soft ground

Thanks f-d.

To progress our solution for my case, I will now undertake some ground improvement to address the issue of settlement. For a depth of 0.7 as you assumed, how much bearing capacity should be provided by the "ground improved" soil? Do I need to make sure the "ground improved" soil has to provide a minimum 200kPa BC as my footing has been designed for or redesign the footing based on reduced BC? What if the depth is 2 metres instead of of 0.7m.?
I understand the critical factor for this case is the settlement issue, not the bearing cap. I just need to tick off this BC part and work out my footing size based on proper BC assessment.

RE: Foundation over very soft ground

It is really amazing how many times we have had to discuss in these forums the difference between "capacity" and "settlement" when it relates to bearing pressures.  I think a lot of this has to do with the "space" that is allocated to bearing capacity when, in reality, it seldom governs.  Further few texts these days ever discuss anything but the "ideal" conditions.

RE: Foundation over very soft ground

Why not replacing the surface soil first, not adding aditional 2 meter layer but instead dig 2m into the soil, fill it and compact the material.
You avoid additional stressing of clay under wich also cancles settlement due to additional fill layer.
You basicly removed the stress from the soil and then after aplied "same" stress due to the self weight of the fill layer.

RE: Foundation over very soft ground

@mar20805 - this may be suitable if the first two meters is a reasonable and suitable fill material - i.e., alluvial sand, etc.  but if the first two meters is a soft clay of high moisture content, it will be problematic to condition the clay to a moisture content suitable for compaction, etc.  In the end, your approach is valid but must take into account the various materials you have to deal with - the other problem that one might have with your solution is a project requirement to have a specific grade elevation.  While the footing may be founded okay with a raised pedestal, it appears that there still must be a fill placement to reach a slab level(?).  In this case, one must consider the raised fill, not just the footing, in causing settlement of a soft clay.

There are solutions that can be utilized - such as the use of EPS - cut 2 m out of the foundation soil as you have indicated, replace with a fill that is suitable to support the footing, use EPS to raise the grade to the underside of the slab.  Railway bridge abutments have been used founding on EPS.

RE: Foundation over very soft ground

In difficult soil sites, we can chase a suitable material and have large excavations and backfill or we can use alternate foundation.  
This is the ideal job for a drilled pier foundation. Ask your geotechnical firm to provide you geotechnical parameters for a drilled pier.

RE: Foundation over very soft ground

Overexcavate to good bearing, preload to limit settlement, raft or matt footing for a more even settlement, or piling for little to no settlement.  These are your basic options.

If as Fattdad and BigH say, there could be a lot of settlement, which only your geotech can confirm.  So I would use the piling option.

Otherwise, you building is liable to wind up in a pile on the ground, instead of bearing on pile in the ground.

Mike McCann
MMC Engineering

RE: Foundation over very soft ground

Yeap, I was aiming at sandy material, proprely graded and compacted.
Im not shure I follow your statement "project requirement to have a specific grade elevation".
You mean the elevetion form the groundfloor slab to surounding ground of the building?
EPS, do you mean polystirol?
Sorry Im not from States...

Adding fill, compacting, and then preloading might be one way to deal with this situation. Offcourse you should calculate the time needed for the consolidation to finish, based on odeometer results. Add drainage bores to speed up the proces.

RE: Foundation over very soft ground

The OP stated that there was going to be a 2m grade rise and his footings were to be in that material.  This implies to me, at least, that the original ground is not at the correct elevation for his project.  if this is the case, then the removal of 2 m of soil and then recompacting it or brought in engineered fill to the OGL, would not be sufficient for the project - of course, it might support the footing, but again, it appears that the grade from OGL was/is to be raised.  By adding the 2 m of fill - some 35 kPa of loading, this might cause significant settlement of the soft clay - the footings might be okay, per se, but overall, the structure would undergo settlement - and if uneven, then may cause structural concerns.

I suggested that EPS might be used (styrofoam - see Utah's I-15 upgrade) which is about 1/100th of the weight of soil fill.  By excavating 2 m of soil, then using 3 m of styrofoam (OGL plus 2 m) and 1 m of covering fill, the net loading on the soft clay would be basically 20 kPa unloading.  Footings can be on EPS (again, research the founding of railroad abutments in Scandanavia on EPS) and behave well.  The addition of the structure weight (unless "heavy") will therefore not really load the underlying soft clay.

On can also consider putting the structure on a raft foundation whereas any differential settlement will be rather uniform - did this for some houses over soft clay in India.  As mar2805 suggested, the use of preload and wick drains (to speed up the settlement) might also be employed.

There are many toys (potential solutions) that can be used - but one needs to see the overall picture . . . and then develop a rational solution.

RE: Foundation over very soft ground

Nicely stated, BigH.

RE: Foundation over very soft ground

BigH has an approach that makes sense.  We're using EPS for a bridge approach in karst and so far so good. . .


¡papá gordo ain't no madre flaca!

RE: Foundation over very soft ground

Have also used cellular concrete and lightweight insulating concrete as fill to keep the loads down.

Cellular will run about 40 pcf (640 kg/m^3) in place.  Lightweight insulating concrete (with perlite or vermiculite aggregate) will be about 25 to 30 pcf (400-480 kg/m^3).

RE: Foundation over very soft ground

@Ron - I was one of the first in Vancouver (I believe - way back in 1983) to suggest the use of Elastizell on a small project where fill had been raised over 25 ft of peat and 25 ft of soft clay for a large truck maintenance shop.  Over the years they kept having to add more asphalt leading up to the buildings entrance to counter settlement (the structure was on wood piles).  We took out the sand and gravel fill at the entrance and placed Elastizell - three years later, I went back and there was no movement at all . . .

RE: Foundation over very soft ground

Elastizell - I never heard of this.
Im from Europe but this seems like the IDEAL material for fills.
I mean no compaction required?! Wow!
Lighter then soil!
Whats the bearing capacity?
Compresive strenght?

At first read I thought you guys where talking aboth XPS (extruded polystirol) wich we use as a insulation material under floor salbs since it has compressive strenght. But these are prefabricated plates that you put on compacted fill material. Again compaction is needed.

RE: Foundation over very soft ground

EPS (expanded polystyrene or white beadboard) is not cheap, but could be the solution.  I have used it in a few places, & you can't beat it for some things.  Settlement of underlying soft soil can be a big problem, & helical piers aren't necessarily the solution; you definitely need more geotech input for them.  I was asked to design new house basements for a few houses in a new subdivision where everyone had basement settling & cracks. At first visit, I thought piers were the ticket, & review of 2 geotech reports seemed to indicate it.  The clay soil was very stiff at the basement level, & for 2 m below it, but deeper than that it was like Jello & both geotech investigations stopped at 6m as it was getting worse.  The problem was that when we actually had deeper investigation done, the Jello went to bedrock at 30+m deep.  End-bearing piers would work, or a raft that would have the whole house settle straight down, or an EPS pad that basically floated the house, but other options that were tried by other consultants & contractors only resulted in lawsuits.

RE: Foundation over very soft ground

Yeap saw it!
Very impressive!

One thing that we usualy always do if we hit clay.
As we agreed geotechical reoport is a must, but before pooring the raft or any other solution we leave the ground for few days to dry out in the sun. Always watch the weather forcast and before any rain comes we poor the slab.
Its nor something that improves the ground but on the summer sun those pores will definetly dry out in the sun.
Only problem is that this will only affect surface part -0,5m under the future slab. But you cerate aditional 0,5m thick soil layer that wont consolidate as much as those with water in them.ž
I know its a long shot but this is something thats been always done (wich doesnt neceserly mean thats its the right thing to do).

RE: Foundation over very soft ground

Sorry sir but I belive that what you said in your last post is very dangerous.
You are altering clays natural water content, wich in future, if exposed to water, will start swelling process.

RE: Foundation over very soft ground

Drying out cohesion type of soils isnt a bad thing to do.
You are evporating small amount of water.
Lovering the pore water pressure.
Your effective stresses are also geting higher since.
sigma(effective)= sigma (total) - u(pore water pressure)
Soils is moved towards it plastic limit and away from its liqid limit.
Agree on the part of swelling, but this also depends on the type of mineral thats dominant in the clay.
So you should cast your foundation before any rain, wich will alter its "dried" state.
If done so, since clay has very low permabilty, it wont be affected later on if you provide drainage around you foundations.
Im interested in hearing other opinions also.

RE: Foundation over very soft ground were ahead of me on the curve.  I was still using it for roofs until about 1990, when we did this large cavity fill project for "a major theme park in Central Florida".  This is just one of many opportunities to port various construction materials across different applications.

Have specified it since for high modulus, lightweight fill for a roadway carrying pulpwood trucks over a tidal swamp.

As far as I know, both have performed well.

RE: Foundation over very soft ground

in Alaska roads are built on muskeg using "hog fuel."  Hog fuel is essentially wood chips that are made in a "hog mill."  The wood chips (i.e., lightweight aggregate) are placed on a reinforcement layer and then the road is built on the hog fuel.

Wood chips are highly frictional and when they sink into the muskeg are not prone to decay.


¡papá gordo ain't no madre flaca!

RE: Foundation over very soft ground

@fattdad - yes, I know it well - many of the roads through Burnaby in Vancouver (major ones) were built of hog fuel or had hog fuel incorporated into them.  I used it once behind a mall that was undergoing settlement due to the raised grade at the rear.  Interesting - might have considered ESP now but the hog fuel was obviously so much cheaper.

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