Foundation over very soft ground
Foundation over very soft ground
(OP)
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.
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
RE: Foundation over very soft ground
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
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
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation over very soft ground
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
As both F-D and I have said...be concerned about the settlement!
RE: Foundation over very soft ground
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
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.
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation over very soft ground
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
RE: Foundation over very soft ground
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
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
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
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
http://mmcengineering.tripod.com
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
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
RE: Foundation over very soft ground
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation over very soft ground
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
RE: Foundation over very soft ground
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?
Price?
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
RE: Foundation over very soft ground
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
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
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
Wood chips are highly frictional and when they sink into the muskeg are not prone to decay.
f-d
¡papá gordo ain't no madre flaca!
RE: Foundation over very soft ground