Foundation over very soft ground 4

[bakal28]

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.

 

[mar2805]

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?
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.

 

[shobroco]

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.

 

[BigH]

@mar2805:

http://www.elastizell.com/

 

[mar2805]

@BigH
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).

 

[AnaAnt]

@mar2805
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.

 

[mar2805]

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.

 

[Ron]

BigH...you 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.

 

[fattdad]

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.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

@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.