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]

This is an interesting problem.

 

[Ron]

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.

 

[fattdad]

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

f-d

¡papá gordo ain’t no madre flaca!

 

[bakal28]

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?

 

[Ron]

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 said...be concerned about the settlement!

 

[bakal28]

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?

 

[fattdad]

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.

f-d

¡papá gordo ain’t no madre flaca!

 

[bakal28]

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.

 

[BigH]

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.

 

[mar2805]

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.

 

[BigH]

@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.

 

[FixedEarth]

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.

 

[msquared48]

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

http://mmcengineering.tripod.com

 

[mar2805]

@BigH
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.

 

[BigH]

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.

 

[Ron]

Nicely stated, BigH.

 

[fattdad]

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

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

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).

 

[BigH]

@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 . . .