MAT FOUNDATION - Not able to reduce the Soil pressure by increasing depth of Mat foundation.

[chaitrar]

I am analyzing Mat foundation using SAFE Model, having a soil pressure of 82 kPa. But the allowable is only 40kPa. I have increased the depth of MAT foundation, yet the model shows same pressure. Can anyone explain me why the SAFE is showing me no change in soil pressure. It will be a great help. Thank you.

 

[KootK]

Perhaps the mat is already so thick that it is effectively rigid and further increases in thickness do not improve pressure distribution?

Does the pressure under the footing look uniform currently? Or lots of localized peaks?

 

[To Madeira]

40kPa is an insanely low bearing capacity for soil - is your mat foundation on a marsh? With this bearing capacity, I suspect the weight of your building plus live loads cannot be supported.

 

[TRAK.Structural]

Check your subgrade modulus constant. If it is very stiff the load will transfer straight down through the mat into the soil below without much spreading. If you have a geotechnical engineer on the project you should consult them for the appropriate value to use for your specific site.

 

[chaitrar]

Perhaps the mat is already so thick that it is effectively rigid and further increases in thickness do not improve pressure distribution?

Does the pressure under the footing look uniform currently? Or lots of localized peaks?

It's almost uniform. It does not have any peaks. I have attached snip for a quick look. The settlement also looks impractical.

 

[chaitrar]

40kPa is an insanely low bearing capacity for soil - is your mat foundation on a marsh? With this bearing capacity, I suspect the weight of your building plus live loads cannot be supported.

Yes, your understanding is right. The Mat foundation is on clay. The Client is not okay with Pile foundation, Geo technical engineer has given me a soil subgrade modulus of 500, which comes to the bearing capacity of 40kPa. What solution I can provide in this situation. The model is also showing me Irregular settlement as shown in the snip & also attached soil pressure distribution for envelope. I have ULS Load combinations + Nonlinear Uplift cases for envelope.

 

[chaitrar]

Check your subgrade modulus constant. If it is very stiff the load will transfer straight down through the mat into the soil below without much spreading. If you have a geotechnical engineer on the project you should consult them for the appropriate value to use for your specific site.

The subgrade modulus is confirmed by geotech engineer. Its very low for this particular Site. There is nothing the client is ready to do for this particular site.

 

[HTURKAK]

If the safe bearing pressure of the soil is only 40kPa and the average soil pressure 65 kPa , you can not reduce the pressure to 40kPa with increasing the mat thickness and /or softening the springs . The average settlement 120 mm corresponding to the 65 kPa implies you have adapted K =550 kN/m3 which is too low .

If the safe bearing pressure 40kPa, and assume SF= 2.5 and corresponding allowable settlement is 10mm
The Winkler spring constant should be around K=40*2.5/0.01=10000 kN/m3,
The structure could be equivalent of six storey bldg and IMO, you have two options ;

- Provide a basement storey to reduce the effective stress ,
- Consider soil improvement or piling .

Say my opinion only,

 

[KootK]

Foundation rigidity is relative to the subgrade modulus. Given your low modulus and uniform soil bearing stress, it appears that your foundation is already rigid in this sense. So it makes sense that increasing the foundation thickness is not improving matters

 

[human909]

I am analyzing Mat foundation using SAFE Model, having a soil pressure of 82 kPa. But the allowable is only 40kPa. I have increased the depth of MAT foundation, yet the model shows same pressure. Can anyone explain me why the SAFE is showing me no change in soil pressure. It will be a great help. Thank you.

Pressure = Force / Area

Once you have approach a rigid condition you can thicken the mat as much as you won't and you won't increase the area. How about widening the foundation? Surely this engineering 101?

 

[TRAK.Structural]

Seems to me that the next steps would be to explore a larger mat as noted above or to push the client towards deep foundations. Client preferences are important, but if the numbers don't work than you have your answer.

 

[EngDM]

The subgrade modulus is confirmed by geotech engineer. Its very low for this particular Site. There is nothing the client is ready to do for this particular site.

That last sentence is your problem. There's some things that just aren't feasible/economical on clay, and the client needs to understand that. We do a lot of work on clay and I feel you, the sites just suck.

What is the load condition? I had a geotech suggest to me to reinforce the slab to stiffen it up to engage more of the soil further in to the "field". My load case was a line load near the edge of my slab however.

 

[Just Some Nerd]

I can't even imagine building on 40 kPa, feels like a recipe for disaster imo. Practically asking for settlement/serviceability issues if you don't take the foundations down to something stiffer

 

[human909]

I can't even imagine building on 40 kPa, feels like a recipe for disaster imo. Practically asking for settlement/serviceability issues if you don't take the foundations down to something stiffer

Depends what you are building and how. If it basic residential build you really don't need much bearing capacity. If you provide a stiff raft then most/all serviceability issues should be covered. Settlement is what it is and can be accommodated. But it can be readily less than a 100kPa or 150kPa strip footing design.

Recently we put 12x1500T silos onto 100kPa soil. The required bearing load for these is an order of magnitude greater than a typical 2-3 story construction.

 

[haynewp]

I would also not build on soil with 835 psf equivalent allowable if the actual pressures are getting close to that under a mat slab. I can’t imagine settlement not being an issue no matter what any models say.