Soil Spring Stiffness for Mat or Pile Foundation 5

[struggle67]

Hi

How do I get the soil spring stiffness/subgrade modulus/Winker constant for Mat foundation or pile modeling in my 3D model? from pile ULT tests? plate load tests? lab triaxial tests? Any quick theoretical formula?

I know that I can get those from a geotechnical engineer but in my region, unless there is a deep excavation in a project, there is no geotechnical engineer involved.

Thank you

 

[HTURKAK]

If you know the allowable soil stress and factor of safety, you can estimate Winkler spring ks

ks= 40*F.S.*q all ( for SI units)..

if F.S = 3 qall= 250 kN/m2

ks= 40*3*250= 30 000 kN/ m3

This eq. for qall which corresponds 25 mm settlement.

Ref. ( Foundation Analysis and Design by BOWLES )

 

[centondollar]

If you read more about the subgrade modulus in the book by Bowles, you'll notice that the formula "ks = 40*FS*qall" has many ridiculous assumptions (e.g., assumed soil pressures, 25mm settlement (time-independent and supposedly also scale-independent (independent of size of loaded area and the characteristics of soil relatively far below the loaded area)) and is not therefore not suitable for any large-scale designs in which a +-200-500% accuracy in subgrade modulus is not acceptable.

Struggle67: I recommend you talk to an experienced geotech about the ways to evaluate the subgrade modulus. The iterative procedure, outlined by "BigRedGeo" in this thread, might provide insights into how subgrade modulus can be more accurately estimated:

https://www.eng-tips.com/viewthread.cfm?qid=63500

Remember, the subgrade modulus is not a parameter that can be directly measured (plate load tests only give short-term results, do not account for the size effect (influence depth of soil stress etc.) and are not usable for soft soils), nor is it easy to estimate its size. It exists only because engineers (Winkler and many after him) realized that such a constant is an essential parameter for the mathematical solution of "elastic foundation"-type problems.

 

[struggle67]

Centondollar, HTURKAK Thanks

Thanks for the link.

 

[Settingsun]

As a counterpoint to centondollar's comments, a lot of cases are fairly insensitive to the value. This is fortunate because you'll only get a rough estimate in many cases. Sensitivity checks are done to cover the expected range.

 

[TLHS]

Honestly, I'd be assuming a few hundred percent potential swing in soil modulus over a large mat foundation even if it were carefully calculated.

 

[centondollar]

Steveh49 and TLHS:

A +-100% swing will not affect bending moments significantly (based om papers I've read that treat the winkler model for beams, the bending moment variance is roughly 10-25% for a +-100% error in subgrade modulus), but several hundred percent swings will cause problems for design. As you all know, reinforced concrete cannot be made "too strong, just to be safe", because that would mean over-reinforcing and brittle failure modes.

 

[patswfc]

I agree with varying the stiffness parameter to get a range of design values.
If getting guidance from a geotech make sure that they are considering all of the soil layers that will be affected by the stress bulb under the mat foundation. I've had goetechs provide a subgrade modulus based on a plate load test only, when questions on this they said they thought the structural analysis software would somehow take account of the different soil layers and the stresses in each!
If you dont have info from a geotech, the attached is from Design of ground-supported concrete industrial floor slabs by British Cement Association and gives some typical values for different soils.
Depending on how the loading on the slab, varying the stiffness can have a significant effect on the forces in the mat.

The stiffness of the support to the mat should also be varied to account for the soil continuing past the footprint of the mat (stiffest in the corners, less stiff along the edges and less stiff again in the middle). E.g. if you have a raft foundation with a uniform load supported on constant stiffness winkler springs, you will not get any bending forces in the slab in analysis software. In reality the stresses in the soil around the perimeter will be lower as the load can spread out through the soil beyond the footprint of the raft, thus the raft will dish in the middle as the soil around the perimeter wont settle as much and bending stresses will develop. There is a publication by IStructE - Soil-Structure interaction, which has a couple of examples which show how using the simplistic winkler spring model can give completely the wrong result (bending moment causing tension in one face, whereas in reality the tension would be in the opposite face!) depending on the stiffness of the raft, supports and loading arrangement.

 

[struggle67]

Thanks, everyone,

I have concluded this. Please correct me if I am wrong.

1. For low-rise / small mat
Use the estimated value with a range +-100%. Assume a low subgrade value for mat reinforcement and high value to get bearing pressure.

2. For large mat or high rise or special cases like mat with thick soft clay layer on one side and thin layer on the other side
Consult geotechnical engineer. Use iterative approach if possible with a few hundred % swing in subgrade modulus value.