Subgrade Modulus (different approaches) 2

[pelelo]

Hello,

I am working on the geotechnical investigation for a 25 story tower project.

After computing the stimated elastic settlements (using sigma - W) (project is located on a weathered limestone), I computed the subgrade modulus.

Subgrade modulus K = applied load / elastic settlement underneath that footing or mat.

In summary, the mass presents few porous to very porous strata (SPT-N 1 - 10). The lowest K value I got was 1.5 kg/cm3. According to the structural engineer this value is way too low as it seems he is getting a very thick mat (around 1.5 - 2 m thick).

Somehow previously, another firm performed another geotechnical investigation for the same project.

The firm computed the subgrade modulus using Bowles´ approach:
K = 40 x FS x qa.

This firm came up with a K value of about 5 kg/cm3, which is 4 times higher than our values. No question the structural engineer would feel more confortable using this value instead of ours.

Now the question is, which one is the best approach?. We both used the same applied load (qa). This other firm used a FS of 3, which I didn´t used at all.

I bet there are lots of other approaches for K. I would like to know in your experience what would be the most suitable approach.

Please let me know.

 

[DST148]

I have Bowles 2nd edition which gives
ks = 12 * FS * qa = 36 qa in kcf or
ks = 40 * FS * qa = 120 qa in kN/m^3, where qa is allowable bearing capacity.
You may also try thread274-27153. There are many other threads on this topic.

 

[FixedEarth]

Pelelo-

The vertical subgrade modulus value does not have much influence if the range is about 50%. However, If you have 300% change in the k value for instance, the pressure changes only about 15%, but the moments change about 45%. So you can imagine, what that would do to the mat thickness & reinforcement. Attached are the values I use and they are a bit conservative. My value for dense Limestone is 4.3 kg/cm^3 (270 kips/ft^3). You may use about 2.5 kg/cm^3 if weathered.

Is the foundation fully compensated (floating)? I would think heave after excavation & subsequent settlement are more of a geotechnical concerns? Maybe you have a pier supported mat? The Foundation Engineering Handbook by Winterkorn & Fang, '75 edition has very nice chapters. ACI SP-152 publication, "Design and Performance of Mat Foundations", 1995, should be on your desk.

 

[mar2805]

@pelelo
Did you used plate load test results for determining load-settlement response?
Im also new to this and have posted few questions in this thread:

http://www.eng-tips.com/viewthread.cfm?qid=318295

User called DRMABZ gave a good intro and guidance, altho a lot is still not clear.

 

[mar2805]

Just a simple question.
In oreder to define "k" value for beams on ground, one should divide defined "k" value with beams width "B".
Is this ok?

 

[fattdad]

I would first make sure that the specifications require that the subgrade be free of soft or loose natural soil, even if that means an undercut and replacement or recompaction of the native soil at the subgrade elevation. No point doing new construction on something that's highly variable.

I would take a representative bulk sample and do a CBR (California Bearing Ratio) test on a remolded sample prepared at the specification requirement (i.e., 95 percent standard or modified proctor) and at the optimum moisture content). I'd then correlate the CBR value to the subgrade modulus.

I'm used to values in the range of 125 to 200 pci range. You can run the conversion to metric.

f-d

¡papá gordo ain’t no madre flaca!

 

[howardoark]

As I understand it, you calculated the settlements and used those to get the springs which the structural engineer then input into their program and they were surprised at how thick a mat they needed. They'll be much more surprised when they get 4 inches of differential settlement across their too-thin mat and a 25-story building gets red tagged and they lose their licenses and life savings in the subsequent lawsuit and have to take jobs as flag persons for the local DOT.

You used the correct method, but you might want to look at the inputs you used in Sigma/W and make sure you're ok with them. You might also look at the mats for other tall buildings in the area and see how thick those mats are.

Why would modulus of subgrade reaction be a function of allowable foundation loads?