Criteria for punching failure footing on compact sand over soft clay

[GeoDDD]

I have a project with a compact sand 4 m over 21 m soft clay. The point load is 1400 kN. The size of the footing is to be 3 m by 3 m. if the footing is too close to the soft clay. It will cause the punching failure of the compact sand and then overstressing the soft clay.

I searched the books. No one seems talking about this. a depth of 2B seeems to be very safe in terms of avoiding punching failure, which will require 6 m thick sand. I searched the China Manual. It says that you do not need to worry about the punching failure with a depth of 0.7 B below the footing, which seeems to be too shallow to me based on the generall failre cures under the footing.

Anyone has any research on this, please share your idea and wisdom.

Regards,

 

[AussieGeoEng]

You first need to check that the failure envelope will extend into the clay. Bowles (1995) has some guidance on this. Bearing capacity is sum of the bearing capacity of the clay plus a contribution due to shearing of the sand - roughly worked out based on the perimeter of the foundation.

Depending on your situation it might be settlement that governs given 21m of soft clay.

 

[FixedEarth]

You don't say the depth to the bottom of your footing. Let us say 0.8m for discussion, check that the allowable bearing pressure at -4m is greater than the induced stress by the footing. Right now you have induced bearing pressure of (1400/9) = 156 kPa & -0.8m For a square shape and using Boussinesq charts with Z/B = (4.0-0.8)/3.0 = 1.07

I am getting approx. 0.3q = (0.3x156) = 47 kPa stress at the top of the soft Clay. Your Soft Clay should have undrained cohesion > 47/2 = 24 kPa

Your major concern, however, will be checking the consolidation of the 21m stratum does not exceed say 25mm.

 

[GeoDDD]

Thanks for quick responses.

We are reviewing a report done by someone else and will probabaly take it over. The N values in the upper sandy silt/silty sand layers are generally around 10. The depth generally ranged from 4 to 5 m. The soft clay is 21 m. Two consolidation tests were done on the samples from top two meters of clay(i.e. 7 and 8 m below ground surface). The Pc were both 210 kPa, indicating the overconsolidation. The water contents of the soft clay 30% in the upper 11 m and 20% in the lower 10 m. The void ratio of soft clay were 0.8 and 0.9. The Cr is 0.03 and 0.04. Field vane testing indicated the undrained shear strengths ranging from 12 kPa to 28 kPa. Based on the local geology, the upper sandy/silty layer and soft clay are bascially outwashs of the glacier. There is no consolidation history in this area from ice shhet. The Cu in the clay are generally same from top to bottom. Water contents varies (30% top 11 m and 20% lower 10m). I dout about the results of the consolidation testing. If the consolidation is from the dicication, the top portion of the clay should have lower water contents.

In the previous report done by someone else, the recommendation were made by lowering the grade to 1.3 m to counter-balance the required floor slab loading of 24 kPa. The footing has to go down 1 m for frost protection. For 4 m sandy/silty layer, there will be only 2 m below the ooting (i.e. lowering grade for 1 m and 1 m for frost and footing thickness). For a 3x3 m footing with 2 m sand (Average N of 10) underneath, my concerns would be that the loading could punch through the 2 m sand, since we are trying to maximize the bearing capacity to reduce the size of the footing (i.e. 150kPa for N value of 10) However, the higher footing loading may punch the sand laer, and if it happens, the loading plus the weight of the 2 m sand will be applied to the soft clay, which will be a disaster. I checked Das's new book for shallow foundation. He avoided talking about the stronger layer thickness in terms of the punching failre. His calculation in layers soil has a stress distribution angel, which will not be true if punching failure is to occur. As per China Manual, the stress transfering angel could be 0 (i.e pounch failre) if the sand thickness if less than 0.75 of the footing width B.

The additional loading from the footings on the soft clay will not exceed the Pc, assuming the Pc from previous report was right. The settlement will not be a big issue. Bearing capacity will not govern the design in most soft caly senarios. But this puch failure concern really bugs me. If I could have had a thickness of B or 1.5 B, I would be faeeling more confortable. But now, we only have 2 m, which is 2/3 B, which is even less than the China's criteria. I would not follow China's standard, they are way more less conservative than North America. Thanks for the lawyears here.

It is a really intersting job and I will keep you guys updated if we can put a hand on further field investigation work.

Please advise if any of you have any study in this punching failure criteria.

Thanks,

Based on my experiences, the field vane testing reqults are usually much lower than actual in-situ strengths. Although the FVT in the previous reports indicates the Cu of 12 to 28 kPa. I would expect higher values when we can go back to do more testing. Any advices on this?