Schmertmann's Method using Cone penetration resistance correlation to SPT 1

[sworddark90]

Greetings , I have been using this equation from Smith's Elements of Soil Mechanics book , and when comparing it with other equations that uses for instance the Modulus of Elasticity (Es) instead of "xCr" , there is a noticeable difference in settlement values , can anyone clarify , to how reliable or accurate this equation ?

where x is modulus factor = 2.5 for square footing and 3.5 for long footing (L/B>10) . Cr = 400 x Nspt(Kpa) proposed by Meyerhof (1956)

 

[EireChch]

Schmertmann CPT correlation of 2.5 and 3.5 x qc is incredibly conservatively. If you look at Robertson and Cabal 2015, their factor is up to 7 generally for SAND and higher for CLAY.

Even now I am working on a ground improvement project and we have 5 3m x 3m zone load tests to 150kPa. Even using Robertson and Cabal we are getting settlements that are 3 times what the ZLT is showing. Essentially most E correlations are highly conservative.

Bowles correlations, which I see you have used in the other post are also very conservative correlations. Stroud 1N=E (MPa) is a far better correlation in my opinion.

 

[sworddark90]

Hello EireChch , thanks for your reply . Can you perhaps explain to me why there is a noticeable difference between these two methods ?

https://www.eng-tips.com/viewthread.cfm?qid=501772

Regards,

 

[pelelo]

Since there are so many correlations of E values, some times engineers are not confident in the settlements they are trying to predict.

FHWA has a E values correlations based on SPT-N values, however I understand they are too conservative. Using moderate loads of 150 kpa to 250 kpa, and N values between 10 - 30 in sands, you get settlements over 1 inch (25 mm) easily, and that doesn't make sense.