Drained and undrained shear strength of clay for shoring design 1

[sango]

Dear all,

Why undrained strengths (or total stress approaches) instead of drained strengths (or effective stress approaches)are adopted in shoring design in CLAY?

How does drained and undrained shear strength of clay affect the design of temporary or permanent sheetpile? which type of parameter is to be used for the design?

I think undrained parameters of the CLAY are used to assess the short term stability, and it does not imply that the long term stability is satisfactory. Therefore, effective parameters of the CLAY are also needed to determine for assessing the long term stability of the shoring.

Thx.

 

[DRC1]

Typically undrained strength of clay controls design for soft to medium normally consolidated clays. As these clays drain, strength typically increases. For stiff over consolidated clays, it is possible for stength to decrease with time. If you have worked in an area for a while and are familar with soil properties for the local, you probably won't do as much testing as you would in an area that you are not as familar. I suspect that if everone in your area designs to short term, it is because it controls. However, it would not take a lot of lab testing to confirm this.

 

[sango]

Thx. DRC1.

Yes, I agree with u.

For N.C. clay, positive pore water pressure will built up during shearing and short term effect for the design is more critical. But, suction in O.C. clay will dissipate with time and long term effect controls the design.

My next question is:

If loading history of the site is not known, any in-situ testings or laboratary testings can be assigned to determine the OCR of the soil?

 

[BigH]

Consolidation testing by oedometer will give a handle on the OCR of the soil. Above posts are correct - short term conditions (e.g., undrained) control normally to lightly overconsolidated clays; long term (e.g., drained) controls overconsolidated to heavily overconsolidated clays.

 

[wshi]

There is an emperical linear relationship between maximun past pressure and plastic index, it is resonable acurate.

 

[sango]

Thx. xixia,

Would u pls tell me the details and the reference of the emperical relationship as u suggested.

I know there is a correlation between the ratio of undrained shear strength of N.C. clay (cu) and effective vertical stress (v') to plasticity index (Ip)as proposed by Skempton.

Cu/v' = 0.11 + 0.0037Ip

But, I wonder the above equation can be used to determine OCR?

 

[BigH]

Such empirical relationships (simply 0.23xeffective vertical stress) can give a good idea of OCR subject to non-sensitivity of the material to significant secondary consolidation which would provide significant quasi-preconsolidation.