Effective strees analyses in slope stability applications
Effective strees analyses in slope stability applications
(OP)
I would like to get some comments or input on the selection of c’ and phi’ for cohesive soil
properties in slope stability analyses using effective stress analyses.
Let us say, that based on a considerable amount of direct shear tests, it was found that the soil has
the following properties (same data, by regression analysis) :
1. Regression - not forcing result to c’=phi’=0
c’= 20 kPa
phi’= 18 deg.
2. Regression - forcing results to c’=phi’=0
c’=0 (forced)
Phi’= 22 deg.
In the steady state seepage analysis (long term), we usually are reluctant to include any c’ component in the effective stress analyses. Adding a small amount of c’ in the stability analyses would affect the results considerably. However, if we choose the properties based on No. 1 above and dropping the c’ to 0, then we “internally” penalize (over conservative) in just using phi’=18 deg.
What about using the properties as shown on No.2 above ? (ie., we accept that c’=0, but another rerun on regression analysis will be carried out, forcing the data to include
c’=phi’=0. This way, we will use the strengths of c’= 0 and phi’= 22 deg.
Of course, if we choose No. 1 above and minimum F.O.S =1.5, it will be too conservative. What about the option of using data from No.2 ?
Any comments ?
PS.
Some people tend to analyze using concept of No. 1 above, but then they add a little cohesion in the analyses, such as using minimal value of c’= 4 kPa (0.5 psi), instead of zero. So, which is the right way for effective stress analyses ?
properties in slope stability analyses using effective stress analyses.
Let us say, that based on a considerable amount of direct shear tests, it was found that the soil has
the following properties (same data, by regression analysis) :
1. Regression - not forcing result to c’=phi’=0
c’= 20 kPa
phi’= 18 deg.
2. Regression - forcing results to c’=phi’=0
c’=0 (forced)
Phi’= 22 deg.
In the steady state seepage analysis (long term), we usually are reluctant to include any c’ component in the effective stress analyses. Adding a small amount of c’ in the stability analyses would affect the results considerably. However, if we choose the properties based on No. 1 above and dropping the c’ to 0, then we “internally” penalize (over conservative) in just using phi’=18 deg.
What about using the properties as shown on No.2 above ? (ie., we accept that c’=0, but another rerun on regression analysis will be carried out, forcing the data to include
c’=phi’=0. This way, we will use the strengths of c’= 0 and phi’= 22 deg.
Of course, if we choose No. 1 above and minimum F.O.S =1.5, it will be too conservative. What about the option of using data from No.2 ?
Any comments ?
PS.
Some people tend to analyze using concept of No. 1 above, but then they add a little cohesion in the analyses, such as using minimal value of c’= 4 kPa (0.5 psi), instead of zero. So, which is the right way for effective stress analyses ?





RE: Effective strees analyses in slope stability applications
I'm not sure that there is a "right way" to do the analysis based solely on the estimated parameters. From speaking with other engineers the best advice I have received is to initially look around the area for existing slopes in similar materials (ie railway or road cuttings etc). This will give you an instant verification of what slopes are achievable in the drained condition. If you have undertaken a series of on site tests then this will give you your total stress parameters. Rather than simply relying on regression analyses of these results alone I would also be tempted to find parameters successfully adopted for other engineering projects in these materials.
The final parameters you choose should ideally then be based on a range of parameters for c and phi and you can check the sensitivity of your results against the risk of slope failure. Whilst this may seem like a lot of work, most software these days takes the grind out of the calculation but you get a good appreciation of the sensitivity of your design and the choice of parameters becomes almost secondary to your engineering judgement.
Regards
Andy Machon
Andy@machona.freeserve.co.uk
RE: Effective strees analyses in slope stability applications