## Shear strength for slope stability analysis

## Shear strength for slope stability analysis

(OP)

Hi, I am doing a preliminary slope stability analysis and would like to hear your opinions.

The slope is 1:2 (V:H), about 10 meters in height and covered by vegetation. Visual inspection indicates that the slope is stable. Water table at 10 below the top of the slope. Soils are sandy silts (ML) however they have about 23% of clays. Sands are about 41%, the remaining are silts. Moisture contents are about 27% with Atterberg limits from NP to LL=43 and PL=30. Degree of saturation is 73%~86%. I did some unconfined compression tests (qu=130~160 kPa) and some UU triaxial tests (c=38~55 kPa and phi=12~28 degrees).

My question is about how do you consider the shear strength for this analysis:

#1. c-phi soil

#2. Total stress/short term analysis (Su= "X" , phi = zero)

#3. Effective stress/long term analysis (Su= zero , phi = "Y"). For this case, perhaps the fully softened strength may be appropriate but based on the actual conditions of the slope, I doubt that wetting-drying process can affect the slope as it is properly covered with vegetation.

Anyways, I am inclined to use #1 above because of the proportions between sand, silts and clays. I was also looking at shear strengths using #2 and # 3, but the factors of safety resulted in less than 1. That is one more reason to use #1.

Talking with one of my seniors, he also suggested to use #1.

What do you think?

The slope is 1:2 (V:H), about 10 meters in height and covered by vegetation. Visual inspection indicates that the slope is stable. Water table at 10 below the top of the slope. Soils are sandy silts (ML) however they have about 23% of clays. Sands are about 41%, the remaining are silts. Moisture contents are about 27% with Atterberg limits from NP to LL=43 and PL=30. Degree of saturation is 73%~86%. I did some unconfined compression tests (qu=130~160 kPa) and some UU triaxial tests (c=38~55 kPa and phi=12~28 degrees).

My question is about how do you consider the shear strength for this analysis:

#1. c-phi soil

#2. Total stress/short term analysis (Su= "X" , phi = zero)

#3. Effective stress/long term analysis (Su= zero , phi = "Y"). For this case, perhaps the fully softened strength may be appropriate but based on the actual conditions of the slope, I doubt that wetting-drying process can affect the slope as it is properly covered with vegetation.

Anyways, I am inclined to use #1 above because of the proportions between sand, silts and clays. I was also looking at shear strengths using #2 and # 3, but the factors of safety resulted in less than 1. That is one more reason to use #1.

Talking with one of my seniors, he also suggested to use #1.

What do you think?

## RE: Shear strength for slope stability analysis

Mike Lambert

## RE: Shear strength for slope stability analysis

Is any correlations to get drained strength parameters for sandy silt soils? Most of the correlations that I know are for clays and involve the PI values.

## RE: Shear strength for slope stability analysis

## RE: Shear strength for slope stability analysis

Mike Lambert

## RE: Shear strength for slope stability analysis

## RE: Shear strength for slope stability analysis

Mike Lambert

## RE: Shear strength for slope stability analysis

I use correlations between atterberg limits and peak/residual friction angles as appropriate. There are many correlations available in the literature. I normally use a friction angle that is one standard deviation below the 'average' value determined by the correlation (assuming that I have that info for the particular correlation) for design purpose.

## RE: Shear strength for slope stability analysis

Mike, I found that chart. See attached. Thanks. I guess that using this chart will be conservative since it says that it is for normally consolidated soils and my soils are overconsolidated.

Bottom line, also from geotechguy1 response, looks like there is an agreement on using the drained strengths. I will look at the peak strengths and run some analysis. Thanks again for your responses.

## RE: Shear strength for slope stability analysis

It is not either/or. Do both #2 (short term) and #3 (long term), with FSS values.

## RE: Shear strength for slope stability analysis

www.PeirceEngineering.com

## RE: Shear strength for slope stability analysis

I am doing both analysis: #2 and #3. For short term (#2), I am using soil parameters from UU triaxial tests that I have done. Since my soils are unsaturated, the UU triaxial tests are giving me c and phi values. I will use both. The USACE manual for slope stability indicates that results from UU tests can be used also for partially saturated soils. Using c and phi values, FOS are larger than 1.5.

For long term (#3), I found that the FSS given by the Stark's correlations are very close to the peak values from the correlations shown in Lambe & Whitman's book as suggested by GeoPaveTraffic. Looks like the L & W correlation is a bit conservative. c will be zero. As PEinc indicated, this case controls.

BTW, I already ordered Duncan's book. I wanted to buy this book form before but forgot about it.

Thanks again for your inputs.

## RE: Shear strength for slope stability analysis

For stiff-fissure clays (e.g., the Potomac in Fairfax, we'd limit the drained strength to fully-softened or residual. Don't want to type too much on that.

We'd also require stability to consider circular and non-circular analyses and in design-build, we'd require reliability analyses ala Duncan.

f-d

ípapß gordo ainÆt no madre flaca!

## RE: Shear strength for slope stability analysis

## RE: Shear strength for slope stability analysis

## RE: Shear strength for slope stability analysis

f-d

ípapß gordo ainÆt no madre flaca!