## Friction Angle for Cut Slopes Underwater

## Friction Angle for Cut Slopes Underwater

(OP)

I have a question regarding dredging a shipping channel. The existing channel will be deepened and widened by box cut dredging. Based on past dredging events in this channel, after the box cut operations are completed, the slope fails and stabilizes anywhere between 11 degrees to 14 degrees (i.e. 5H:1V and 4H:1V). The soils are very soft to medium stiff lean clay [CL] and fat clay [CH]. To determine the angle the slope will stabilize at, I'm thinking this would be similar to the angle of repose. I am hung up on laboratory testing methods to determine the drained friction angle for the clay soils. CU with pore pressure measurement or CD to determine peak drained friction angle, fully softened strength from direct shear remolded at LL, or residual strength. I don't know if tests to determine the angle of repose of sands also apply to clays.

After the slopes are cut, then fail, I don't think the peak drained friction angle applies. The clays would be somewhat remolded, absorb water when negative pore pressures dissipate, and I think would be close to the fully softened strength. I know the likelihood of reaching the fully softened strength increases with higher PI, higher moisture, higher clay size fraction, lower sand and silt content, presence of fissures or shrinkage cracks, higher activity, etc.; similar to my situation. But since the slope has failed, would we be closer to the residual strength? I want to say yes, but I'm not sure if the failure plane would be oriented parallel to the angle the slope eventually stabilizes at.

Slope stability analyses need to be performed to prove the new cut slopes will stabilize close to the existing slope angles of 5H:1V.

Your thoughts and experiences would be appreciated.

After the slopes are cut, then fail, I don't think the peak drained friction angle applies. The clays would be somewhat remolded, absorb water when negative pore pressures dissipate, and I think would be close to the fully softened strength. I know the likelihood of reaching the fully softened strength increases with higher PI, higher moisture, higher clay size fraction, lower sand and silt content, presence of fissures or shrinkage cracks, higher activity, etc.; similar to my situation. But since the slope has failed, would we be closer to the residual strength? I want to say yes, but I'm not sure if the failure plane would be oriented parallel to the angle the slope eventually stabilizes at.

Slope stability analyses need to be performed to prove the new cut slopes will stabilize close to the existing slope angles of 5H:1V.

Your thoughts and experiences would be appreciated.

## RE: Friction Angle for Cut Slopes Underwater

https://www.researchgate.net/publication/276078531...'s_Slope_Stability_Charts_Revisited

## RE: Friction Angle for Cut Slopes Underwater

If this is coastal then daily tides and changes in water velocity continually erode and redeposit material. The issue becomes how wide a shipping channel do you intend to excavate and what rate of redeposition can you live with??? Alternatively

how often do you intend re-dredging ??? Annually or every 50 years???

## RE: Friction Angle for Cut Slopes Underwater

Also as you posted this is a shipping channel previously dredged it will need to be dredged again, just a matter of time.

## RE: Friction Angle for Cut Slopes Underwater

Some of the research I have done so far shows use of undrained shear strengths, but the slopes are temporary. The underwater cut slopes in the San Francisco Bay Mud with work by Duncan (factors of safety and reliability in geotechnical engineering) shows use of undrained shear strengths, but they were cutting slopes and backfilling with sand. Never mentioned drained strengths. Those slopes failed (if I remember correctly) because of creep and dissipation of excess negative pore water pressures due to permeable sands below the bay mud.

## RE: Friction Angle for Cut Slopes Underwater

https://www.ussdams.org/resource-center/publicatio...

When people say drained strength they infer effective stress and when people say undrained strength they infer total stress, this is not correct.

The most recent version of Canadian guidance I see says to use effective stress with no pore pressure change and undrained strengths. That is for contractive materials and soft clay. Dilative sand materials should be modelled with a residual friction angle and stiff clay with a fully softened strength using effective stress.

If you play around with the stability programs long enough you will see that putting in friction angles or su/sigma' will show you shallow sloughs as the answer. This is because the program interprets T=tan14xsigma'+0 and Su=0.25xsigma' as base shear resistance = 0.25x sigma' which is the same.

Putting in cohesion only will show deep circles that go to the base of the material and come back up as rotational failures.

For cut slopes in clay you need to consider the change in stress state. I am familiar with geostudio a sigma analysis which is a finite element mesh using consolidation parameters before and after cut and long term deformation, if acceptable deformation is predicted. Then a limit equilibrium analysis with those stress states can be completed.

## RE: Friction Angle for Cut Slopes Underwater

The following table may be of some use.

## RE: Friction Angle for Cut Slopes Underwater

## RE: Friction Angle for Cut Slopes Underwater