Dam Slope Stability
Dam Slope Stability
(OP)
Hello
I am calculating the slope stability for an earth dam.
The material has been mixed with cement and has a cu=190kpa. When the slope is modeled in an undrained state for seimic load of 0.33g it holds up with an FS over 3.
However the drained steady state model c'=7 and phi=37 (assumed as the triaxial is not back yet) gives an FS of 1.15, codes ask for 1.5.
Is something wrong here the dam is ok undrained in an earthquake but not ok drained in its steady state?
Thanks
I am calculating the slope stability for an earth dam.
The material has been mixed with cement and has a cu=190kpa. When the slope is modeled in an undrained state for seimic load of 0.33g it holds up with an FS over 3.
However the drained steady state model c'=7 and phi=37 (assumed as the triaxial is not back yet) gives an FS of 1.15, codes ask for 1.5.
Is something wrong here the dam is ok undrained in an earthquake but not ok drained in its steady state?
Thanks
RE: Dam Slope Stability
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RE: Dam Slope Stability
As high as your drained strength is and as low as your FS is, I have to conclude that your slopes are quite steep, and/or you have a homogeneous dam without any internal drainage (hence, seepage exiting the downstream slope).
190 kPa is numerically equal to 7 kPa + 243 kPa tan(37). At 100 kPa your assumed drained strength is less than half of the undrained. Is it reasonable to expect that drained strength is so much lower than undrained strength at sigma' less than 243?
RE: Dam Slope Stability
RE: Dam Slope Stability
RE: Dam Slope Stability
Thank you for the replies, the mix is 5% cement and has a tested k=3.9x10-7.
The dam was 3:1 upstream and 2:1 downstream. I have changed the downstream slope to 2.5:1 and the FS is still below 1.5.
The dam is homogeneous.
If I follow correctly my assumed c' is too low for a soil with an undrained shear strength of 190kPa?
Thanks,
RE: Dam Slope Stability
Ordinarily, I would tell you that unfiltered homogeneous dams should never be built above where people live, but it is POSSIBLE (I don't know for certain) that 5% cement might make the fill resistant to internal erosion. I'm thinking primarily of erosion in cracks or at poorly bonded lift lines, and that the actual mixing is never as thorough as in the lab, even with a pug mill. Anybody have experience with internal erosion in cement mod'd soil, or know of research data?
RE: Dam Slope Stability
RE: Dam Slope Stability
RE: Dam Slope Stability
f-d
¡papá gordo ain't no madre flaca!
RE: Dam Slope Stability
There are no buildings/infrastructure within the flood zone. The matrix material which is borrow from nearby is Sandy Clay SILT 20/20/60. Good point about the strength increase of the cured mix, so the dam will probably be strong enough after 28 days or less. I will add a toe drain and see what happens. However the phreatic line is about halfway up the downstream face.