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Design of a 200m high slope in clay

Design of a 200m high slope in clay

Design of a 200m high slope in clay

(OP)
We are doing an investigation for a mine where one of the walls will be in transported unconsolidated sediments. The envisaged height of this wall will be up to 200m.

The profile consists of 0-60m-Gravelly sandy CLAY; 60-100m - Sandy silty Clay and silty clays; 100-140m- Organic Silty Clay; 140-160 - Silty Clay and extremely weathered shale (silty Cly) from 160-200m.

The water table sits at a depth of 70m from the surface.

What types of laboratory tests should be conducted?
What cell pressures should be used on Consolidated undrained tests?

RE: Design of a 200m high slope in clay

If you don't have a lot of experience in soil, "unconsolidated sediments", slope stability; then I strongly suggest you contract with an experienced geotechnical engineer who is.  This is nothing against your experience, I'm sure you have forgotten more about mining than I'll ever know.  But soil slope stability is nothing like rock slope stability.

Several things about your situation cuase all kinds of alarm bells.  First, 200m soil slope is significant no matter what kind of slopes you work on.  Second, "extremely weathered shale" could be a real trouble maker depending on the beding angle, type of material, etc.  Will likely need to perform direct shear testing parallel to the beding plane in that material.  Last, you have a relatively high water table.

Now for you specific questions.  First the cell pressures should cover the range from at least 20 to 100 psi.  Second type of tests should include index tests, CU with pore pressure measurement, UU, direct shear and dry density.  The index tests would include natural moisture content, Atterburg limits, and possibly some grain size.

Good luck.

RE: Design of a 200m high slope in clay

And definitely do not forget about inherent structures in the weathered shale or any residual soils you might have . . .  I learned that recently from another's design!

RE: Design of a 200m high slope in clay

I think direct Shear tests are the way to go.  Direct shear is better for remolded samples since the sample can be molded directly in the shear box.  Using CU tests, the pore water pressure measurements can be unreliable since saturation can be hard to achieve.  Using direct shear, the fine-grained samples can be sheared over several weeks if necessary to develop a nice effective stress envelope.  Also, with direct shear, the shear box can be large enough to accomdate the large particles to fairly represent the benefit of these particles in the matrix.  Whatever method you choose, it will be difficult to model the true condition since the overburden pressure is off the scale of what can be achieved in the laboratory.

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