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When I model an open up excavation in the computer model, I found that when the surcharge applied on slope is long enough , the soil movement (downwards under the surcharge region) is lower. When I model the short region of surcharge, I found that the soil movement underneath the region of surcharge is higher.

I just don't understand this situation, can someone help to explain it?

The top region is soft clay, excavation happens in soft clay, underlain is hard layer of soil.type 1 type 2

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2 Answers 2

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The program is not capable of discretizing the line load as the real soil medium, thus for a long uniform load, it mobilized/engaged more soil below as opposed to the finite soil mass under the shorter uniform load. I suggest reviewing "failure wedge analysis" for slop stability.

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  • $\begingroup$ Slope stability isn'y my concern here, I ran FOS slope stability analysis in the software, nothing wrong in both cases. I am having problem with the soil displacement. When the applied surcharge is short, the soil displacement is higher than the soil displacement when the applied surcharge is long. I am not sure whether this is correct ot not. Can someone enlighten me on this ? $\endgroup$
    – gx6336
    Commented Jan 1, 2022 at 5:51
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    $\begingroup$ The intensity of soil pressure decreases as the mobilized soil increases, the slope stability can remain constant. $\endgroup$
    – r13
    Commented Jan 1, 2022 at 18:40
  • $\begingroup$ so, my analysis model is correct ? When the width of applied surcharge is long, more soil is mobilised? Hence, the pressure spreaded to larger area of soil, and lead to smaller smaller soil displcement underneath the surcharge? $\endgroup$
    – gx6336
    Commented Jan 2, 2022 at 0:03
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    $\begingroup$ In theory, it is correct, because the longer the loaded area, the less influence from the load far away, which is similar to the effect from a concentrated load and a uniformly spread load. But usually, you are interested in the most critical case, so you shall set the loading length realistically and play with different scenarios. $\endgroup$
    – r13
    Commented Jan 2, 2022 at 0:19
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    $\begingroup$ The longer the uniform load, the equivalent load is farther away from the edge. It is more complicated than that, but doesn't it make sense? $\endgroup$
    – r13
    Commented Jan 3, 2022 at 20:00
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It may be similar to a bearing capacity problem, when you bury a foundation versus not bury it.... when you bury the foundation, it settles less because the soil around the footing provides surcharge and prevents heaving of the soil around the footing and your bearing capacity is more and settlement is less. And here, when the surcharge load is long, it prevents the soil that would heave, if the surcharge load was short, thus your settlement with long surcharge load is less... Does the program produce heaving of soil outside of the surcharge load area when your surcharge load is shorter length? Even if it didn't, the term of surcharge in the bearing capacity equation can be the cause, but I don't know how the program makes its calculations, I am just thinking out loud here, and the reason might be something else...

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  • $\begingroup$ I still dont undrestand this . This is excavation. It's exacavation of clay, so, heaving of soil at excavated part and the settlement of soil at the part where surcharge is applied is the problem here. When the length of applied surcharge is long, shouldnt the heaving more noticable at the exacavted part ? (Because more vertical surcharge as action that cause heaving of soil) ? $\endgroup$
    – gx6336
    Commented Jan 3, 2022 at 15:20

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