Correct Calculation of Vertical Stress in Soil

[tzx9633]

Homework Statement

In this question , i suspect the total vertical stress at the center of the clay is incorrect .

Homework Equations

The Attempt at a Solution

In the figure , we can see that the water table is art 2m below the sand . So , the 2m sand thick should be dry , am i right ? Only the 1m thick sand is moist / saturated with water .

so , when we are calculating the vertical stress , we have to divide the soil into 2 parts , which are (dry unit weight of soil x 2m ) + ( saturated unit weight of soil x 1m) + ( saturated unit weight of clay x 2m)

 

[Chestermiller]

Homework Statement

In this question , i suspect the total vertical stress at the center of the clay is incorrect .

Homework Equations

The Attempt at a Solution

In the figure , we can see that the water table is art 2m below the sand . So , the 2m sand thick should be dry , am i right ? Only the 1m thick sand is moist / saturated with water .

so , when we are calculating the vertical stress , we have to divide the soil into 2 parts , which are (dry unit weight of soil x 2m ) + ( saturated unit weight of soil x 1m) + ( saturated unit weight of clay x 2m)

Well, the problem statement says that, in determining the vertical stress, "the sand above ground level may be assumed to be saturated." So, you are correct in your objection. But the question is, how much of an error does this approximation introduce? Why don't you assume a sand porosity of 30% and quantify the magnitude of the error introduced by this approximation in the vertical stress and in the effective stress at the center of the clay layer?

 

[tzx9633]

Well, the problem statement says that, in determining the vertical stress, "the sand above ground level may be assumed to be saturated." So, you are correct in your objection. But the question is, how much of an error does this approximation introduce? Why don't you assume a sand porosity of 30% and quantify the magnitude of the error introduced by this approximation in the vertical stress and in the effective stress at the center of the clay layer?

Do you mean this is a special case where the soil is assumed to be saturated above the groundwater table ??

 

[Chestermiller]

Do you mean this is a special case where the soil is assumed to be saturated above the groundwater table ??

Did I say that? No I didn't.

 

[tzx9633]

Did I say that? No I didn't.

then , why the soil is assumed to be saturated ?

 

[Chestermiller]

then , why the soil is assumed to be saturated ?

As you already pointed out, this is only an approximation. I would like for you to solve it with and without the approximation so you can compare the results. Do you think you can do that?