Finding Youngs modulus in layered soil.

[Arun2505]

"Currently, I am trying to calculate the SBC (Safe Bearing Capacity) of soil using SPT N-values. To find that, I require the c, φ, and E values of each layer. I have calculated the individual c, φ, and Young's modulus values and then used a weighted average to calculate SBC. The current challenge I face is that, once I take the weighted average, the values are found to be c = 3.5 t/m² and φ = 27°. However, in the case of sand, c = 0, and in the case of clay, φ = 0. So, I feel this is an error because c = 3.5 > 0 is unrealistic and vice versa.

Kindly provide your opinion on this.

I feel I need to have some equivalent values representing c and φ based on the N-value for either cohesive or cohesionless soil. By multiplying the N-value with that equivalent value, we can obtain the Young's modulus of the soil.
Es= (fn() or equivalent value) * N value

 

[EireChch]

Tell us what the layering and thicknesses are. The size of your foundation needs to be considered too.

 

[1503-44]

I think you can calculate settlement using a factored value, but it does not seem to be logical to calculate SBC that way. Weighted average will always be higher. If layer 2 fails, so does layer 1, or visa versa. Why would you not need the lowest SBC value?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Arun2505]

Kindly dont get confused with unit weight, these are just assumed values, i will change later

My foundation starts from 5.9m below ground level

 

[Arun2505]

Yes , I understand we should go for lower value, currently my SBC is based on Settlement criteria. so , my result wont gets affected in current project. but , i feel something can be done on this shear creteria.

 

[1503-44]

Is this what you want to do?

https://civilweb-spreadsheets.com/f...ring-capacity-calculation-from-spt-value-xls/

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Arun2505]

almost like this but i have made using reference of Indian standard

 

[Arun2505]

Actually based your 1st comment, I got a idea. I am thinking to find shear strength of soil
S= C + eff. stress * tan(phi).==>Mohr Columb

Based on this, let's find the critical layer below the foundation.

suppose, if clay layer is found to have less shear strength than sand , i can take only c value of 15.5 t/m^2 as my strength throughout all layers neglecting higher strength and vise versa.

by this way , we are neglecting the impact of weighted avg.

Do you think , this can be a possible solution.

 

[geotechguy1]

How are you planning on using young's modulus to calculate bearing capacity?

 

[EireChch]

Your soil descriptions do not match your SPTs / parameters.

You are needlessly over complicating things. At 5.9m depth you will have a huge bearing capacity in SAND. Granular soils are dependent on effective stress.

Cohesive soils are not dependent on effective stress. Give how close you are to the CLAY at 7.5m, just do a calculation based on your Clay layer. 15.5t/m2 is 150kPa. Crudely, you have an allowable bearing capacity of (5.14*150)/3 = 250kPa ish.

If you wanted to do it correctly, weight average or shear stress is not the right way. You need to use a layered bearing capacity assessment using the Meyerhoff and Hanna method.

In terms of settlement and Young's modulus. You need to do a settlement calculations as per your table above. Calculate the settlement in each of your layers separately.

 

[Arun2505]

i will be use young's modulus in settlement criteria,
Elastic immediate settlements (S[sub]i[/sub]) = (p*Beff*(1-μ)[sup]2[/sup]) I)/E

 

[Arun2505]

Eirechch, let me try your suggestion and get back to you.

You are correct the soil descriptions and spt n value doesnt match. i will edit it.

 

[geotechguy1]

Unless you're planning on doing something rigorous with finite element modeling, I agree with EireChch. You need to use a layered bearing capacity equation and also consider whether you have a local shear-type or punching-type failure mode. Then, do a separate calculation for settlement.