Hi Sumit,

I interpreted that you'd like to calculate the bearing capacity of a square footing design on a cohesive soil. The question of whether the soil behaves in an undrained or a drained manner is governed by the soil properties and the loading and drainage conditions. For cohesive soil, it is recommended that you perform both drained and undrained analyses.

For undrained analysis, you take phi'=0 and perform tests e.g. vane shear test to obtain the undrained shear strength cu or su. These types of tests give crude approximation of the undrained shear strength (half of the Mohr's circle). Bearing capacity can be estimated as multiples of the undrained shear strength.

For drained analysis, you need an estimate of the effective friction angle phi' which is mobilised when you load the soil with the footing until it fails. There is much discussion on this effective friction angle in the literature. Even the point of "failure" needs to be clearly defined for a discussion on it to be meaningful. In practice, phi' is usually estimated using correlations. Once an estimate of phi' is known, you can use Terzaghi's equation to estimate the ultimate bearing capacity.

In both drained and undrained analysis, you would need to account for the effect of shape of the footing. You also need to consider other factors (if and when they become relevant) such as loading inclination, ground water flow, proximity to a slope, inclination of the ground, etc. Many of these factors are addressed in standard soil mechanic textbooks.

Regards,

Liem

dear sir

this case it's undrained bearing capacity (friction angle =0, dilatance angle=0, poissin's ratio=0.49, E/C=500-100).

Best regards 

Messaoud

Dear sir Thanm Liem Vo,

Thank you so much for your valuable suggestion. Your useful comments have made me clear on so many issue in the practical scenario. 

Sir Messaoud, I am thankful to you as well for clearly presenting the parameters for undrained bearing capacity.

Thanking u much,

Sumit

Hi Sumit,

Because you use the total stress analysis approach (phi=0, c=Su), please be careful when calculating the term q.Nq in the bearing capacity equation. You should calculate the q (overburden pressure) with saturated unit weight of soils, instead of the effective unit weight of soils. Thanks.

Best,

Aswin

Hi Sumit Maskey

You can found, for example:

cu = c'·cosΦ' + sinΦ'·[½(σ1 + σ3) - u]

u – pore pressure

1 – main stress max

3 – main stress min

cu - undrained shear strength

c' – effective  cohesion (value of bearing capacity)

Φ' – effective angel of friction (value of bearing capacity)

Attention, soil mechanical tests (triaxial technique) and note its implementation.

 Good luck

Vane shear test is used in soft saturated clays. It measures the undrained strength, Su or cu. phiu = 0. Undrained strength is used in short term bearing capacity calculations. There is only the first term in the bearing capacity equation. The value of Nc = 5.14.

If you have to construct a traffic line, I would used the equation above and would this do adapt with different soil mechanical tests under the boundary conditions. It is cheeper as the construction must be abort later.

The values of bearing capacity are c' and Φ', so the second term is zero only by Φ'=0 or if the construction is without inner friction.

Are you sure that's so ??? Has a trafffic line a short term stability???

Good luck

Dear sir Lim, Mike and Kaniraj,

I really appreciate your valuable comments. I shall incorporate the suggestion in my work. 

I think as said by Mr. Kaniraj, the Vane shear test and the following bearing capacity can produce the result for short term bearing capacity calculation. 

Thank you for your well wishes 

You can choose undrained cohesion value obtained from UU triaxial test if the construction of the structure is rapid, say 6-12 months. For such situations, you may use cohesion value obtained from unconfined compression strength test as well provided the degree of saturation is at least 90%.

However, if the construction speed is slow enough, say 2-3 years, you may use the strength parameters (cohesion and friction) obtained from CU or CD test, as the case may be applicable.

Please note that even for cohesive soil, you may obtain angle of friction only, if the soil is normally consolidated. For over consolidated soils, you will obtain both cohesion and friction angle from CU and CD tests.

I would love to answer more of your queries on the issue.

Thank you for the very informative reply Abdul Jabbar. I have a couple of queries to add to your comment. 

1. Can you kindly quote some reference mentioning the use of UU for construction time of around 6-12 months (as stated by you), and CU/CD for longer construction times.

2. Also for degree of saturation > 90%, I think it would be very difficult to perform unconfined compression test considering the undisturbed sample might collapse during extrusion. 

Thanks.

Dear Sir Khan and Irfan,

I highly appreciate your valuable comments and your generosity for your help in future.

As per sir Irfan, I would be glad enough if you could quote some reference so that we can go thru it and learn more. I highly acknowledge all of your comments, thank you very much sir!

Hi Sumit Maskey,

enclosed a solution for your problem. Have a look.

Regards

Michael Lersow

Dear sir Naveen and sir Michael,

Thank you so much for your support. 

Cheers,

Sumit

In code and standard , the effective stress can not be' used, only total stress. It means not the Terzaghi formula.