Shear strength of soil depends on cohesion and angle of internal friction between soil grains. Shear strength of soil in general , = cohesion + Normal stress x tan ( angle of friction)

In case of non cohesive soil, angle of internal friction can be determined at field by pouring dry soil on leveled surface and angle of friction is angle made by soil mass with ground . The height and spread of heap can be easily measured to find angle of friction.

Angle of friction = tan ( Height of heap of soil mass/ radius of heap)

The cohesion in soil mass depends on clay percentage and moisture content. For non cohesive soil like sand, gravel etc cohesion can be taken as zero. The measurement of unit cohesion at field is difficult. The field procedure to measure cohesion is given below.

The clay content in soil mass is separated out by washing and drying slurry sample into a cylinder. One can pore slurry sample into a small pit and allow slurry to dry. A thin metal wedge with wedge angle about 15 to 20 degree can be used for determining force ( F) required to insert into dry slurry.

Then cohesion of soil = (F X tan ϴ /2) / surface area

One can prepare a stand of four vertical legs of height 400 mm ,made from steel plates of 5 mm thick and 50 mm wide and height about 300 mm . The ends of legs should be wedge shaped to avoid effect of bearing . One can put weight in increasing order on the stand and observe static load ( F) responsible for uniform penetration of legs. Take moisture content of soil.

Repeat the same procedure for finding out cohesion for various moisture content of soil.

Lets us discuss what moisture content should be considered for determination of acceptable value of cohesion. In my opinion, for soil containing approximately clay content up to 30 percent, cohesion can be taken corresponding to 10 % moisture content of soil.

For clayey soil, cohesion may be taken corresponding to higher moisture content as more than 30 to 40 %

You can do it easily if you have the field standard penetration number (N-value). For example, if you have the N value, you can calculate cohesion, Cu=KN, where K=constant=3.5-6.5 kN/sq. m and N= field N-value. Again phi=27.1+0.3N-0.00054N*N.

You can refer to books " Principles of foundation engineering" by B. M. Das for numerous empirical equations for the estimation of shear strength parameters without Lab Test.

Md. Mahmud Sazzad Hello, coul you please share the references for K=constant=3.5-6.5 kN/sq. m?

Tks