Unconfined Compression Test of Soil Specimen

In an unconfined compression test, a cylindrical soil specimen is loaded axially until failure occurs. The test is conducted on a soil sample that is not confined laterally and the stress conditions are uniaxial. The test measures the compressive strength of the soil specimen.

Given Data

Cohesion C = 100 kN/m²

Angle of internal friction φ = 10°

Determination of Failure Plane

In an unconfined compression test, the soil sample will fail along a plane that is perpendicular to the axial load. The angle that the failure plane makes with the vertical axis can be determined using Mohr's Circle.

Mohr's Circle is a graphical method used to determine the stresses acting on a material at a point. The circle is constructed using the normal and shear stresses acting on the material. The angle of the failure plane can be determined from the point of intersection between the Mohr's Circle and the Mohr-Coulomb failure envelope.

Calculation of Angle of Failure Plane

Using the given data, we can calculate the normal stress and shear stress acting on the soil specimen. The normal stress is equal to the axial load divided by the cross-sectional area of the specimen. The shear stress is equal to the normal stress multiplied by the tangent of the angle of internal friction.

Normal stress = Load / Area = P / A

Shear stress = Normal stress x tan φ

Using Mohr's Circle, the normal and shear stresses can be plotted. The point of intersection between the Mohr's Circle and the Mohr-Coulomb failure envelope gives us the angle of the failure plane.

The angle of the failure plane can be calculated using the following equation:

tan θ = 2C / (σ1 - σ3)

Where θ is the angle of the failure plane, σ1 is the major principal stress, σ3 is the minor principal stress, and C is the cohesion.

Substituting the given values, we get:

θ = tan⁻¹(2C / (σ1 - σ3))

θ = tan⁻¹(2 x 100 / (P/A))

θ = tan⁻¹(200A/P)

Where P is the axial load and A is the cross-sectional area of the specimen.

Conclusion

In conclusion, the angle which the failure plane of the soil specimen makes with the vertical axis can be determined using Mohr's Circle. The angle is calculated using the cohesion and angle of internal friction of the soil specimen, as well as the axial load and cross-sectional area of the specimen.

A soil specimen with dimensions, 100 mm x 100 mm x 100 mm, is subjected to the

forces shown in Figure.

8 KN

2 kN

12 kN

Determ
ine

I. The minor principal stress (63) in KPa.

-4 kN

II. The major principal stress (al) in KPa

III. The shear stress on a plane inclined at 20 ^ 0 clockwise from the x-plane (in KPa).

20 deg

IV. The normal stress on a plane inclined at clockwise from the x-plane (in KPa).