Through a point in a loaded soil mass, innumerable planes passes and stress component on each plane depends upon direction of plane.

The consolidated-undrained tests are performed by two methods: i) remoulded specimens are sheared under a cell pressure. ii) moulded specimens are consolidated under same cell pressure and sheared with different cell pressure.

Shear-box test is most commonly used apparatus for direct shear test.

As compared to tri axial system, re is a little control on drainage of soil in direct shear test.

When soil is in state of stress defined by equation ?1?= ?3?tan2 ?? + 2c? tan ?? as principal stress relationship, it is said to be in plastic equilibrium.

The approximate value of pore pressure parameter at failure for very loose, fine saturated sand, saturated clays is 2 to 3.

The ory of failures was first expressed by coulomb in 1776 and later generalized by Mohr.

Concept:

The following table shows different tests and ir use:

Additional Information

Difference between Standard penetration test and Static cone penetration test:

The standard penetration test is carried out in a borehole, while DCPT (Dynamic cone penetration test) and SCPT (Static cone penetration test) are carried out without a borehole.

Concept:

Using relation

${\sigma }_1={\sigma }_3{\tan }^2\left({45}^{?}+\frac{?}{2}\right)+2c\tan ?\left({45}^{?}+\frac{?}{2}\right)$

Where,

σ1 = Major principal stress

σ3 = all round cell pressure

? = Angle of internal friction of soil

c = cohesion of soil

Calculation:

For a sandy soil

C = 0

Now,

$\begin{array}{l}{\sigma }_1={\sigma }_3{\tan }^2\left({45}^{?}+\frac{?}{2}\right)+2c\tan ?\left({45}^{?}+\frac{?}{2}\right)\\ 50={\sigma }_3{\tan }^2\left({45}^{?}+\frac{30}{2}\right)+0\end{array}$

50 = σ₃ tan² (60°)

$\frac{50}{{\tan }^2\left({60}^{?}\right)}={\sigma }_3$

In tri axial test, major principle stress ?1 is applied in vertical direction, and or two principal stresses ? and ? are applied in horizontal direction by fluid pressure round specimen.