Vane shear test: 

It is used to determine undrained shear strength of soils, especially soft clays. This test can be done in a laboratory or in field directly on ground. Vane shear test gives accurate results for soils of low shear strength (less than 0.3 kg/cm2).

Direct shear test or Box shear test:

It is used to determine shear strength of soil. It is more suitable for cohesionless soils. Using a direct shear test, one can find out cohesion and angle of internal friction of soil. This test can be done only in  laboratory.

The triaxial test:

It is one of most versatile and widely performed geotechnical laboratory tests, allowing us to assess mechanical property of soli. These mechanical properties include shear strength, cohesion, and dilatancy stress. This test is suitable for all types of soli. Advantages over simpler procedures, such as direct shear test, include ability to control specimen drainage and take measurements of pore water pressures.

The Unconfined Compression Test: 

It is a laboratory test used to derive  Unconfirmed Compressive Strength (UCS) of saturated, cohesive soils recovered from thin-walled sampling tubes and rock specimens. Unconfirmed Compressive Strength (UCS) stands for maximum axial compressive stress that a specimen can bear under zero confining stress. Due to fact that stress is applied along longitudinal axis, Unconfined Compression Test is also known as Uniaxial Compression Test.

Explanation:

Unconfined compression test apparatus 

• The unconfined compression test apparatus is used to determine compressive strength of a soil sample without applying any lateral confinement.
•  The apparatus typically consists of a loading device, loading platen, base plate, sample holder or mold, and a displacement measurement device.
• The soil sample is prepared in a cylindrical shape and is subjected to axial load until failure.

Additional InformationDirect shear test apparatus

• To calculate shear strength of soil, Direct shear test is performed on a soil sample that is contained in a metal box with a square cross-section that is split horizontally at mid-height.
• A small gap exists between two halves of box.
• By moving top half of box relative to bottom half, soil is sheared along a predetermined plane.
• The box is typically square in plan, measuring 60 mm x 60 mm.

Tri-axial shear test apparatus

• In a cell, triaxial test is performed on a cylindrical soil sample with a length-to-diameter ratio of 2 to calculate shear strength of soil.
• The standard dimensions are 76 mm x 38 mm and 100 mm x 50 mm.
• Three principal stresses are applied to soil sample.
• Out of three, two principal stresses are equal and are caused by water pressure inside confining cell.
• The third principal stress, which differs from or two principal stresses, is applied by loading ram through top of cell.

Vane shear test apparatus

• In this test, re is no mechanism to measure pore water pressure and re is no drainage facility provided.
• Hence, vane shear tests can be conducted only under undrained conditions.
• The Vane Shear test, for calculating shear strength of soil, is suitable for soft saturated clay like marine clay to find undrained shear strength.
• This test can be conducted in lab as well as in field.
• The mechanism is same, but difference is only in size of vane.

The circle drawn from two dimensional stress systems is known as Mohr?s circle of stress (Mohr, 1870).

Given,
Af=2181.7 mm²; Pf=150 N
qu = Pf/ Af = 150/2181.7
= 68.75 kN/m²
Shear resistance = qu/2 = 68.75/2
= 34.38 kN/m²
Therefore, Shearing resistance = 34.38 kN/m².

When a material is subjected to three dimensional stresses, intermediate principal stress does not have any influence on strength of material.

?f= qu/2
Where, qu=unconfined compressive strength at failure.

Pore pressure developed in specimen during test can be measured with help of a separate pore pressure measuring equipment such as Bishop?s pore pressure apparatus developed by Bishop in 1950, 1961.

The unconfined compression test is a special case of tri axial compression test in which ?2 = ?3 = 0.

The shear strength in cohesion less soils from inter granular friction, while in or soils it results both from internal friction as well as cohesion.

In plastic failure, no definite maximum load is indicated. In such a case, load corresponding to 20 % strain is arbitrarily taken as failure load.