The direct shear test is a strain-controlled test: the rate at which the soil will be strained is
controlled. A specimen of soil will be placed into a shear box, and consolidated under an
applied normal load. The shear box is made of two separate halves, an upper and a lower.
After the application of the normal load, these two halves of the be moved relative to one
another, shearing the soil specimen on the plane that is the separation of the two halves.

The direct shear test was formerly quite popular, but with the development of the triaxial test
which is much more flexible, it has become less popular in recent years.

The advantages of the direct shear test are:
1- Cheap, fast and simple – especially for sands
2- Failure occurs along a single surface which is approximately the shear type failures in natural soils.

The Disadvantages of the test include:
1- Difficult or impossible to control drainage, especially for fine-grained soils.
2- Failure plane is forced (it may not be the critical plane in the field)

Because the drainage conditions during all stages of the test markedly influence the shear
strength of soils, the direct shear test is only applicable for relatively clean sands which are free draining during shear. For clay soils, some unknown amount of consolidation could occur during shear, which would give larger shear strength than actual. Therefore the test is not generally recommended for cohesive soils.


Procedure

1. Become familiar with the apparatus, see figure.1.


• How to apply normal loads
• The function of each screw
• Preparing the specimen (using the fixing screws, lock the 2 halves of the shear box together, placing the bottom frictional plate and the top porous stone)



When the specimen is setup, the frictional plate will be at the bottom of the shear box. Sand
will then be placed on top of this, and on top of the sand will be a porous stone.



2. Set up the specimen.
•??Place the shear box in the direct shear device.
•??Make sure the fixing screws (screws that locks the shear box together) are locked in place and the set screws (screws that adjust the gap space between the shear box halves) are at the bottom of their travel.
•??Place the bottom frictional plate into the bottom of the shear box with the rough side up.
•??Pour the sand in rather loosely.
•??Place the porous stone on top of the sand. It should be sticking up out of the shear box at this point, see figure.2.



3. Place the loading cap and ball bearing on top of the porous stone.
4. Adjust all dial gages. Take initial readings at this time.
5. Use the consolidation curve to approximately determine the time u have to start shear test.
6. Remove the fixing screws
7. Shear the specimen.
•??Start the horizontal (shear) loading and take readings of the load, shear displacement, and vertical displacement
•??The shearing is complete when load is determined to be dropping (the residual strength of the soil has been reached