View PDF file of  Direct Shear Test.  

Testing objectives:

·          Estimate the angle of internal friction (Φ) and cohesion (C) from plots with different normal loads for different soils under drained and undrained conditions.

·          Determination of Shear parameters for over and normally consolidated clays.

·          Controlled stress and controlled strain

·          Computation of residual shear by prescribing a slow rate of shear with a maximum displacement limit.

·          Testing conforms to ASTM D3080/T236, ASTM D2435/T216.

Some Advantages:

• simple and fast especially for sands.

• Failure occurs along a single surface, which approximates observed slips or shear type failure in natural soils.

Disadvantages:

• Difficult or impossible to control drainage especially for fine_grained soils.

• Failure plane is forced. it may not be the weakest in the field.

Device:

·          DigiShear^TM is a series of interconnected modules that are connected to a computer via a serial (COM) port.

·          The analog to digital conversion technology is used to handle low level (mV) signals from strain gage type sensors without any signal conditioning.

·          Load and displacement are controlled by a servo controlled closed loop, automated with Distributed Data Acquisition Control (DDAC^®) to overcome electrical noise.

·          The included software allows spread sheet analysis for real time graphs generated simultaneously during the testing and computes the peak shear strength at failure.

·          The vertical and horizontal load cells are calibrated for loads up to 2000lb with transducers.

Photos:

Direct Shear Equipment	Fixing screws are locked all the way and set screws are at the bottom of their travel.	Top and bottom halves of the shear box
Sand is poured in rather loosly and the top porous stone is placed. in order for the porous stone to become flush with the top of the shear box, tap on the box.	Water will be added to saturate the soil.	The loading cap and ball bearing on top of the porous stone are placed.
Box is moved horizontally to align load cell and loading cap. Lower the reaction bar until it just touches the loading cap.	All sensors are adjusted and will be taken zero.	Fix the clamp.
Tighten the nut.	Make sure you remove the fixing screws. Make sure you remove the fixing screws.	Screws are fastened at the end of the test to take the box out.
Disassemble the shear box after the test is done.

 

 

Final Results taken from Direct Shear test :

At least 3 tests are done and for each test shear stress versus horizontal displacements is taken whereby Shear stress at peak is obtained.

For each test vertical stress is constant, so with three pairs of vertical stresses and peak shear stresses failure envelope is found.