Shear Strength versus Normal Stress


 The ‘Soil Shear Boards’ are incorporated into this lecture again to demonstrate the importance of normal stress on the shear strength of the soil. To create another interactive but simple way of driving this point home, the instructor asks for three volunteers (two small students and one larger student, if possible). A small student will sit on one chair and a larger student will sit on an identical chair next to them. Additionally, the larger student will hold a bucket of soil in their lap to increase their own vertical load. The third person will then act as the shear force, pushing each person in their chair across the floor. The shear force volunteer will be asked to comment on how easy or hard it was to push each person. The instructor will point out that the frictional resistance between the chair and the floor is the same for each (same  factor) so in this analogy, the shear strength (ability to resist) is dependent on the vertical load. While this point seems obvious for two people pushed on chairs, the same concept is not intuitively understood when it comes to the fundamentals of   shear strength in soils so it is a simple demonstration that drives an important point home to the students.



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