Following data are given for a direct shear test conducted on dry sand:
• Specimen dimensions: 63 mm x 63 mm x 25 mm (height)
• Normal stress: 105 kN/m2
• Shear force at failure: 300 N
a. Determine the angle of friction, ϕ'
b. For a normal stress of 180 kN / m2, what shear force is required to cause failure?