Step 1: Write the Mohr-Coulomb equation.
The shear strength of a soil is \[ \tau = c + \sigma \tan\phi \] where \(c\) is cohesion, \(\sigma\) is normal stress and \(\phi\) is the friction angle.
Step 2: Note the soil type.
The soil is purely cohesive, like saturated clay. Such soil has no internal friction.
Step 3: Set the friction angle.
For a pure cohesive soil, \(\phi = 0\), so \(\tan\phi = \tan 0 = 0\).
Step 4: Simplify the equation.
The friction term vanishes, leaving \[ \tau = c + \sigma \times 0 = c \]
Step 5: Insert the cohesion value.
Given \(c = 40 \ \text{kPa}\), the shear strength equals this cohesion directly.
Step 6: Conclusion.
So \(\tau = 40 \ \text{kPa}\).
\[ \boxed{\tau = 40 \ \text{kPa}} \]