Step 1: Active earth pressure in cohesive soil.
The general expression for active earth pressure at depth $z$ is:
\[p_a = \gamma z K_a - 2C \sqrt{K_a},
\]
where $K_a = \tan^2\left(45^\circ - \frac{\phi}{2}\right)$ and $C$ represents cohesion.
Step 2: Pressure at the top of the wall ($z=0$).
At $z=0$, the active earth pressure is:
\[p_a = -2C \sqrt{K_a}.
\]
This negative value indicates tension at the top, which is not physically realistic.
Step 3: Application of surcharge $q$.
The expression for active earth pressure, modified to include a surcharge $q$, becomes:
\[p_a = q K_a - 2C \sqrt{K_a}.
\]
Step 4: Condition for zero pressure at the top.
For the pressure at the top to be zero ($p_a = 0$), the following condition must be met:
\[q K_a = 2C \sqrt{K_a}.
\]
Solving for $q$:
\[q = \frac{2C}{\sqrt{K_a}} = 2C \tan \alpha.
\]
Step 5: Conclusion.
Therefore, the required uniform surcharge intensity to achieve zero pressure at the top of the wall is $2C \tan \alpha$.