Two different soil types (Soil $1$ an Soil $2$) are used as backfill behind a retaining wall as shown in the figure, where $\gamma_t$ is total unit weight, and $c’$ and $\phi’$ are effective cohesion and effective angle of shearing resistance. The resultant active earth forceper unit length (in kN/m) acting on the wall is
- $31.7$
- $35.2$
- $51.8$
- $57.0$