$2$D Seepage $(10$ points$)$

A sheet pile wall is installed along the entire thickness of a sand layer. The bottom sand layer is twice as permeable as the top one, and below the bottom sand layer there is impermeable soil $($note: drawing shown below is not to scale$).$ Both sand layers have a total unit weight of $18k\frac{N}{m^3}.$

I. Draw the flow net and calculate the flow rate across the sand layers

II$.$ Investigate the possibility of boiling on the lower sand layer

III. If the design is not stable, propose an alternative that would eliminate the possibility of boiling $($note: a previous engineering analysis ruled extending the sheet pile into the impermeable soil unfeasible so do not propose that$).$ Provide calculations that show how boiling does not take place with your improved design.