A river 5 m deep flows over a sand deposit. The saturated unit weight of the sand is 18 kN/m^3 and the unit weight of water is 9.8 kN/m^3. At a depth of 5 m below the river bed:
1) Determine the total vertical stress, the pore water pressure, and the effective vertical stress.
2) If the river level falls to the river bed, what would be the effective vertical stress?