$[35$ points$]$ You have been asked to review the proposed construction plan for the caissons supporting the main$-$span bridge piers for this project. The construction contractor for the project has proposed to excavate an open$-$cut, braced sheetpile cofferdam $($as shown$)$ to form the caissons $"$in the dry". The contractor intends to provide a sump pit at the bottom of the excavation and remove any infiltrating ground water from the bottom of the excavation using a lift pump. Within the sheetpile cofferdam, this will maintain a water level that is essentially

$20$

equal to the bottom of the excavation. You have determined that the clayey silt at the bottom of the caisson excavation is hydraulically connected to the river channel and have drawn the flow net shown below.

Applicable construction safety codes specify that a factor of safety of $3$ must be maintained against liquefaction or heave of the excavation bottom. If the factor of safety is not provided $($e$.$g$.,$ due to high levels in the adjacent river channel, then the contractor must flood the excavation with water and then resume construction when the river stage has fallen to safer levels.

a$)$ At the high water level of el$.+65\mathrm{.}0,$ what is the pore water pressure at Point $"$A$"?$

b$)$ Shade the flow element exhibiting the largest seepage velocity, regardless of river stage.

c$)$ At the high water level of el$.+65\mathrm{.}0,$ what is the rate of seepage into the excavation, assuming the excavation is $40$ feet wide $($into the page$)?$

d$)$ At what river stage should the contractor begin flooding the excavation to maintain the required factor of safety against liquefaction?