sheet pile

Problem 540
Timbers 8 inches wide by 12 inches deep and 15 feet long, supported at top and bottom, back up a dam restraining water 9 feet deep. Water weighs 62.5 lb/ft³. (a) Compute the center-line spacing of the timbers to cause f_b = 1000 psi. (b) Will this spacing be safe if the maximum f_b, (f_b)_max = 1600 psi, and the water reaches its maximum depth of 15 ft?
 

Problem 539
Timbers 12 inches by 12 inches, spaced 3 feet apart on centers, are driven into the ground and act as cantilever beams to back-up the sheet piling of a coffer dam. What is the maximum safe height of water behind the dam if water weighs = 62.5 lb/ft³ and ( f_b )_max = 1200 psi?