A 400mm square concrete pile of length 20m is to be built at a site with two layers of soil. The top layer is 8m thick a

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A 400mm square concrete pile of length 20m is to be built at a
site with two layers of soil. The top layer is 8m thick and
consists of sands and gravels with bulk unit weight of 22kN/m3 and
angle of friction of 𝜙𝑐𝑟𝑖𝑡 ′ = 30°. The bottom layer is composed of
clay with bulk unit weight of 18kN/m3 and undrained shear strength
of 60kPa at a depth of 8m that increases linearly by 3kPa with each
m of depth. Determine the safe vertical load immediately following
construction assuming that the water table is at a depth of 3m from
the ground surface involved. The average adhesion factor may be
taken as 0.35 for the clay layer. Assume that the horizontal
effective stress at any depth is equal to (1 − sin𝜙𝑐𝑟𝑖𝑡 ′ ) times
the vertical effective stress at the same depth, and that the angle
of friction between the concrete and the sand is equal to 0.6𝜙𝑐𝑟𝑖𝑡
′ . It may be assumed that the end bearing capacity factor is equal
to 9 for the present conditions. The shaft resistance safety factor
may be taken as 2.0, while the end bearing safety factor may be
taken as 2.5. (answer: Safe vertical load ܳ=392kN)