Question #196907

A 8m thick layer of medium sand overlies a deep deposit of dense gravel. A series of standard penetration tests carried out through the depth of the sand has established that the average blow count is 26. Further tests show that the gravel has a standard penetration value of N = 52 in the region of the interface with the sand. A precast pile of diameter 300mm is to be driven down through the sand and to penetrate sufficiently into the gravel to give good end bearing. Adopting a safety factor of 3.0, determine the allowable load that the pile will be able to carry.

Expert's answer

given that,

Average blow count "=26"

diameter of forecast file "=300mm=0.3m"

standard penetration value (N)"=52"

factor of safety(fs)"=3"

length of file "=8m=8000mm"

allowable load"=?"

Qu"=400NA_b+2\\bar{N}A_s"

where,

"A_b-" gross sectional area

i.e "A_b=\\frac{\\pi}{4}\\times d^\n2"

"=\\frac{\\pi}{4}(300)^2"

"=70685.8mm^2" (Q)"0.07068"

"A_s=" lateral surface area

i.e, "A_s=\\pi\\times" dia of file "\\times" length of file

"=\\pi \\times d\\times l"

"=\\pi\\times300\\times8000"

"=\\pi\\times2400000"

"=7539822.37mm^2" (Q) "7.54m^2"

Qu"=(400)(52)(0.0707)+(2)(26)(7.54)"

Qu"=14705.6+392.08"

allowabe load"=\\frac{Qu}{0.5}"

"=1509.77\/3"

"=502.25KN"

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