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PILES there are two categories of piles according to the method of installation: A. Driven Piles 1.t imber, steel, precast concrete, piles formed by driving tubes or shells and then casting concrete 2.d riven piles cause displacement and disturbance of adjacent soil 3.D riving can be done by repeatedly raising and dropping a weight on the pile head or anvil or in favourable conditions vibration can be used

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a) timber or precast reinforced concrete, b) steel H pile, c) precast concrete shell pile, d) concrete pile cast as driven tube withdrawn (slip form) Driven Piles Craig, 6 th Ed.

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B. Shaft Construction 1.s oil removed by boring or drilling to form a shaft 2.c oncrete cast in a casing or without casing – depends on soil conditions 3.t ip can be expanded by underreaming 4.n o disturbance to surrounding soil

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e) bored pile (cast in-situ), f) underreamed bored pile (cast in-situ) Shaft Construction Craig, 6 th Ed.

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area of pile tip x pressure at tip Q f = Base Resistance + Shaft Resistance Ultimate Load, Q f area of shaft x frictional resistance of shaft Q f = q f A b + f s A s where qf = ultimate bearing capacity of soil at base tip of pile Ab = cross-sectional area of pile at base tip fs = skin friction between pile and soil As = surface area of pile shaft

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For Sands where N = N-Value of soil at pile tip Db = length of pile (embedment) B = diameter (width) of pile = average N-Value over length of pile shaft (300N for non-plastic silts) (use for bored piles) (use for bored piles and H piles)

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For Clays where cu = undrained shear strength of clay at base tip of pile Nc = Skempton’s value of Nc for = average undrained shear strength of undisturbed clay over the pile length = adhesion factor (0.3 – 1.0); depends on type of clay, installation method and pile material 4 9

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Negative Skin Friction a pile can be pulled down: a pile can be pulled down: a) if the fill surrounding the pile is settling b) if fill is placed over a clay layer and causes it to consolidate it to consolidate

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Pile Groups piles are rarely placed in isolation it is much more common to place an array of pilesand place a pile cap over top

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Pile Group Criteria piles in a pile group are assumed to have all the same dimensions and properties in order to act as a pile group the spacing between piles should be between 2B and 4B where B is the diameter of the piles

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Pile Group Shaft Area ffffor a single pile, As is the perimeter of the pile multiplied by its length ffffor a pile group, As is the perimeter of the pile group multiplied by the length BB s s s s B 8s + B

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Pile Group Tip Area ffffor a single pile, Ab is the area of the pile tip ffffor a pile group, Ab is the area of the entire space enclosed by pile group perimeter s s s s B

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Pile Group Efficiency If the ultimate load of a pile group of n piles is Qfgroup and the ultimate load of a single pile of the same dimensions as those in the pile grouphas an ultimate load, Qfpile, then the pile group efficiency, EPG is the average ultimate load per pile of the pile group divided by the single pile ultimate load

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