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Session 25 – 26 DRILLED SHAFT And CAISSON FOUNDATION Course: S0484/Foundation Engineering Year: 2007 Version: 1/0

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DRILLED SHAFT And CAISSON FOUNDATION Topic: Types of Drilled Shaft Design Method of Drilled Shaft Installation Method of Drilled Shaft Types of Caisson Foundation Design Method of Caisson Foundation

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TYPES OF DRILLED SHAFT

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DESIGN METHOD OF DRILLED SHAFT ESTIMATION OF LOAD BEARING CAPACITY - GENERAL Where: Q u = ultimate load Q p = ultimate load- carrying capacity at the base Q s = frictional (skin) resistance

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DESIGN METHOD OF DRILLED SHAFT Ultimate Base Load Net load-carrying capacity at the base Where: N c *, N q *, N * = the bearing capacity factor q’ = vertical effective stress at the level of the bottom of pier D b = diameter of the base A p = area of the base = /4. D b 2 (In most cases, the third term is neglected)

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DESIGN METHOD OF DRILLED SHAFT Friction or Skin resistance, Q s Where: p = shaft perimeter = .D s f = unit frictional (skin) resistance

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DESIGN METHOD OF DRILLED SHAFT - SAND Net load-carrying capacity at the base Friction or Skin resistance Where: p = shaft perimeter = .D s f = unit frictional (skin) resistance = K. v ’.tan K = earth pressure coefficient K o = 1 - sin v ’ = effective vertical stress at any depth z Net allowable load

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DESIGN METHOD OF DRILLED SHAFT - CLAY Net load-carrying capacity at the base Friction or Skin resistance Where: c u = undrained cohesion N c * = bearing capacity factor = 9 p = perimeter of the shaft cross section * = varies between 0.3 to 1.0 or

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SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD S = S 1 + S 2 + S 3 Where: S = total pile settlement S 1 = elastic settlement of pile S 2 = settlement of pile caused by the load at the pile tip S 3 = settlement of pile caused by the load transmitted along the pile shaft

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Where: Q wp = load carried at the pile point under working load condition Q ws = load carried by frictional (skin) resistance under working load condition A p = area of pile cross section E p = modulus of elasticity of the pile material L = length of pile = the magnitude which depend on the nature of unit friction (skin) resistance distribution along the pile shaft. SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD

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Where: q wp = point load per unit area at the pile point = Q wp /A p D = width or diameter of pile E s = modulus of elasticity of soil at or below the pile point s = poisson’s ratio of soil I wp = influence factor = r SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD

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Where: Q ws = friction resistance of pile L = embedment length of pile p = perimeter of the pile I ws = influence factor SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD

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UPLIFT CAPACITY OF DRILLED SHAFT

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NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN SAND

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UPLIFT CAPACITY OF DRILLED SHAFT

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NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN SAND 1.Determine L, D b, and L/D b 2.Estimate (L/D b ) cr and hence L cr 3.If (L/D b ) (L/D b ) cr, obtain B q from the graph and 4. If (L/D b ) >(L/D b ) cr Frictional resistance developed along the soil-shaft interface from z = 0 to z = L – L cr and is similar to:

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UPLIFT CAPACITY OF DRILLED SHAFT

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NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY

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UPLIFT CAPACITY OF DRILLED SHAFT

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NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY 1.Determine c u, L, D b, and L/D b 2.Estimate (L/D b ) cr and obtain L cr 3.If (L/D b ) (L/D b ) cr, obtain B c from the graph and 4. If (L/D b ) >(L/D b ) cr, B c = 9 and

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UPLIFT CAPACITY OF DRILLED SHAFT The skin resistance obtained from the adhesion along the soil-shaft interface and is similar to With

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DRILLED SHAFT INSTALLATION

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TYPES OF CAISSONS

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DESIGN METHOD OF CAISSONS FOUNDATION THICKNESS OF CONCRETE SEAL IN OPEN CAISSONS (b). Rectangular Caisson LoLo BoBo BiBi LiLi

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DESIGN METHOD OF CAISSONS FOUNDATION 1. Check for Perimeter Shear at Contact Face of Seal and Shaft TWO OTHER CONDITIONS SHOULD BE CHECKED FOR SAFETY: The Perimeter shear, , should be less than the permissible shear stress, u

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DESIGN METHOD OF CAISSONS FOUNDATION 2. Check for Buoyancy TWO OTHER CONDITIONS SHOULD BE CHECKED FOR SAFETY: If the shaft is completely dewatered, the bouyant upward, F u is The downward force, F d, is caused by the weight of the caisson and the seal and by the skin friction at the caisson-soil interface If F d > F u the caisson is safe from bouyancy If F d < F u dewatering the shaft completely will be unsafe and the thickness of the seal should be increased by t, or

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