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**Session 25 – 26 DRILLED SHAFT And CAISSON FOUNDATION**

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

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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: Qu = ultimate load Qp = ultimate load-carrying capacity at the base Qs = frictional (skin) resistance

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**DESIGN METHOD OF DRILLED SHAFT**

Ultimate Base Load (In most cases, the third term is neglected) Net load-carrying capacity at the base Where: Nc*, Nq*, N* = the bearing capacity factor q’ = vertical effective stress at the level of the bottom of pier Db = diameter of the base Ap = area of the base = /4 . Db2

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**DESIGN METHOD OF DRILLED SHAFT**

Friction or Skin resistance, Qs Where: p = shaft perimeter = .Ds 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 = .Ds f = unit frictional (skin) resistance = K.v’.tan K = earth pressure coefficient Ko = 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: cu = undrained cohesion Nc* = 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 = S1 + S2 + S3 Where: S = total pile settlement S1 = elastic settlement of pile S2 = settlement of pile caused by the load at the pile tip S3 = settlement of pile caused by the load transmitted along the pile shaft

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**SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD**

Where: Qwp = load carried at the pile point under working load condition Qws = load carried by frictional (skin) resistance under working load condition Ap = area of pile cross section Ep = 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.

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**SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD**

Where: qwp = point load per unit area at the pile point = Qwp/Ap D = width or diameter of pile Es = modulus of elasticity of soil at or below the pile point s = poisson’s ratio of soil Iwp = influence factor = r

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**SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD**

Where: Qws = friction resistance of pile L = embedment length of pile p = perimeter of the pile Iws = influence factor

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**UPLIFT CAPACITY OF DRILLED SHAFT**

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**UPLIFT CAPACITY OF DRILLED SHAFT**

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN SAND

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**UPLIFT CAPACITY OF DRILLED SHAFT**

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**UPLIFT CAPACITY OF DRILLED SHAFT**

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**UPLIFT CAPACITY OF DRILLED SHAFT**

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

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**UPLIFT CAPACITY OF DRILLED SHAFT**

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**UPLIFT CAPACITY OF DRILLED SHAFT**

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY

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**UPLIFT CAPACITY OF DRILLED SHAFT**

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**UPLIFT CAPACITY OF DRILLED SHAFT**

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY Determine cu, L, Db, and L/Db Estimate (L/Db)cr and obtain Lcr If (L/Db) (L/Db)cr, obtain Bc from the graph and 4. If (L/Db) >(L/Db)cr, Bc = 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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**DRILLED SHAFT INSTALLATION**

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TYPES OF CAISSONS

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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 Lo Bo Bi Li

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**DESIGN METHOD OF CAISSONS FOUNDATION**

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

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**DESIGN METHOD OF CAISSONS FOUNDATION**

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

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