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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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Presentation on theme: "Session 25 – 26 DRILLED SHAFT And CAISSON FOUNDATION Course: S0484/Foundation Engineering Year: 2007 Version: 1/0."— Presentation transcript:

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

2 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

3 TYPES OF DRILLED SHAFT

4 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

5 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)

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

7 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

8 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

9 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

10 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

11 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

12 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

13 UPLIFT CAPACITY OF DRILLED SHAFT

14 NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN SAND

15 UPLIFT CAPACITY OF DRILLED SHAFT

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17 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:

18 UPLIFT CAPACITY OF DRILLED SHAFT

19 NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY

20 UPLIFT CAPACITY OF DRILLED SHAFT

21 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

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

23 DRILLED SHAFT INSTALLATION

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25 TYPES OF CAISSONS

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

28 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

29 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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