Session 25 – 26 DRILLED SHAFT And CAISSON FOUNDATION

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Session 25 – 26 DRILLED SHAFT And CAISSON FOUNDATION. Course : S0484/Foundation Engineering Year : 2007 Version : 1/0. DRILLED SHAFT And CAISSON FOUNDATION. Topic: Types of Drilled Shaft Design Method of Drilled Shaft Installation Method of Drilled Shaft Types of Caisson Foundation

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

Course : S0484/Foundation Engineering

Year : 2007

Version : 1/0

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
DESIGN METHOD OF DRILLED SHAFT

ESTIMATION OF LOAD BEARING CAPACITY - GENERAL

Where:

Qp = ultimate load-carrying capacity at the base

Qs = frictional (skin) resistance

DESIGN METHOD OF DRILLED SHAFT

(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

DESIGN METHOD OF DRILLED SHAFT

Friction or Skin resistance, Qs

Where:

p = shaft perimeter = .Ds

f = unit frictional (skin) resistance

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

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

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

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.

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

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

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:

UPLIFT CAPACITY OF DRILLED SHAFT

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY

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
UPLIFT CAPACITY OF DRILLED SHAFT

The skin resistance obtained from the adhesion along the soil-shaft interface and is similar to

With

(b). Rectangular Caisson

Li

Bo

Bi

Lo

DESIGN METHOD OF CAISSONS FOUNDATION

THICKNESS OF CONCRETE SEAL IN OPEN CAISSONS

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

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