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

TYPES OF DRILLED SHAFT

DESIGN METHOD OF DRILLED SHAFT

ESTIMATION OF LOAD BEARING CAPACITY - GENERAL

spu QQQ Where:

Qu = ultimate load

Qp = ultimate load-carrying capacity at the base

Qs = frictional (skin) resistance

DESIGN METHOD OF DRILLED SHAFT

*** ..).3.0('.. NDNqNcAQ bqcpp

** '.. qcpp NqNcAQ

)1('.''.. ****)( qcpqcpnetp NqNcAqNqNcAQ

Ultimate Base Load

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

(In most cases, the third term is neglected)

DESIGN METHOD OF DRILLED SHAFT

Friction or Skin resistance, Qs

1

0

..L

s dzfpQ

Where:

p = shaft perimeter = .Ds

f = unit frictional (skin) resistance

DESIGN METHOD OF DRILLED SHAFT - SAND

)1('''. **)( qpqpnetp NqAqNqAQ

11

0

'

0

.tan.sin1....L

vs

L

s dzDdzfpQ

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

FS

QQQ snetp

netall

)(

)(

DESIGN METHOD OF DRILLED SHAFT - CLAY

*)( .. cupnetp NcAQ

1

0

* ...LL

Lus LpcQ

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

2

*

/3.101

125.021.0

mkNpressurecatmospherip

c

p

a

u

a

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

pp

wswp

EA

LQQS

.

.1

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

wpss

wp IE

DqS .1

. 22

Where:qwp = point load per unit area at the pile point = Qwp/Ap

D = width or diameter of pileEs = modulus of elasticity of soil at or below the pile points = poisson’s ratio of soilIwp = influence factor = r

SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD

wsss

ws IE

D

pL

QS .1 2

3

Where:

Qws = friction resistance of pile

L = embedment length of pile

p = perimeter of the pile

Iws = influence factor

D

LIws 35.02

SETTLEMENT OF DRILLED SHAFT AT WORKING LOAD

UPLIFT CAPACITY OF DRILLED SHAFT

UPLIFT CAPACITY OF DRILLED SHAFT

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN SAND

UPLIFT CAPACITY OF DRILLED SHAFT

UPLIFT CAPACITY OF DRILLED SHAFT

UPLIFT CAPACITY OF DRILLED SHAFT

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN SAND

WLABT pqug

1. Determine L, Db, and L/Db

2. Estimate (L/Db)cr and hence Lcr

3. If (L/Db) (L/Db)cr, obtain Bq from the graph and

4. If (L/Db) >(L/Db)cr

crLL

uvspqug dzKDWLABT0

'' tan

Frictional resistance developed along the soil-shaft interface from z = 0 to z = L – Lcr and is similar to:

UPLIFT CAPACITY OF DRILLED SHAFT

UPLIFT CAPACITY OF DRILLED SHAFT

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY

UPLIFT CAPACITY OF DRILLED SHAFT

UPLIFT CAPACITY OF DRILLED SHAFT

NET ULTIMATE UPLIFT CAPACITY OF DRILLED SHAFT IN CLAY1. Determine cu, L, Db, and L/Db

2. Estimate (L/Db)cr and obtain Lcr

3. 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

DRILLED SHAFT INSTALLATION

DRILLED SHAFT INSTALLATION

TYPES OF CAISSONS

TYPES OF CAISSONS

DESIGN METHOD OF CAISSONS FOUNDATION

THICKNESS OF CONCRETE SEAL IN OPEN CAISSONS(b). Rectangular Caisson

Lo

Bo Bi

Li

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

DESIGN METHOD OF CAISSONS FOUNDATION

2. Check for BuoyancyTWO OTHER CONDITIONS SHOULD BE CHECKED FOR SAFETY:

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