Course
Year
Version
: S0484/Foundation Engineering
: 2007
: 1/0
Session 25 – 26
DRILLED SHAFT And CAISSON FOUNDATION
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
Qu  Q p  Qs
Where:
Qu = ultimate load
Qp = ultimate loadcarrying capacity at the
base
Qs = frictional (skin)
resistance
DESIGN METHOD OF DRILLED SHAFT
Ultimate Base Load

 A c.N
Qp  Ap c.N  q'.N  (0.3). .Db .N
Qp
p
*
c
*
q
*
c
 q'.N q*
*


(In most cases, the third term is neglected)
Net load-carrying capacity at the base



Qp ( net )  Ap c.Nc*  q'.Nq*  q'  Ap c.Nc*  q' ( Nq* 1)

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
L1
Qs   p. f .dz
0
Where:
p = shaft perimeter = .Ds
f = unit frictional (skin) resistance
DESIGN METHOD OF DRILLED SHAFT - SAND
Net load-carrying capacity at the base



Qp( net )  Ap q'.Nq*  q'  Ap q' ( Nq* 1)

Friction or Skin resistance
L1
L1
0
0
Qs   p. f .dz   .Ds .1  sin    v' . tan  .dz
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
Qall( net ) 
Q p ( net )  Qs
FS
DESIGN METHOD OF DRILLED SHAFT - CLAY
Net load-carrying capacity at the base
Qp ( net )  Ap .cu .N
*
c
Friction or Skin resistance
Qs 
Where:
L  L1
*

 .cu . p.L
L 0
cu = undrained cohesion
Nc* = bearing capacity factor = 9
p = perimeter of the shaft cross section
p 
* = varies between 0.3 to 1.0 or  *  0.21  0.25 a   1
c 
 u 
pa  atmospheric pressure  101.3 kN / m 2
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
S1

Q

wp
  .Qws L
Ap .E p
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
S2 
qwp .D
Es
1   .I
2
s
wp
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
 Qws  D
2

S3  
1   s .I ws
 pL  Es

Where:
Qws = friction resistance of pile
L = embedment length of pile
p = perimeter of the pile
Iws = influence factor
I ws
L
 2  0.35
D

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
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
Tug  Bq ApL  W
4. If (L/Db) >(L/Db)cr
Tug  Bq ApL  W 
L  Lcr
'
'



D

K
 s v u tan  dz
0
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 CLAY
1. 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
Li
Bo
Bi
Lo
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