Deep Foundations

Deep Foundation are used because:

 Upper soils are weak and/or structural loads are
high that spread footings are unsuitable.
 Upper soils are subjected to scouring or
undermining – bridges.
 Foundation must penetrate through water, e.g., a
pier.
 A large uplift capacity is needed.
 A large lateral capacity is needed.
 There will be a future excavation adjacent to the
foundation, and this excavation may undermine
shallow foundations.

2
 Shallow and Deep Foundations

In shallow foundations, In deep foundations, side

load transfer is by resistance becomes the
lateral spreading dominant load transfer
i.e., end bearing through mechanism compared to
passive resistance of soil end bearing.

Deep foundations transfer most of the applied structural loads to deeper strata

3
Types of Deep Foundations

 Piles
 Drilled shafts (Bored piles)
 Caissons
 Mandrel-driven thin shells filled with concrete
 Auger cast piles
 Pressure-injected footings
 Anchors

4
Pile Material

 Timber – Up to 25 tons
 Concrete – Up to 100 tons
 Steel – Over 25 tons

5
Timber Piles

Typically, of 5 - 8m length and 75 mm to

100 mm diameter tapering to between 50
mm and 75 mm and 100 mm to 175 mm
square sections. They are usually treated
with preservatives for durability.

6
Concrete Piles
Conditions Minimum Concrete
Grade
Hard and very hard driving conditions 40
for all piles and in marine works
Normal and easy driving conditions 30

Typical splices for concrete piles (Precast/Prestressed Concrete Institute)

Normal and easy driving conditions

7
 Steel Piles

Comparison between typical wide flange (WF) and H-Pile (HP) sections

Pipe piles are either welded or

seamless steel pipes which may be
driven either open-end or closed-end.

Pipe piles are often filled with

concrete after driving, although in
some cases this is not necessary A steel H-pile

8
Definitions

Parts of a deep foundation (a) straight foundations; (b) tapered foundations; (c)
foundations with an enlarged base.

9
Load Transfer

Transfer of structural loads from a deep foundation into the ground (a) axial compressive loads;
(b) axial tension loads; (c) lateral loads

10
Types of Piles according to the method of
installation
Three main types:
1. Driven piles,
2. Cast-in-situ piles and
3. Driven and cast-in-situ piles.

Driven Piles
Piles may be of timber, steel or concrete and are driven
using a pile hammer.
When a pile is driven into a granular soil, the soil so
displaced, equal to the volume of the driven pile,
compacts the soil around the sides since the displaced
soil particles enter the soil spaces of the adjacent mass
which leads to densification of the mass.
11
Advantages
1. Piles can be precast to the required specifications.
2. Piles of any size, length and shape can be made in advance and
used at the site. As a result, the progress of the work will be rapid.
3. A pile driven into granular soil compacts the adjacent soil mass
and as a result the bearing capacity of the pile is increased.
4. The work is neat and clean. The supervision of work at the site
can be reduced to a minimum. The storage space required is very
much less.
5. Driven piles may conveniently be used in places where it is
advisable not to drill holes for fear of meeting ground water under
pressure.
6. Driven piles are the most favored for works over water such as
piles in wharf structures or jetties.

12
 Disadvantages
1. Precast or prestressed concrete piles must be properly reinforced
to withstand handling stresses during transportation and driving.
2. Advance planning is required for handling and driving.
3. Requires heavy equipment for handling and driving.
4. Since the exact length required at the site cannot be determined in
advance, the method involves cutting off extra lengths or adding
more lengths. This increases the cost of the project.
5. Driven piles are not suitable in soils of poor drainage qualities. If
the driving of piles is not properly phased and arranged, there is
every possibility of heaving of the soil or the lifting of the driven
piles during the driving of a new pile.
6. Where the foundations of adjacent structures are likely to be
affected due to the vibrations generated by the driving of piles,
driven piles should not be used.

13
Cast-in-situ Piles
Cast-in-situ piles are concrete piles. These piles are
distinguished from drilled piers as small diameter
piles.
They are constructed by making holes in the ground to
the required depth and then filling the hole with
concrete.
Straight bored piles or piles with one or more bulbs at
intervals may be cast at the site also called under-
reamed piles.
Reinforcement may be used as per the requirements.

14
Advantages
1. Piles of any size and length may be constructed at
the site.
2. Damage due to driving and handling that is common
in precast piles is eliminated in this case.
3. These piles are ideally suited in places where
vibrations of any type are required to be avoided to
preserve the safety of the adjoining structure.
4. They are suitable in soils of poor drainage qualities
since cast-in-situ piles do not significantly disturb the
surrounding soil.

15
Disadvantages
1. Installation of cast-in-situ piles requires careful
supervision and quality control of all the materials used
in the construction.
2. The method is quite cumbersome. It needs sufficient
storage space for all the materials used in the
construction.
3. The advantage of increased bearing capacity due to
compaction in granular soil that could be obtained by a
driven pile is not produced by a cast-in-situ pile.
4. Construction of piles in holes where there is heavy
current of ground water flow or artesian pressure is very
difficult.

16
Driven and Cast-in-situ Piles

A. A plug of sand / stone is placed in the piling tube and compacted with the hammer.
B. The tube is driven by applying blows of the drop hammer to the plug which arches in the tube
and draws the tube into the ground.
C. On reaching the founding level the tube is held by extracting gear while the plug is expelled
using blows of the hammer.
D. Measured quantities of relatively dry concrete are expelled from the toe of the tube thus
forming an enlarged base.
E. The reinforcing cage is placed in the tube which is then filled with high slump concrete.
F. The tube is extracted by means of the extraction gear. On deeper piles, the concrete level may
have to be topped up during extraction.
G. The completed pile. 17
The factors that govern the selection of piles

1. Length of pile in relation to the load and type of soil

2. Character of structure

3. Availability of materials

4. Type of loading

5. Factors causing deterioration

18
Vertical Load Bearing Capacity Of A Single Vertical Pile
The bearing capacity of groups of piles subjected to
vertical or vertical and lateral loads depends upon the
behavior of a single pile. The bearing capacity of a single
pile depends upon.
1. Type, size and length of pile,
2. Type of soil,
3. The method of installation.

The bearing capacity depends primarily on the method

of installation and the type of soil encountered. The
bearing capacity of a single pile increases with an
increase in the size and length. The position of the water
table also affects the bearing capacity.
19
Vertical Load Bearing Capacity Of A Single Vertical Pile
In order to be able to design a safe and economical pile
foundation, we have to analyze the interactions between
the pile and the soil, establish the modes of failure and
estimate the settlements from soil deformation under
dead load, service load etc.
The design should comply with the following
requirements.
• It should ensure adequate safety against failure; the factor of
safety used depends on the importance of the structure, the
reliability of the soil parameters and the loading systems used
in the design.
• The settlements should be compatible with adequate behavior
of the superstructure to avoid impairing its efficiency.
20
Performance Requirements

 Strength
 Geotechnical
 Structural
 Serviceability
 Constructibility
 Economic

21
Structural Considerations

 Piles are similar to columns in a

superstructure except that they are confined
within the ground.
 Two possible mode of failure: compression
and buckling
 However, even the softest soils provide
enough lateral support to prevent
underground buckling in axially loaded deep
foundations.
22
Structural Considerations (cont’d)
 Slender deep foundations subject to both
axial and lateral loads may experience
underground buckling if the upper soils are
very soft.
 Above-ground buckling may be a problem in
piles that extend above ground surface.
 Buckling is a greater concern during pile
driving, especially in long, slender piles
driven through water.

23
Comparison with Superstructure Design
Since buckling is not a concern in piles, the
structural design is similar to structural design of
short columns except:
 Construction tolerances for foundations are much larger and
quality control is more difficult.
 Piles can be damaged during driving, so as build capacity may be
less than expected.
 Residual stresses may be locked into piles during driving, so the
actual stresses in the piles after the structure is completed may be
greater.
 Concrete in drilled shafts and other cast-in-place foundations is not
placed under ideal conditions, and thus may experience problems of
aggregate segregation, contamination from the soil, and other
problems.

24
Loads and Stresses
Structural design must consider:
 Axial loads (compression and tension)
 Lateral loads (shear and moment)
 Torsion loads in special circumstances
 Driving stresses (Driveability)
Structural engineers use two different methods of designing structural members:
• The allowable stress design (ASD) method (also known as the working stress design
method), which is based on the stresses induced in the structural member when
subjected to the design loads. The engineer compares these working stresses with the
allowable stress, which is the strength divided by a factor of safety, to determine if
the design is satisfactory.
• The load and resistance factor design (LRFD) method (also known as the ultimate
strength design method), which increases the design loads through the use of load
factors, then compares these to the ultimate load-bearing capacity.

25
Loads and Stresses
Structural engineers use two different methods of designing structural
members:
• The allowable stress design (ASD) method (also known as the working
stress design method), which is based on the stresses induced in the
structural member when subjected to the design loads. The engineer
compares these working stresses with the allowable stress, which is the
strength divided by a factor of safety, to determine if the design is
satisfactory.
• The load and resistance factor design (LRFD) method (also known as
the ultimate strength design method), which increases the design loads
using load factors, then compares these to the ultimate load-bearing
capacity. 26
Axial load
When using ASD, the axial tension or compression
stress at a depth z in a pile foundation subjected to an
axial load is: P
fa 
A

where fa  average normal stress caused by axial load

P  unfactored axial tension or compression force in the
foundation at depth z
A  cross- sectionalarea of pile
fa  Fa (allowable axial stress)

27
Flexural load
The maximum fibre stress caused by a moment M in the
foundation is:
M
fb 
S
where f b  normal stress in extreme fiber caused by flexural load
M  moment in the foundation at depth z based on unfactored loads
S  elastic section modulus of pile
B3
S for solid circular cross sections of diameter B
32
B3
S for square cross sections of width B.
6

f b  Fb (allowable flexural stresses)

28
Shear load
The shear stress caused by a shear load V in the foundation is:
V
fv 
A
For a working stress analysis, the shear stress, f v , must not
exceed the shear capacity, Fv

f v  Fv

where f v  shear stress infoundation at depth z

V  shear force in foundation at depth z based on
unfactored loads
A  cross- sectionalarea of pile
Fv = shear capacity
29
Interaction effects
In Allowable Stress Design (ASD)Approach:
The allowable axial and flexural stresses are Fa and Fb,
respectively, and the design must satisfy the following
condition:
fa f b
 1
Fa Fb
No interaction between fv and fa or fb
The allowable axial stress in steel piles, Fa, is typically 0.35Fy to 0.50Fv (87-124 Mpa)
for either tension or compression. AASHTO is more conservative and uses Fa = 0.25
Fy to 0.33 Fy.

For flexural bending, allowable shear stress of Fb= 0.40 Fy

30
Example 1 – The maximum compression loads in a drilled shaft foundation
are as follows:
PD = 1334 kN
PL = 1156 kN
Assuming the drilled shaft is nominally reinforced concrete of grade 30,
determine the required pile diameter for structural capacity with factor of
safety of 4.
f ' 30000
Allowable compressive stress, Fa    7500 kPa
4 4
Unfactored design load, P  PD  PL  13341156  2490 kN

P
Need to satisfy: fa   Fa
A
2490kN
i.e.  7500kPa
 B 
2
 
 4 
B  650 mm

31
Example 2 – A large sign is to be supported on a single, 400mm diameter free-
head steel pipe pile with 10-mm wall thickness. The sign will impose a vertical
downward load of 20 kN, a shear load of 12 kN, and an overturning moment
of 95 kN-m onto the top of the pile. The pile is made of A36 steel.
Is this design adequate?
Solution
Because this is a free-head pile, the maximum moment is equal to the applied
moment.
Check axial and flexural stresses at the top of the pile.
 x 0.42  x 0.382
A   0.0123m2
4 4
20
fa   1, 600 kPa = 1.6 Mpa
0.0123
 x 0.44  x 0.384 Check shear stresses using half of the
I=   2.33 x 10-3 m3
64 64 cross-sectional area
2 I 2(2.33 x 10-4 m 4 )
S   1.17 x 10-3 m3 12
B 0.4m fv   1950 kPa = 1.95 Mpa
0.0123 x 0.5
M 95
fb    81,500 kPa = 81.5 Mpa
S 1.17 x 10-3 Fv = 0.4 Fy = (0.4)(250) = 100 Mpa
Fy  250 Mpa
f v  Fv OK
Fa = Fb = 0.35 Fy = (0.35)(250) = 87 Mpa
f a f b 1.6 81.5 The design is satisfactory
    0.96  1.0 OK
Fa Fb 87 87
32
The structural design of piles must consider
each of the following loading conditions:

 Handling loads
 Driving loads

 Service loads

Using specified pickup points to keep handling stresses within

tolerable limits: (a) single-point pickup; (b) double-point pickup.

Piles can be damaged during driving

33
Other pile elements
 Pile caps

Battered piles being used in combination with vertical piles to resist

combined vertical and horizontal loads.
34
Performance Requirements

 Strength
 Geotechnical
 Structural
 Serviceability
 Constructibility
 Economic

35
Axial Capacity of Single Pile

 Full-scale static load tests

 Analytic methods based on either soil
properties obtained from laboratory or in-situ
tests
 Dynamic methods based on dynamics of pile
driving or wave propagation

36
Load Transfer – Compressive Load
 F  0,
y

Ultimate loadcapacity,
Pult  Pt  Wf  Ps  Pt'Ps

Allowable load capacity,

P
Pa  ult where F is the factor of safety
F

Rewriting in terms of unit tip and side resistances :

q t 'A t   f s A s
Pa 
F

Transfer of axial loads from a deep foundation into

the ground by side friction and toe bearing
37
 Example 1 – An 800 kN compressive load is to be imposed on a 400-mm
diameter, 15-m long steel pipe pile driven into the soil profile shown in t h e
Figure shown. The net unit toe-bearing and unit side-friction resistances are as
shown. Compute the downward load capacity using a factor of safety of 3
and determine if the design is acceptable.

Assume pipe pile is closed- ended,

q t' A t   f sA s
Pa 
F
40000.2 2   250.44  1000.410 8000.41

3
50412612571024

3
 970 kN  P  800 kN

Proposed pipe pile for the example

38
Load Transfer – Tensile Load
 F  0,
y

Ultimate tensile load capacity,

Pupward  Wf  Ps

Allowable tensileload capacity,

P  P
upward a
upward
where F is the factor of safety
F

Rewriting in terms of unit tip and side resistances :

P  W  f A
upward a
f s s

Transfer of axial loads from a deep foundation into

the ground by side friction and toe bearing
39
If part or all of pile is submerged below the
groundwater table, computation of Wf must
consider buoyancy effects, i.e.

Wf’ = Wf – Submerged volume x w

40
Contact Areas At and As

In open-ended piles, the soil

plug may be considered
rigidly embedded if

D/B > 10 to 20 for clays

D/B > 25 to 35 for sands

Soil plugging in open-ended steel pipe piles and steel H-piles

41
 q ult_ net
Allowable Bearing Capacity q a _ net 
F

Factors affecting the design factor of safety, and typical values of F

42
Allowable Pile Capacity Pa 
Pult
F

Type and Number of soil tests performed High Low

Availability of on-site or nearby full-scale static load test Yes No

results
Availability of on-site or nearby dynamic test results Yes No

Anticipated level and methods of construction inspection High Low

and quality control

Downward or upward loading Downward Upward

6.0

43
Conventional Static Load Tests - Purpose

 To develop the load-settlement curve or load-

heave curve for the determination of the
ultimate load capacity.

44
Conventional Static Load Tests - Equipment

Use of a hydraulic jack reacting against dead weight to develop the test load in
a static load test
45
Conventional Static Load Tests - Equipment

Use of a hydraulic jack reacting against beam and reaction piles to develop the
test load in a static load test
46
Pull-out Test

47
Conventional Static Load Tests -
Procedure

 Controlled stress tests (maintained load or

ML tests)
 Controlled strain tests – reverse of ML tests

48
Controlled stress tests (maintained
load or ML tests)
 Test load is applied in increments, usually 25,
50, 75, 100, 125, 150, 175 and 200% of design
load and generate the load-displacement curve

 Slow ( 1 to 2 hrs per load increment, 24 hrs per

test pile)

 Fast (load increment is 10% of design load each

time and the load is held for 2.5 to 15 minutes,
2-5 hrs per test pile)

49
Conventional Static Load Tests -
Interpretation
Many methods of interpretations:

• Davisson’s (1973) method

• Chin’s (1970) method

Typical load-settlement curves: Curve A is typical in soft clayey soils and curve B
is typical of intermediate, stiff clay and sandy soils (ever-increasing load)
50
Davisson’s Method

  P
  
E D AE
PD

AE

51
Chin’s Method

Developed for footings and precast floating piles

Chin observed that load-settlement curve can be

assumed as a hyperbola

i.e. a plot of settlement, s, versus the ratio s/P

gives a linear relationship when the pile
approaches failure.

52
Example 3 – The load-settlement data shown in the Figure shown were obtained
from full-scale static load test on a 400-mm square, 17-m long concrete pile (fc’ =
40 MPa). Use Davisson’s and Chin’s method to compute the downward load
capacity.
Davisson' smethod:
E  4700 f c '  4700 40 MPa
4 mm + B/120  30000MPa
B  PD
4 mm 
120 AE
400 mm (P kN)17000 mm
PD/AE  4 mm  
120 
0.4m 2 30x106 kPa 
 7.3 mm  0.0035P

Pult 1650 kN

Found to work best for quick ML tests, overly

conservative for slow ML tests
Static load test data for the example

53
 Example 3 – The load-settlement data shown in Figure 13.11 were obtained from
full-scale static load test on a 400-mm square, 17-m long concrete pile (fc’ = 40
MPa). Use Davisson’s and Chin’s method to compute the downward load
capacity.

0.012
Chin's method:
Settlement / Applied Load (mm/kN)

0.01

1/Pult =0.000375 1
0.008 Pult   2665kN
0.000375
0.006

0.004

0.002
Typically used for slow ML tests
0
0 5 10 15 20 25

Settlement (mm)

54
Mobilization of Soil Resistance

5 to 10 mm is sufficient to
fully mobilize side
resistance

About 0.1B settlement is

needed to fully mobilize
toe-bearing resistance

Load-displacement relationship for side-friction and toe bearing under

downward loads

55
Osterberg Load Tests
Hydraulic fluid is pumped into the
jack and both pressure and
volume are noted. The jack
expands and pushes up the
shaft. The shaft movement is
measured by a dial gauge. Thus,
a plot of side friction capacity vs
axial movement is obtained.
If a telltale rod is included at the
bottom of the jack, the downward
movement at the bottom can be
used to obtain a plot of toe-
bearing pressure vs axial
movement.

Osterberg load test device (Loadtest, Inc.)

56
When and Where to Use Full-Scale Static
Load Tests
 Many foundations are to be installed, a small
savings in each will result in a significant reduction
in construction cost
 Soil conditions are erratic or unusual, and thus
difficult to assess with analytic methods
 The pile is supported in soils that are prone to
dramatic failures
 The structure is especially important or sensitive to
settlements
 The engineer has little or no experience in the area
 The foundations must resist uplift loads

57
When and Where to Use Full-Scale Static
Load Tests (cont’d)
 Ideally, static load tests are constructed well
before construction. However mobilization
cost before construction may be expensive.

 As a proof test for the first production piles.

58
Eurocode 7 -Correlation factors for pile foundations:

Table A.NA.9- Correlation factors to derive characteristic

values of the resistance of axially loaded piles from static pile
load tests (n – number of tested piles)

 for n = 1 2 3 4 5
X1 (min) 1.55 1.47 1.42 1.38 1.35
X2 (mean) 1.55 1.35 1.15 1.15 1.08

59