Foundation Analysis

PILE FOUNDATIONS
Part 1
Introduction to Pile Foundations
Point Load Capacity of Pile
 INTRODUCTION
• Piles are structural members that are made of steel,
concrete, or timber. They are used to build up
foundations which are deep and which cost more than
shallow foundation.

• The use of piles is often necessary to ensure structural

safety.

• The following list identifies some of the conditions that

require pile foundations:
 INTRODUCTION
Conditions that require pile foundations:

• Top layers of soil are highly compressible for it to support

structural loads through shallow foundations. Rock level
is shallow enough for end bearing pile foundations
provide a more economical design.
• Lateral forces are relatively prominent.
• In presence of expansive and collapsible soils at the site.
• Offshore structures.
• Strong uplift forces on shallow foundations due to shallow
water table can be partly transmitted to piles.
• For structures near flowing water (bridge abutments, piers,
etc.) to avoid the problems due to erosion.
 CLASSIFICATION OF PILES
Based on material

STEEL CONCRETE TIMBER

 Pipe piles  Pre-cast piles

 Rolled steel H-section piles  Cast-in-situ piles
 CLASSIFICATION OF PILES
Steel Piles: General Facts

 Usual length: 15 m – 60 m
 Usual Load: 300 kN – 1200 kN
 Advantages:
 Relatively less hassle during installation and easy to achieve cutoff level.
 High driving force may be used for fast installation
 Good to penetrate hard strata
 Load carrying capacity is high
Allowable Structural Capacity
 Disadvantages: 𝑄𝑎𝑙𝑙 = 𝐴𝑠 𝑓𝑠
 Relatively expensive
 Noise pollution during installation 𝐴𝑠 is the cross-sectional area of steel
 Corrosion 𝑓𝑠 is the allowable stress of steel
 Bend in piles while driving (~0.33 – 0.5𝑓𝑦 )
 CLASSIFICATION OF PILES
Concrete Piles: General Facts

 Concrete Piles:
 Usual length: 10 m – 15 m
 Usual Load: 300 kN – 3000 kN
 Pre-cast Piles:
 Usual length: 10 m – 45 m
 Usual Load: 7500 kN – 8500 kN
 Cast-in-situ Piles:
 Usual length: 5 m – 15 m
 Usual Load: 200 kN – 500 kN
 CLASSIFICATION OF PILES
Concrete Piles: General Facts

 Advantages:
 Relatively cheap
 It can be easily combined with concrete superstructure
 Corrosion resistant Allowable Structural Capacity
 It can bear hard driving 𝑄 =𝐴 𝑓 +𝐴 𝑓
Cased: 𝑎𝑙𝑙 𝑠 𝑠 𝑐 𝑐
 Disadvantages:
Uncased: 𝑄𝑎𝑙𝑙 = 𝐴𝑐 𝑓𝑐
 Difficult to transport
 Difficult to achieve proper cutoff
𝐴𝑠 is the cross-sectional area of steel
𝐴𝑐 is the cross-sectional area of concrete
𝑓𝑠 is the allowable stress of steel
𝑓𝑐 is the allowable stress of concrete
 CLASSIFICATION OF PILES
Timber Piles: General Facts

 Usual length: 5 m – 15 m
 Usual Load: 300 kN – 500 kN
 Three classes [ASCE Manual of Practice, No. 17 (1959)]:
 Class A piles carry heavy loads. Butt dia. ≥ 356 mm
 Class B piles are used to carry medium loads. Butt dia. ≥ 305 to 330 mm
 Class C piles are used in temporary construction work. Butt dia. ≥ 305 mm

Allowable Structural Capacity

𝑄𝑎𝑙𝑙 = 𝐴𝑝 𝑓𝑤

𝐴𝑝 is the average cross-sectional area of pile

𝑓𝑤 is the allowable stress on the timber
CLASSIFICATION OF PILES
Based on cross-sectional area
Circular
Square
H
Octagonal
Tubular
Based on size
Micro pile dia. < 150 mm
Small pile dia. >150 mm and < 600 mm
Large pile dia. > 600 mm
CLASSIFICATION OF PILES
Based on inclination
Vertical Piles
Inclined Piles

Based on load transfer mechanism

End bearing piles
Friction/Floating piles
Compaction piles

Based on method of construction/installation

Driven Piles (Displacement Piles)
Bored Piles (Non-displacement Piles)
 INSTALLATION OF PILES
Category of pile due to nature of placement

Displacement piles – considered solid; more movement on

surrounding soil during installation.
Ex. driven piles, concrete piles, close end piles

Non-displacement piles – are of hollow or outline shape

and displace little or no soil during installation.
Ex. H-piles, bored piles
 INSTALLATION OF PILES
Displacement Piles
 In loose cohesionless soils
• Densifies the soil up to a distance of 3.5 times the pile
diameter (3.5D) which increases the soil’s resistance to
shearing.
• The friction angle varies from the pile surface to the limit of
compacted soil.
 In dense cohesionless soils
• The dilatancy effect decreases the friction angle within the zone
of influence of displacement pile (3.5D approx.).
• Displacement piles are not effective in dense sands due to
above reason.
 In cohesive soils
• Soil is remolded near the displacement piles (2.0D approx.)
leading to a decrease value of shearing resistance.
• Pore-pressure is generated during installation causing lower
effective stress and consequently lower shearing resistance.
• Excess pore-pressure dissipates over time and soil regain its
strength.
 INSTALLATION OF PILES
Non-displacement Piles

 Due to no displacement during installation, there is no

heave on the ground.
 Cast-in-situ piles may be cased or uncased (by removing casing
as concreting progresses). They may be provided with
reinforcement if economical with their reduced diameter.
 Enlarged bottom ends (three times pile diameter) may be
provided in cohesive soils leading to much larger point
bearing capacity.
 Soil on the sides may soften due to contact with wet concrete
or during boring itself. This may lead to loss of its shear strength.
 Concreting under water may be challenging and may result in
waisting or necking of concrete in squeezing ground.
 INSTALLATION OF PILES
Driven Piles
Most piles are driven into the ground by means of hammers or vibratory
drivers. In special circumstances, piles can also be inserted by jetting or partial
augering.

The types of hammer used for pile driving include

(a)the drop hammer
(b)the single-acting air or steam hammer
(c) the double-acting and differential air or steam hammer
(d)the diesel hammer

In the driving operation, a cap is attached to the top of the pile. A cushion
may be used between the pile and the cap. The cushion has the effect of
reducing the impact force and spreading it over a longer time; however, the
use of the cushion is optional. A hammer cushion is placed on the pile cap.
The hammer drops on the cushion.
INSTALLATION OF PILES
Driven Piles
INSTALLATION OF PILES
Bored Piles
Dry Method of Construction
INSTALLATION OF PILES
Bored Piles
Casing Method of Construction
INSTALLATION OF PILES
Bored Piles
Wet Method of Construction
INSTALLATION OF PILES
Bored Piles
Wet Method of Construction
ESTIMATING PILE LENGTH
 LOAD TRANSFER MECHANISM
OF PILES

With the increasing load on a pile initially the resistance is

offered by the side friction and when the side resistance is
fully mobilized to the shear strength of soil, the rest of the
load is supported by pile end. At certain load the soil at
the pile end fails usually in punching shear, which is
defined as the ultimate load capacity of pile.
 LOAD TRANSFER MECHANISM
 The frictional resistance
per unit area at any
depth

 Ultimate skin friction

resistance of pile

 Ultimate point load

 Ultimate load capacity

in compression

 Ultimate load capacity

in tension
 POINT LOAD CAPACITY OF PILE
General Bearing Capacity Approach

 Ultimate bearing capacity of soil considering general

bearing capacity equation is,
𝒒𝒑𝒖 = 𝒄′𝑵∗𝒄 + 𝒒′ 𝑵∗𝒒 + 𝟎. 𝟓𝜸𝑫𝑵∗𝜸
 Shape, depth and inclination factors are included in
bearing capacity factors.
 Since pile diameter is relatively small, the third term may
be dropped out
𝒒𝒑𝒖 = 𝒄′𝑵∗𝒄 + 𝒒′𝑵∗𝒒
 Hence, pile load capacity is,
𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝒄′𝑵∗𝒄 + 𝒒′ 𝑵∗𝒒 . 𝑨𝒑
 POINT LOAD CAPACITY OF PILE
General Bearing Capacity Approach

𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝒄′𝑵∗𝒄 + 𝒒′ 𝑵∗𝒒 . 𝑨𝒑

where,
𝐴𝑝 is the area of the pile tip
𝑐′ is the cohesion of the soil supporting the pile tip
𝑞𝑝 is the unit point resistance
𝑞′ is the effective vertical stress at the level of the pile tip
𝑁𝑐∗ , 𝑁𝑞∗ are the bearing capacity factors
 FRICTIONAL RESISTANCE

𝑸𝒔𝒖 = (𝒑. ∆𝑳. 𝒇)

where,
𝑝 is the perimeter of the pile section
∆𝐿 is the incremental pile length over which p and f are
taken to be constant
𝑓 is the unit friction resistance at any depth z
 ALLOWABLE LOAD
After the total ultimate load-carrying capacity of a pile
has been determined by summing up the point bearing
capacity and the frictional (or skin) resistance, a
reasonable factor of safety should be used to obtain the
total allowable load for each pile, or
𝑄𝑢
𝑄𝑎𝑙𝑙 =
𝐹𝑆
𝑄𝑎𝑙𝑙 is the allowable load-carrying capacity for each pile
𝐹𝑆 is the factor of safety

The factor of safety generally used ranges from 2.5 to 4,

depending on the uncertainties surrounding the
calculation of ultimate load.
 POINT LOAD CAPACITY OF PILE
 Methods in estimating 𝑄𝑝𝑢

 Meyerhof’s method (1976)

 Janbu’s method (1976)
 Vesic’s method (1977)
 Coyle and Castello’s method (1981)
 Using correlation with SPT and CPT
 POINT LOAD CAPACITY OF PILE
Meyerhof’s Method (1976)

 Granular Soils
Point bearing capacity of pile increases with depth in
sands and reaches its maximum at an embedment ratio
𝐿 𝐿𝑏
= . Therefore, the point load capacity of pile is,
𝐷 𝐷 𝑐𝑟
𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝒒′𝑵∗𝒒 . 𝑨𝒑 < 𝒒𝒖𝒍 . 𝑨𝒑
𝒒𝒖𝒍 = 𝟎. 𝟓𝒑𝒂 𝑵∗𝒒 𝒕𝒂𝒏∅′
where,
𝑝𝑎 is the atmospheric pressure (≈100 kPa)
∅′ is the effective soil friction angle of the bearing stratum
 POINT LOAD CAPACITY OF PILE
Meyerhof’s Method (1976)

 Granular Soils
𝐿𝑏
value typically ranges from 15D for loose to
𝐷 𝑐𝑟
medium sand to 20D for dense sands.

 Saturated Clays
𝑸𝒑𝒖 = 𝒄𝒖 . 𝑵∗𝒄 . 𝑨𝒑 = 𝟗. 𝒄𝒖 . 𝑨𝒑
where,
𝑐𝑢 is the undrained cohesion of the soil below the pile tip
POINT LOAD CAPACITY OF PILE
Janbu’s Method (1976)
 POINT LOAD CAPACITY OF PILE
Vesic’s Method (1977)
 Granular Soils
Pile point bearing capacity based on the theory of
expansion of cavities
𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝝈′𝒐 𝑵𝝈∗ 𝑨𝒑
where,
𝜎𝑜′ is the mean effective normal ground stress at the level
of the pile point
′
1 + 2𝐾𝑜
𝜎𝑜 = 𝑞′
3
𝐾𝑜 is the earth pressure coefficient at rest = 1 − 𝑠𝑖𝑛∅′
𝑁𝜎∗ is the bearing capacity factor
 POINT LOAD CAPACITY OF PILE
Vesic’s Method (1977)
 Granular Soils
𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝝈′𝒐 𝑵𝝈∗ 𝑨𝒑
3𝑁 ∗
𝑞
𝑁𝜎∗ =
(1 + 2𝐾𝑜 )
𝑁𝜎∗ = 𝑓 𝐼𝑟𝑟
where,
𝐼𝑟𝑟 is the reduced rigidity index for the soil
𝐼𝑟 𝐸𝑠 𝐺𝑠
𝐼𝑟𝑟 = ; 𝐼𝑟 = ′
= ′
1 + 𝐼𝑟 ∆ 2 1 + 𝜇𝑠 𝑞 𝑡𝑎𝑛∅′ 𝑞 𝑡𝑎𝑛∅′
where,
∆ is the average volumetric strain in the plastic zone below
the pile point
 POINT LOAD CAPACITY OF PILE
Vesic’s Method (1977)
 Granular Soils

Approximations by Chen and Kulhawy, 1994

𝐸𝑠
=𝑚
𝑝𝑎
100 𝑡𝑜 200 (𝑙𝑜𝑜𝑠𝑒 𝑠𝑜𝑖𝑙)
𝑚 = 200 𝑡𝑜 500 (𝑚𝑒𝑑𝑖𝑢𝑚 𝑑𝑒𝑛𝑠𝑒 𝑠𝑜𝑖𝑙)
500 𝑡𝑜 1000 (𝑑𝑒𝑛𝑠𝑒 𝑠𝑜𝑖𝑙)
∅′ − 25
𝜇𝑠 = 0.1 + 0.3 𝑓𝑜𝑟 250 ≤ ∅′ ≤ 450
20
∅′ − 25 𝑞′
∆= 0.005 1 −
20 𝑝𝑎
POINT LOAD CAPACITY OF PILE
Vesic’s Method (1977)
 POINT LOAD CAPACITY OF PILE
Vesic’s Method (1977)

 Saturated Clays
𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝒄𝒖 𝑵𝒄∗ 𝑨𝒑
∗
4 𝜋
𝑁𝑐 = 𝑙𝑛𝐼𝑟𝑟 + 1 + + 1
3 2
𝐸𝑠
𝐼𝑟 =
3𝑐𝑢
𝑐𝑢
𝐼𝑟 = 347 − 33 ≤ 300 (𝑂′ 𝑁𝑒𝑖𝑙𝑙 𝑎𝑛𝑑 𝑅𝑒𝑒𝑠𝑒, 1999)
𝑝𝑎
For saturated clay with no volume change, ∆= 0. Hence,
𝐼𝑟𝑟 = 𝐼𝑟
POINT LOAD CAPACITY OF PILE
Vesic’s Method (1977)
 POINT LOAD CAPACITY OF PILE
Coyle and Castello’s Method (1981)

 Granular Soils
𝑸𝒑𝒖 = 𝒒𝒑𝒖 . 𝑨𝒑 = 𝒒′𝑵∗𝒒 𝑨𝒑
where,
𝑞′ is the effective vertical stress at the pile tip
𝑁𝑞∗ is the bearing capacity factor which is a function of 𝐿/𝐷
𝐿 is the length of pile below the ground level.
POINT LOAD CAPACITY OF PILE
Coyle and Castello’s Method (1981)
POINT LOAD CAPACITY OF PILE
Correlations with SPT and CPT

 Granular Soils
Correlation of limiting point resistance with SPT-N
value (Meyerhof, 1976)
𝑳
𝒒𝒑𝒖 = 𝟎. 𝟒𝒑𝒂 (𝑵𝟔𝟎 ) ≤ 𝟒𝒑𝒂 𝑵𝟔𝟎
𝑫
where,
𝑁60 is the average value of the standard penetration
number near the pile point (about 10D above and 4D
below the pile point)
POINT LOAD CAPACITY OF PILE
Correlations with SPT and CPT

 Granular Soils
Correlation of limiting point resistance with SPT-N
value (Briaud et al., 1985)
𝒒𝒑𝒖 = 𝟏𝟗. 𝟕𝒑𝒂 (𝑵𝟔𝟎 )𝟎.𝟑𝟔

Meyerhof (1956) also suggested that,

𝒒𝒑𝒖 ≈ 𝒒𝒄
where,
𝑞𝑐 is the cone penetration resistance
 Problem Set 6
1. Consider a 15 m long concrete pile with a cross section
of 0.45 m x 0.45 m fully embedded in sand. For the sand,
unit weight, γ = 17 kN/m3 and soil friction angle, ϕ’ = 35o.
Estimate the ultimate point 𝑄𝑝𝑢 with each of the following:
1.1 Meyerhof’s method (1014 kN)
1.2 Vesic’s method (1754 kN)
1.3 Coyle and Castello’s method (2479 kN)
1.4 Based on the results from 1.1, 1.2, and, 1.3, adopt a value for 𝑄𝑝𝑢 .
 Problem Set 6
2. Consider a pipe pile with flat driving point having an
outside diameter of 406 mm. The embedded length of the
pile in layered saturated clay is 30 m. The following are the
details of the subsoil:
Depth from Saturated unit weight
𝑐𝑢 , 𝑘𝑁/𝑚2
ground surface, m 𝛾, 𝑘𝑁/𝑚3
0–5 18 30
5 – 10 18 30
10 – 30 19.6 100
The groundwater table is located at a depth of 5 m from
the ground surface. Estimate 𝑄𝑝𝑢 by using:
2.1 Meyerhof’s method (116.5 kN)
2.2 Vesic’s method (149.0 kN)
 Problem Set 6
3. Consider a concrete pile that is 0.305 m x 0.305 m in cross
section in sand. The pile is 15.2 m long. The following are
the variations of 𝑁60 with depth. Estimate 𝑄𝑝𝑢 by using:
3.1 Meyerhof’s correlation equation (893 kN)
3.2 Briaud et al. correlation equation (575.4 kN)
 Problem Set 6
Depth below ground surface, m 𝑁60
1.5 8
3.0 10
4.5 9
6.0 12
7.5 14
9.0 18
10.5 11
12.0 17
13.5 20
15.0 28
16.5 29
18.0 32
19.5 30
21.0 27
 POINT LOAD CAPACITY OF PILE
Goodman (1980)
 Piles resting on rock
𝑸𝒑𝒖 = 𝒒𝒖 (𝑵∅ +𝟏)𝑨𝒑
where,
𝑞𝑢 is the unconfined compression strength of rock
𝑁∅ = 𝑡𝑎𝑛2 (450 + ∅′ /2)
∅′ is the drained friction angle

To consider the influence of distributed fractures in rock

which are not reflected by the compression tests on small
samples, the compression strength for design is taken as,
(𝑞𝑢 )𝑙𝑎𝑏
(𝑞𝑢 )𝑑𝑒𝑠𝑖𝑔𝑛 =
5
 POINT LOAD CAPACITY OF PILE
Goodman (1980)
 Piles resting on rock