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PILE FOUNDATIONS

Foundation Analysis 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:

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.

INTRODUCTION

Based on material

CLASSIFICATION OF PILES

STEEL CONCRETE TIMBER

Pipe piles

Rolled steel H-section piles

Pre-cast piles

Cast-in-situ 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

Disadvantages:

Relatively expensive

Noise pollution during installation

Corrosion

Bend in piles while driving

𝑄𝑎𝑙𝑙 = 𝐴𝑠𝑓𝑠

Allowable Structural Capacity

𝐴𝑠 is the cross-sectional area of steel

𝑓𝑠 is the allowable stress of steel

(~0.33 – 0.5𝑓𝑦)

Concrete Piles: General Facts


Concrete Piles:



Pre-cast Piles:



Cast-in-situ Piles:



Concrete Piles: General Facts


Advantages: Relatively cheap

It can be easily combined with concrete superstructure

Corrosion resistant

It can bear hard driving

Disadvantages: Difficult to transport

Difficult to achieve proper cutoff

𝑄𝑎𝑙𝑙 = 𝐴𝑠𝑓𝑠 + 𝐴𝑐𝑓𝑐


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

𝑄𝑎𝑙𝑙 = 𝐴𝑐𝑓𝑐

Cased:

Uncased:

Timber Piles: General Facts




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

𝑄𝑎𝑙𝑙 = 𝐴𝑝𝑓𝑤


𝐴𝑝 is the average cross-sectional area of pile

𝑓𝑤 is the allowable stress on the timber

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


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)


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

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 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 Non-displacement 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.

Driven Piles


Driven Piles


Bored Piles

Dry Method of Construction


Bored Piles

Casing Method of Construction


Bored Piles

Wet Method of Construction


ESTIMATING PILE LENGTH

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 OF PILES

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′𝑵𝒄

∗ + 𝒒′𝑵𝒒∗ + 𝟎. 𝟓𝜸𝑫𝑵𝜸

∗

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′𝑵𝒄

∗ + 𝒒′𝑵𝒒∗

Hence, pile load capacity is,

𝑸𝒑𝒖 = 𝒒𝒑𝒖. 𝑨𝒑 = 𝒄′𝑵𝒄∗ + 𝒒′𝑵𝒒

∗ . 𝑨𝒑

POINT LOAD CAPACITY OF PILE General Bearing Capacity Approach

𝑸𝒑𝒖 = 𝒒𝒑𝒖. 𝑨𝒑 = 𝒄′𝑵𝒄∗ + 𝒒′𝑵𝒒

∗ . 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

𝑄𝑎𝑙𝑙 = 𝑄𝑢𝐹𝑆

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

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.

ALLOWABLE LOAD

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

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

𝐷=

𝐿𝑏

𝐷 𝑐𝑟. Therefore, the point load capacity of pile is,

𝑸𝒑𝒖 = 𝒒𝒑𝒖. 𝑨𝒑 = 𝒒′𝑵𝒒∗ . 𝑨𝒑 < 𝒒𝒖𝒍. 𝑨𝒑

𝒒𝒖𝒍 = 𝟎. 𝟓𝒑𝒂𝑵𝒒∗ 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

𝐷 𝑐𝑟 value typically ranges from 15D for loose to

medium sand to 20D for dense sands.

Saturated Clays

𝑸𝒑𝒖 = 𝒄𝒖. 𝑵𝒄∗ . 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 = 𝝈𝒐

′𝑵𝝈∗𝑨𝒑

where,

𝜎𝑜′ is the mean effective normal ground stress at the level

of the pile point

𝜎𝑜′ =

1 + 2𝐾𝑜3

𝑞′

𝐾𝑜 is the earth pressure coefficient at rest = 1 − 𝑠𝑖𝑛∅′

𝑁𝜎∗ is the bearing capacity factor


Granular Soils

𝑸𝒑𝒖 = 𝒒𝒑𝒖. 𝑨𝒑 = 𝝈𝒐′𝑵𝝈

∗𝑨𝒑

𝑁𝜎∗ =

3𝑁𝑞∗

(1 + 2𝐾𝑜)

𝑁𝜎∗ = 𝑓 𝐼𝑟𝑟

where,

𝐼𝑟𝑟 is the reduced rigidity index for the soil

𝐼𝑟𝑟 =𝐼𝑟

1 + 𝐼𝑟∆; 𝐼𝑟 =

𝐸𝑠2 1 + 𝜇𝑠 𝑞

′𝑡𝑎𝑛∅′=

𝐺𝑠𝑞′𝑡𝑎𝑛∅′

where,

∆ is the average volumetric strain in the plastic zone below the pile point


Granular Soils

𝐸𝑠𝑝𝑎

= 𝑚

𝑚 =

100 𝑡𝑜 200 (𝑙𝑜𝑜𝑠𝑒 𝑠𝑜𝑖𝑙)200 𝑡𝑜 500 (𝑚𝑒𝑑𝑖𝑢𝑚 𝑑𝑒𝑛𝑠𝑒 𝑠𝑜𝑖𝑙)

500 𝑡𝑜 1000 (𝑑𝑒𝑛𝑠𝑒 𝑠𝑜𝑖𝑙)

𝜇𝑠 = 0.1 + 0.3∅′ − 25

20 𝑓𝑜𝑟 250 ≤ ∅′ ≤ 450

∆= 0.005 1 −∅′ − 25

20

𝑞′

𝑝𝑎

Approximations by Chen and Kulhawy, 1994


Saturated Clays

𝑸𝒑𝒖 = 𝒒𝒑𝒖. 𝑨𝒑 = 𝒄𝒖𝑵𝒄∗𝑨𝒑

𝑁𝑐∗ =

4

3𝑙𝑛𝐼𝑟𝑟 + 1 +

𝜋

2+ 1

𝐼𝑟 =𝐸𝑠3𝑐𝑢

𝐼𝑟 = 347𝑐𝑢𝑝𝑎

− 33 ≤ 300 (𝑂′𝑁𝑒𝑖𝑙𝑙 𝑎𝑛𝑑 𝑅𝑒𝑒𝑠𝑒, 1999)

For saturated clay with no volume change, ∆= 0. Hence,

𝐼𝑟𝑟 = 𝐼𝑟

POINT LOAD CAPACITY OF PILE Coyle and Castello’s Method (1981)

Granular Soils

𝑸𝒑𝒖 = 𝒒𝒑𝒖. 𝑨𝒑 = 𝒒′𝑵𝒒∗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)

𝒒𝒑𝒖 = 𝟎. 𝟒𝒑𝒂(𝑵𝟔𝟎)𝑳

𝑫≤ 𝟒𝒑𝒂𝑵𝟔𝟎

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

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:

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

Depth from ground surface, m

Saturated unit weight

𝛾, 𝑘𝑁/𝑚3 𝑐𝑢, 𝑘𝑁/𝑚

2

0 – 5 18 30

5 – 10 18 30

10 – 30 19.6 100

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

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)𝑑𝑒𝑠𝑖𝑔𝑛 =(𝑞𝑢)𝑙𝑎𝑏

5

POINT LOAD CAPACITY OF PILE Goodman (1980)

Piles resting on rock

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