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
CLASSIFICATION OF PILES
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
CLASSIFICATION OF PILES
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
Concrete Piles: General Facts
CLASSIFICATION OF PILES
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
Allowable Structural Capacity
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
CLASSIFICATION OF PILES
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
𝑄𝑎𝑙𝑙 = 𝐴𝑝𝑓𝑤
Allowable Structural Capacity
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
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)
CLASSIFICATION 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
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
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
INSTALLATION OF PILES
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
POINT LOAD CAPACITY OF PILE Vesic’s Method (1977)
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
POINT LOAD CAPACITY OF PILE Vesic’s Method (1977)
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
POINT LOAD CAPACITY OF PILE Vesic’s Method (1977)
POINT LOAD CAPACITY OF PILE Vesic’s Method (1977)
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 Vesic’s Method (1977)
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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