Pile & Pier Foundation Analysis & Design

byPeter J. Bosscher

University of Wisconsin-Madison

5Topic Outline

Overview Axial Load Capacity Group Effects Settlement

6Overview

Shallow vs Deep Foundations A deep foundation is one

where the depth of embedment is larger than 2X the foundation width.

7Historic Perspective

one of the oldest methods of overcoming the difficulties of founding on soft soils Alexander the Great, 332BC in Tyre

Amsterdam, die oude Stadt, is gebouwed op palen, Als die stad eens emmevelt, wie zal dat betalen? an old Dutch nursery rhyme

If in doubt about the foundation, drive piles. 1930-1940 practice methodology

8Contrast in Performance

Example deep clay

cu = 500 psf

Load = 340 kips Factor of Safety = 2

Settlements at working load Pad Single Pile Pile & Pad 4-Pile Grp.Immediate 4.1 0.9 2.3 0.8Consolidation 1.2 0.1 0.4 0.2Total 5.3 1.0 2.7 1.0

9Modern Uses weak upper soils

shallow (a) deep (b)

large lateral loads (c) expansive &

collapsible soils (d) uplift forces (e) bridge abutments &

piers (f)

Foundation Design Process(FHWA)

Foundation Design Process

Continued(FHWA)

10

FoundationClassification

11

Pile Types

Timber Piles Steel H-Piles Steel Pipe Piles Precast Concrete

Piles Mandrel-Driven Piles Cast-in-Place

Concrete Piles

Composite Piles Drilled Shafts Augered, Pressure

Injected Concrete Piles

Micropiles Pressure Injected

Footings

12

Timber Piles

13

Steel H-Piles

14

Steel Pipe Piles

15

Precast Concrete Piles

16

Mandrel-Driven Piles

17

Cast-in-place Concrete Piles

Composite Piles

Drilled Shafts

Augered, Pressure Injected Concrete Piles

Micropiles

Pressure Injected Footings

18

Evaluation of Pile Types Load Capacity & Pile Spacing Constructability

soil stratigraphy need for splicing or cutting driving vibrations driving speed (see next slide)

Performance environmental suitability (corrosion)

Availability Cost

21

Soil Properties for Static Pile Capacity

Proper subsurface investigations yield critical information regarding stratigraphy and also provide quality soil samples.

Boring depths minimally should extend 20 feet beyond the longest pile. Looking for critical information such as soft, settlement prone layers, or other problem soils such as cobbles. Want additional information from in-situ field tests (SPT and CPT). Location of groundwater table is critical.

22

Soil Properties for Static Pile Capacity, cont.

From soil samples, determine shear strength and consolidation properties. For clays, both quick and long term strengths (from UU and CU/CD) should be determined. For sands, only CD tests are used.

For clays, the pile capacities in the short and long terms should be compared and the lower of the two cases selected for use. If the design is verified by pile load tests, these results will usually dominate the final design.

23

Factor of Safety

Depends on many factors, including: type and importance of the structure spatial variability of the soil thoroughness of the subsurface investigation type and number of soil tests availability of on-site or nearby full-scale load

tests anticipated level of construction monitoring probability of design loads being exceeded

during life of structure

24

Classification of Structure & Level of Control

Structure: monumental: design life > 100 years permanent: design life >25 yrs and < 100 yrs temporary: design life < 25 yrs

Control:Control

Subsurface Conditions

Subsurface Exploration

Load Tests

Construction Monitoring

Good Uniform Thorough Available Good

NormalSomewhat

variable Good None AveragePoor Erratic Good None VariableVery Poor V. Erratic Limited None Limited

25

Factors of Safety for Deep Foundations for Downward Loads

Design Factor of Safety, F

Classification of Structure

Acceptable Probability of

FailureGood

ControlNormal Control

Poor Control

Very Poor Control

Monumental 1E-05 2.3 3.0 3.5 4.0Permanent 1E-04 2.0 2.5 2.8 3.4Temporary 1E-03 1.4 2.0 2.3 2.8

Expanded from Reese and ONeill, 1989.

26

Methods for Computing Static Pile Capacity

Allowable Stresses in Structural Members Pile Capacity

Many different methods (, , , Meyerhof, Vesic, Coyle & Castello, etc).

Soil Type (Cohesionless, Cohesive, Silt, Layered Soils) Point Bearing Skin Resistance

Normal (Positive) Skin Friction Negative Skin Friction

Settlement of Piles

Allowable Stresses in Structural Members

Any driven pile has to remain structurally intact and not be stressed to its structural limit during its service life under static loading conditions as well as under dynamic driving induced loads. Therefore, material stress limits are placed on:

The maximum allowable design stress during the service life. The maximum allowable driving stresses.

Additional material stress limits, beyond the design and driving stress limits, may apply to prevent buckling of piles when a portion of the pile is in air, water, or soil not capable of adequate lateral support. In these cases, the structural design of the pile should also be in accordance with the requirements of Sections 8, 9, 10, and 13 of AASHTO code (1994) for compression members.

See excerpt from FHWAs Design and Construction of Driven Pile Foundations

27

Axial Pile Capacity

In general:

Three general cases shown (from Das)

30

FAfAq

FPPP sseesea

+=

+=

31

Methods of Evaluating Axial Load Capacity of Piles

32

Full-Scale Load Tests

Most precise way to determine axial load capacity. All other methods are indirect.

Quite expensive thus use judiciously. Two types: controlled stress or controlled

strain, also quick and slow versions. Results are open to interpretation:

9 methods to analyze results

33

When to use Full-scale Load Tests many piles to drive erratic or unusual soil conditions friction piles in soft/medium clay settlement is critical engineer is inexperienced uplift loads on piles

34

How many load tests?

From Engel (1988):Length ofPiling (ft)

Length ofPiling (m)

Number ofLoad Tests

0-6000 0-1800 06000-10000 1800-3000 1

10000-20000 3000-6000 220000-30000 6000-9000 330000-40000 9000-12000 4

35

Static Methods(Based on Soil Tests or In-situ Tests)

More difficult to interpret than load tests: pile driving changes soil properties soil-structure interaction is complex

Less expensive than load tests Used for:

preliminary analysis to plan pile load testing extend results of pile load testing design purposes on small projects

36

Cohesionless Soil no excess pore pressure End Bearing:

many use shallow bearing capacity formulas

use but real piles do not behave

like shallow foundations where capacity increases linearly with depth.

( )q N BNe D q' .= + 1 0 5

37

Max Limit on End Bearing? Some suggest a limit on end

bearing to match experience. Problems with that approach:

more complex than that; need to consider both strength and compressibility of the soil

friction angle varies with effective stress

overconsolidation causes changes in bearing capacity

Vesic/Kulhawy Method Based on Vesics work, Kulhawy gives the

two bearing capacity factors:

38

( ) tan12 DsrEI+

=

( ) tan12 DsrEI+

=

39

Coyle & Castellos Method

Based on 16 pile load tests

Based on and D/B.

CAUTION: No effect of pile material, installation effects, and initial insitu stresses

40

Cohesionless Soil

Skin (Side) Friction use a simple sliding model:

where

often rewrite using K varies with:

amount of soil displacement soil consistency construction techniques

f s h s= tan =

=

h horizontal effective stress

tan coef. of friction between soil and piles = h vK

41

General Method (Kulhawy)

rewrite equation:

Suggest using:

f KKKs v

s=

0 0

tan

Pile & Soil Types s/Sand/Rough concrete 1.0Sand/Smooth concrete 0.8-1.0

Sand/Rough steel 0.7-0.9Sand/Smooth steel 0.5-0.7

Sand/timber 0.8-0.9

Foundation Type &Construction Method

K/K0

Jetted pile -2/3Drilled shaft 2/3 - 1

Pile-small displacemnt -1Pile-large displacement 1 1.2

( ) = sin0 sin1 OCRK

42

Simplistic Method lumps K and tan into one term: =Ktans can develop site-specific or use empirical

formulas in literature. Eg: for large displacement piles in sand,

Bhushan (1982)suggests: = +018 0 65. . D

Dr

rwhere is the relative density in decimal form

43

Coyle & Castellos Method

empirical correlation of fs to and z/B.

z is depth to midpoint of strata.

CAUTION: No effect of pile material, installation effects, and initial insitustresses

44

Cohesive Soil excess pore pressures produced by soil

displacement during driving takes time to dissipate. This means capacity increases with time. Usually assume full capacity is achieved by the time the full dead load is applied.

but usually need to consider live load too. end bearing affected by live load (soil compression)

use undrained strength if significant live load side friction not affected

use drained strength always

45

End Bearing

most engineers use:

not adhesion but rather frictional behavior could use cohesionless equation but

problems again with K0 therefore use method.

=q ss

e u

u

9where = undrained shear strength

Skin Friction

46

Method for Clay use Randolph

and Wroth (1982):

upper limit:

+ tan2 45 2

47

Traditional Methods

a large number of engineers still use adhesion concepts.

The and methods are based onundrained strength. See Sladen (1992) for an analysis of these methods.

These methods have wide scatter, sometimes being as low as 1/3 or as high as 3 times the actual capacity.

48

In-Situ Soil Test Methods

can determine or su and then use previous methods or can use direct correlation methods.

direct in-situ methods especially important for sand as sampling and testing is difficult.

In-situ tests: SPT & CPT

49

Standard Penetration Test

SPT is inconsistent thus correlation is less reliable than CPT.

Two methods (for sand only): Meyerhof &Briaud

SPT does not seem reliable for clays

Meyerhof Method

End Bearing:For sands and gravels:

For nonplastic silts:

=

=

q NDB

N

q NDB

N

e r r

e r r

0 40 4 0

0 40 30

60 60

60 60

. .

. .

For large displacement piles:

For small displacement piles:

f N

f N

sr

sr

=

=

50

100

60

60

Skin Friction:

NOTE NN

r: =

1 6060

tsf; = SPT N corrected for field procedures;= SPT N corrected for field procedures and overburden stress

50

51

Briaud Method

based on regression analyses:

( )( )

=

=

q N

f N

e r

s r

19 7

0 224

60

0 36

60

0 29

.

.

.

.

52

CPT Correlations

the CPT is very similar to driving piles therefore this test is a good predictor of capacity.

unfortunately, the test is rarely run in the U.S. because of the inertia of the engineering community.

for correlations based on CPT see Coduto(1994)

53

From Karl Terzaghi, 1943

The problems of soil mechanics may be divided into two principal groups - the stability problems and the elasticity problems.

Bearing capacity is a stability problem, settlement is an elastic problem.

54

Pile Settlement

Isolated piles designed using the previously mentioned methods usually settle less than 0.5 inches at their working loads. Pile groups may settle somewhat more but generally within acceptable limits. Most engineers do not conduct a settlement analysis unless:

the structure is especially sensitive to settlement, highly compressible strata are present, sophisticated structural analyses are also being used.

55

Why put piles in groups?

Single pile capacity is insufficient Single pile location may not be sufficiently

accurate to match column location To build in redundancy Increased efficiency gained by multiple

piles driven in close proximity

Group characteristics

Common C-C spacing: 2.5 to 3.0 diameters Efficiency: ( ) = = +

Group CapacitySum of Individual Piles

P FN P P

ag

e swhere:group efficiency factor

net allowable capacity of pile groupfactor of safetynumber of piles in groupnet end bearing capacity of single pileskin friction capacity of single pile

==

=

=

=

=

PFNPP

ag

e

s

56

57

Individual vs Block Failure Mode

s

Individual Failure Mode Block Failure Mode

Group characteristics Do not use Converse-Labarre formula for

group efficiency (not accurate) From ONeill (1983):

in loose cohesionless soils, > 1 and is highest at s/B = 2. Increases with N.

in dense cohesionless soils at normal spacings(2 < s/B < 4), is slightly greater than 1 if the pile is driven.

in cohesive soils, < 1. Cap in contact w/ ground increases efficiency but large settlement is required. 58

59

Design Guidelines

Use engineering judgment - no good recipes Block failure not likely unless s/B2, eventual

1.0 but early values range from 0.4 to 0.8. In cohesionless soils, design for between 1.0

and 1.25 if driven piling w/o predrilling. Ifpredrilling or jetting used, efficiency may drop below 1.0.

60

Negative skin friction

Occurs when upper soils consolidate, perhaps due to weight of fill.

61

Negative skin friction The downward drag due to negative skin friction

may occur in the following situations: consolidation of surrounding soil placement of a fill over compressible soil lowering of the groundwater table underconsolidated soils compaction of soils

This load can be quite large and must be added to the structural load when determining stresses in the pile. Negative skin friction generally increases pile settlement but does not change pile capacity.

62

Methods to reduce downdrag

Coat piles w/ bitumen, reducing s Use a large diameter predrill hole, reducing

lateral earth pressure (K) Use a pile tip larger than diameter of pile,

reducing K Preload site with fill prior to driving piling

Laterally Loaded Deep Fnds

Deep foundations must also commonly support lateral loads in addition to axial loads.

Sources include: Wind loads Impacts of waves & ships on marine structures Lateral pressure of earth or water on walls Cable forces on electrical transmission towers

From Karl Terzaghi, 1943

The problems of soil mechanics may be divided into two principal groups - the stability problems and the elasticity problems.

Ultimate lateral load capacity is a stability problem, load-deformation analysis is similar to an elasticity problem.

Ultimate Lateral Load

Dependent on the diameter and length of the shaft, the strength of the soil, and other factors.

Use Broms method (1964, 1965) Divide world into:

cohesive & cohesionless free & fixed head 0, 1, or 2 plastic hinges

Cohesive Soil Diagrams

LateralResistance

Free-HeadDistributions Fixed-Head

Distributions

Cohesionless Soil Diagrams

Free-Head DistributionsFixed-Head Distributions

Summary Instructionsfor

Laterally Loaded Pilesby

B. BromsCohesive Soil:

Cohesionless Soil:

(a)

(b)

Short-Free: ( )Hdg c

e d fuu

=

+ +

2 2515 05

2.

. .

or Fig (a)

where f Hc d

u

u

= 9 and L d f g= + +15.

If M dg cyield u 2 25 2. then pile has one plastichinge and is long.

Long-Free: ( )HM

e d fuyield

=

+ +15 05. . or Fig (b)

Check if ( )M H L dyield u> +0 5 0 75. . . If so, pile isshort, else pile is intermediate or long.Then if M c dgyield u> 2 25 2. then pile isintermediate, else pile is long.

Short-Fixed: ( )H c d L du u= 9 15. or Fig (a)Intermediate-Fixed: H

c dg Md fu

u yield=

+

+

2 2515 05

2.

. .

Long-Fixed: HM

d fuyield

=

+

215 0 5. . or Fig (b)

Short-free: HdK L

e Lup

=

+

0 5 3. or Fig (a)

Long-free: HM

e fuyield

=

+ 0 67. or Fig (b)

where f HdKu

p= 0 82.

Check if M dK Lyield p> 3 . If so, pile is short,else pile is intermediate or long.Then if M yield > the moment at depth f, thenpile is intermediate, else pile is long.

Short-fixed: H L dKu p= 15 2. or Fig (a)

Interm.-fixed: H L dKM

Lu pyield

= +05 2.

Long-fixed: HM

e fuyield

=

+

20 67. or Fig (b)

Load-Deformation Method

Due to the large lateral deflection required to mobilize full lateral capacity, typical design requires a load-deformation analysis to determine the lateral load that corresponds to a certain allowable deflection.

Considers both the flexural stiffness of the foundation and the lateral resistance from the soil.

Main difficulty is accurate modeling of soil resistance.

p-y Method Can handle:

any nonlinear load-deflection curve variations of the load-deflection curve w/ depth variations of the foundation stiffness (EI) w/ depth elastic-plastic flexural behavior of the foundation any defined head constraint

Calibrated from full-scale load tests Reese (1984, 1986) are good references. Requires computer program

COM624P COM624P -- Laterally Loaded Pile Analysis Program for

the Microcomputer, Version 2.0. Publication No. FHWA-SA-91-048.

Computer program C0M624P has been developed for analyzing stresses and deflection of piles or drilled shafts under lateral loads. The technology on which the program is based is the widely used p-y curve method. The program solves the equations giving pile deflection, rotation, bending moment, and shear by using iterative procedures because of the nonlinear response of the soil.

p-y Method: Chart solutions

Evans & Duncan (1982) developed chart solutions from p-y computer runs.

Advantages: no computer required can be used to check computer output can get load vs max moment and deflection

directly

Group Effects

Complexities arise: load distribution amongst piles in group differences between group effect and single pile

ONeill (1983) has identified an important characteristic: pile-soil-pile interaction (PSPI). Larger interaction in closely spaced piles.

Lateral deflection of pile group is greater than single isolated pile subjected to proportional share of load.