Statik Load

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GUIDELINES

for the

INTERPRET TION ND

N LYSIS

of the

ST TIC LO DING TEST

Continuing Education Short Course Text

DEEP FOUND TIONS INSTITUTE


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TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. EXECUTION OF THE STATIC LOADING TEST 1

.

Introduction 1

.2 Tes t ing A rrangem ent 2

.3 ASTM Testing Pro ced ure s 4

.4 Rep orting of Re sults 8

3.

SAFETY CONSIDERATIONS 8

. Safety to persons 8

.2 Safety to the Test 10

.3 Point of w arn ing 10

4. INTERPRETATION OF FAILURE LOAD 11

5.

FACTOR OF SAFETY AND ACCEPTANCE CRITERIA 19

.

Factor of Safety 19

.2 A cce pta nc e Criteria 2 0

6. INSTRUMENTATION OF THE PILE 2 1

.

Introduction 2 1

.2 Telltale Instru m enta tion 2 2

.3 Strain Ga ge Instrum entation 2 5

.4 Load Cells 2 6

7. DETERMINATION OF 'ELA STIC MODULUS 2 7

. Basic principles of stress-strain analysis 2 7

.2 Actual te st resu lts 2 8

.3 M athematical relations 3 0

.4 Example from a pile with non -con stant m odu lus 3 2

8. INTERPRETATION AND EVALUATION OF TELLTALE DATA 3 4

.

Basic analysis 3 4

.2 Leonards-Lovell m etho d for load distribution 3 5

.3 Exam ple of Leonards-Lovell an aly sis 3 8

9. INFLUENCE OF RESIDUAL COM PRESSION 3 9

.1 Residual co m pre ssi on in a Leonards-Lovell ana lysis 3 9

.2 Residual com pres sion from a push-pull tes t com bination . . . 4 1

11.

REFERENCES 4 2

12.EXAMPLES 4 5

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1.

INTRODUCTION

The design of pile foundations is much more commonly verif ied by

m ea ns of a full-scale tes t, than is th e desig n of other found ation units .

The reason is not that our knowledge of pile behavior is more uncertain

than our knowledge of other foundation types making the verif ication

necessary, but more that the loads in a s tructure are more concentra ted

to single foundations in a s tructure founded on piles as opposed to s truc

tures on footings or mats . Therefore, should a pile cap fail or move, the

adverse consequence of this is often drastic, as the piled structure has

lit t le freedom to transfer i ts need for support to other foundation units .

Con sequently, it be co m es im portant to assu re the des ign of piled founda

tions.

In many, may be in m ost in sta nc es, th e s tatic pile loading test is routine

and geared toward determining an at least capacity, only. However, in

these times there is an ever increasing liabili ty of the professional, de

mands for increased economy of the foundation, and frequent lack of the

involvement in the test of the experienced old-timer exercising good

judgment. Furthermore, modern pile design is leaving the s ingle, s imple

concept of capacity and requires more information from the test to assis t

in determining aspects of long-term behavior and settlement. Therefore,

even the straight forward, routine static loading test requires improved

planning, execution, and analysis .

These guidelines are written with the objective of presenting views

on the execution and analysis of the s tatic loading test as i t should be

performed in rout ine s i tuat ions and what to cons ider when expanding

the tes t to provide more answers to the des ign engineer than jus t ad

dressing the total capacity of the pile.

2 .

EXECUTION OF THE STATIC LOADING TEST

2.1 Introduction

For many good reasons, a s tatic loading test must be carried out in

accordance with good engineer ing pract ice and under exper ienced

supervision. In North America and in most parts of the world, this

m eans tha t the tes t mus t be in agreem ent wi th the recom m enda t ions

in s ta nd ar ds published by th e Am erican Society for Testing a nd Materi

als,ASTM. For static axial test ing of piles, th e ASTM h as p ub lishe d tw o

standards, one for testing in compression (push) and one for testing in

tens ion (pull) with des ign at ion s D -1143-81 and D -36 89 -83 , respec

tively

1

) . The asp ec ts p rese nte d in this guide apply in equal d eg re e to

testing in push as well as in pull.

1

The ASTM h as also published stan dards for lateral and dynam ic testing of p iles

with the designations

D

3966 and

D

4945, respectively.

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Fig. 2.1 Illustration of error of

applied load

and the jack pressurewhich must be recordedserves as a back-up

value.

As a representative example of what, to expect from the equipment

used by the industry today, Fig. 2.1 sh ow s the difference be tw een the

load determined from the jack pressure and the load determined by

the load cell, as plotted against the load given by the load cell.

The reasons for the load error is that the jacking system is required

to do two things at the same time, i.e., both provide the load and

measure it, and the jack having moving parts is considerably less accu

rate than w ithout moving parts. Also, to exten d the jack p iston, friction

has to be overcome and part of the jack pressure is used for this

purpo se. Many m easurem ent results similar to tho se sho wn in Fig. 2.1

make it obviou s that if one w an ts to ensure that the error in the applied

load is not too large, a load cell m ust be used. A calibration of the jack

and pressure gage (manometer) for one pile is not applicable when

performed on even a neighboring test pile. When calibrating testing

equipment in the laboratory, it is ensured that no eccentric loadings,

bending m om ents, or temperature variations influence the calibration.

However, in the field, all of these factors are at hand to influence the

test results. The extent of the error will be unknown unless a load cell

is used.

Naturally, many structures are safely supported on piles which have

been tested with erroneous loads, and, as long as we are content to

stay with the old rules, loads, and piling systems, we do not need to

improve the precision. The error is included in the safety factor. That

is why factors as large as 2.0 and 2.5 are applied and such numbers

are really more ignorance factors than safety factors. However, if we

want to economize and continue to increase the allowable loads, as

our geotechnical know-how increases, we cannot accept potential er-

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rors as large as 20%. Therefore, use of a load cell to monitor the load

applied in a loading test is imperative.

The fact that a load cell is used is no guarantee for precise loads.

Some load cells are very sensitive to eccentric loading and to tempera

tur e variation and are , the refo re, un suita ble for field us e.

I t must be remembered that the minimum dis tances f rom the sup

ports of reference beam to the pile and the platform, etc. , as recom

mended in the ASTM Designation, are really minimum values, which

m ost often do no t give errors of much con cern for ordinary testing , but

which are too short for research or investigative testing purposes.

The meas uring of mov em ent of the pile head is normally de term ined

in relation to two reference beams. The most common shortcoming of

a test is that the reference beam is not arranged in accordance with

the ASTM stand ard s : the sup po rts of the b eam s, and therefore a lso the

m eas ure d m ov em en ts, are influenced by the reaction load or reaction

system; the sun is let to shine on the beams; the two beams are con

nected ins tead of independently supported; no smooth bear ing sur

face, such as glass , is used for the dial-gages; the gage stems are too

short; all gages are adjusted simultaneously causing a loss of test

continuity, etc. Before s tarting the test, the person in charge and re

sponsible for the test must ensure and verify that the test set-up is

in conformity with all aspects of the recommendations given in the

applicable ASTM standard.

2 3 The ASTM Testing Procedu res

Until recently in North America, the most common test procedure

has been the s low maintained-load p roc edu re referred to as the s tan

dard loading pr oce du re in the ASTM D-1143 Sta nda rd in which the

pile is loaded in eight equal increments up to a maximum load, usually

twice the predetermined a l lowable load.

The s tand ard loading pro ced ure is often though t of as

th

ASTM

procedu re . However , the ASTM D 1143 -81 and D 36 89 -8 3 S tan dard s

present s ix additional procedures of applying the load. Of these, the

firs t three are variations of the s low maintained load procedure. The

rem aining th ree a re: th e constan t-rate-of-p enetratio n (C.R.P.) pro ce

dure ,

the quick- tes t proce dure , and the cons tant-m ovem ent- incremen t

procedure .

In the s tan dard loading proc edu re , each increm ent is mainta ined

until a minimum movement is reached, commonly referred to as the

zero m ove m ent . The minimum m ove m ent is defined as 0.01 in/h or

0. 0 0 2 in /1 0 min). The final load, the 2 0 0 p erce nt load, is main tained

for 24 hours . The s tand ard p roc ed ure is very t ime consu min g requir

ing from 3 0 to 70 ho urs to co m ple te. It shou ld b e realized that th e

wo rds zero m ov em ent are very mis leading, as the movem ent ra te of

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0. 01 in/h is equal to a m ovem ent as large as 7 ft/yr, well beyo nd any

conceivable yearly settlement rate.

The standard procedure , also called Slow Maintained-Load Test

or just Slow Test , can be sp ee d ed up by using the method pro posed

by Mohan et al. (1967), where the load (jack pressure) is allowed to

reduce to and stabilize at a lower value rather than being maintained

by pumping. The stabilized value is taken as the load applied to the

pile.

Housel (1966) proposed that each of the eight increments be main

tained exactly on e hour wh ether or not the zero m ove m ent has bee n

reached (called the constant-time-interval-loading procedure).

Applying the load at equal time intervals allows an analysis of move

m ent with time, which is not po ssib le with the standard procedure :

For each load increment, plot the magnitude of movement obtained

during the last 30 minutes of the one-hour load duration versus the

applied load. Initially, the values will fall on a more or less straight line.

At one load level, however, two approximately straight lines will be

obtained. Provided that the test has approached failure, that is. The

intersection of the two lines is termed yield value.

A tes t according to H ousel's procedu re tak es a full day to perform.

The points on the curve are still very few, but Housel's procedure is a

definite improvem ent of the standard procedure and it is on e of the

sev en op tional pro cedures in the ASTM Designation D -1 14 3. H owever,

it is better to apply, say, 16 equal increments of a half hour duration

as op po sed to the standard 8 equal increm ents of on e hour duration;

the rate of loading is the same, but the load-movement curve is better

defined. A yield value similar to the on e obtained from the m ovem ent

during the last 3 0 minutes of the one-hour increment can be evaluated

from the m ovem ent during the last 15 minutes of the 30 -minute incre

ment provided that readings are taken often eno ugh and that they are

accurate. But why stop at 16 increments applied at every 20 minutes,

when 32 increments are applied every 15 minutes determine the load

deformation curve even better? The load is still applied at a constant

rate in terms of tons per hour and no principal change is made. An

additional benefit is that a small increment will not shock the soil and

change the load transfer characteristics in contrast to the effect of a

large increment applied quickly.

Actually, the duration of each load is less important, be it one hour

or 15 minutes. The importance is that the duration of each load is the

same. From this realization, we can progress to the one that even

shorter time intervals are possible without impairing the test. Further,

by using as short time intervals as practically possible, the influence

on the results of time dependency is reduced. When it is desirable to

study the time dependency, drained test conditions, creep aspects,

etc., the test duration should be measured in weeks, months, or even

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years. A 48-hour or 72-hour test d oe s not give any information on time-

dependent behavior of the pile and results only in confusion.

A test which c on sists of load increm ents applied at con stan t time

intervals of 5, 10, or 15 minutes, is called Quick Maintained-Load

Test or just Quick Test and is from both tech nica l, practical, and

economical views superior to the Slow Test. This procedure is also

included in the mentioned two ASTM standards.

A Quick Test should aim for at least 20 load increments with the

maximum load determined by the amount of reaction load available or

the capacity of the pile. In routine proof tests, the maximum test load

is commonly chosen to 200 percent of the intended allowable load.

For most tests, however, it is preferable to carry the test beyond the

200 percent value.

As to time intervals, for ordinary test arrangements, where only the

load and the pile head movement are monitored, time intervals of 5

minutes are suitable and allow for taking 2 to 4 readings for each

increment. The ASTM standards permit intervals of time between load

increments as short as 2 minutes. While no technical disadvantage is

associated with a very short time interval as long as the intervals are

equal, unless data acquisition apparata are employed having a rapid

scanning capability, practical difficulties arise when using intervals

shorter than 5 minutes.

W hen testing instrumented piles, wh ere the instruments take a while

to read (scan), the time interval may have to be inc reased . To go beyond

15 minutes, however, should not be necessary. Nor is it advisable,

because of the potential risk for influence of time dependent move

ments, which may impair the test results. Usually, a Quick Test is

completed within two to five hours.

A tes t which has ga ined much use in Europe is the constant-rate-of-

penetration test (C.R.P. test), first proposed internationally for piles by

Whitaker (1957; 1963) and Whitaker and Cooke (1961). Manuals on

the C.R.P. test have been published by the Swedish Pile Commission

(1970) and New York Department of Transportation (1974). In the

C.R.P. test, the pile head is forced to move at a predetermined rate,

normally 0. 0 2 in/min (0.5 m m/m in), and the load to achiev e the mov e

ment is recorded. Readings are taken every two minutes and the test

is carried out to a total movement of the pile head of two to three

inches (50 to 75 mm) or to the maximum capacity of the reaction

arrangement, which means that the test is completed within two to

three hours.

The C.R.P. tes t h as the ad vantage over the Quick Test that it en ab les

an even better determination of the load-movement curve. This is of

particular value in testing shaft bearing piles, when sometimes the

force needed to achieve the penetration gets smaller after a peak

value has b een reached . It also a gr ees with the testing in mo st other

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engineering fields, which regularly use the C.R.P. procedure to deter

mine strength and stress-strain relations. A C.R.P. test is best per

formed with a mechanical pump that can provide a constant and non-

pulsing flow of oil. Ordinary pumps with a pressure holding device,

manual or mechanical, are less suitable because of unavoidable load

ing variations. Also, the a bsolute requirement of simu ltaneou s reading

of all load and deformation gages (changing continuously) could be

difficult to achieve without a well trained

staff.

For the se reaso ns, the

Quick Maintained-Load Test is preferable. For instrumented piles, a

C.R.P. test is not suitable unless used with a very fast data acquisition

unit.

A fourth te st p rocedure is cyclic test ing . For deta ils cyclic proce

dures, see Fellenius (19 75 ), and referen ces containe d therein. In rou

tine tests, cyclic loading, or even single unloading and loading phases

must be avoided. It is a common misconception that unloading a pile

every now and then according to som e more or less logical sch em e

will provide information on the toe movement. That it will not, but it

will result in a destruction of the chances to analyze the test results

and the pile load-movement behavior. In non-routine tests and for a

specific purpo se, cyclic testing can b e use d, but thenafter completion

of an initial test and when having the pile instrumented with at least a

telltale to the pile toe.

To emphasize: there is absolutely no logic in believing that anything

of value can be ob tained from cyclic testing con sisting of one or a few

occasional unloadings, or one or a few resting periods at certain load

levels, wh en considering that w e are testing a unit that is subjected to

the influence of several soil types, is already under stress of unknown

m agnitude, exhibits progressive failure, etc., and wh en all we know is

what we apply and measure at the pile head, while we really are inter

ested in what happens at the pile toe.

The constant-movem ent-increment-loading procedure is rather spe

cial and of little interest to engineering practice.

Unloading procedure

When unloading the pile, a simple procedure

is recommended, as follows: Reduce (leak) the pressure in the pump

in decrem ents and take readings of the pressure and dial g a g es valu es

at each level of reduced load as obtained. It is important that the jack

piston d oes not reverse its direction of travel, that is, the pressure mu st

not be increased even if the desired pressure or load level is missed.

The first two decrements are to be small in order to enable the influ

en ce of the piston friction, if any, to be determ ined. Then the unload ing

con tinue s in so m e fourorfive larger decr em en ts until only a small load

still is on the pile head, which is then unloaded in two small decre

ments. Before removing the gages, a final reading is taken after the

pile has been under zero load for about five minutes.

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2.4 Reporting of Re sults

The results of a static loading test must be presented in a report

conforming to the applicable requirements of the ASTM standards.

More specifically, the immediate test results should be provided in a

table showing pertinent pile identification and the times for start and

finish of the test, and, for each load increment, the load cell readings

and load cell loads, the jack pressure readings and jack loads, move

ments measured for each dial gage and averaged head movements ,

and other recorded data.

The load-movement readings should be presented in a diagram in

th e first qua dra nt with the load on a linear scale on th e or din ate (vertical

axis) and the movement on a linear scale on the abscissa (horizontal

axis).

To facilitate the interpretation of the test results, the diagram scales

should be selected so that the l ine representing the calculated elast ic

line of th e pile (the colu m n line ) will be inclined a t an an gl e of a bo ut

20 degrees to the load axis. The slope of the elast ic l ine is computed

from the following expression:

(2.1)

where

= calculated elast ic shortening

Q = applied load

L = pile leng th

A = pile cross sectional area

E = elastic modulus of the pile material

The calculation is best performed inserting all parameters in base

units:

Q in N (lb), L in m (inch), A in m

2

(in

2

), and E in Pa (psi), which

gives d in N/m (lb/inch). Then, division by 1 000 000 (2,000) gives d

in KN/mm (tons/inch) and the elastic line is simple to draw.

Often the time-movement curve is of interest. Also this curve should

be draw n in the first qu ad ran t. Plot time in a linear scal e on the ab sc issa

and movement in a l inear scale on the ordinate.

3 . SAFETY CONSIDERAT IONS

Safety should be foremost in mind when performing a static loading

test. First of all , the safety of the persons present at the test, and, then,

the safety of the test

itself.

3.1 Sa fety to Person s

There are numerous accidents occurring at s tat ic tests , which have

caused serious injury and death to persons. Common for them all

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is that with some foresight and precautions they could have been

avoided.

The immediate and obvious detail to consider is the stack of sepa

rate items placed between the pile head and the main reaction beam

consisting of a jack, a load cell, a swivel plate, and some spacing

plates. It is im po ssible to ensure that the pile is perfectly perpen dicular

to the main b eam , that the jack, load cell, swivel plate, and sp ac ers are

placed absolutely concentric and in perfect alignment with each other

and the pile, or that the geometric center of the system coincides with

the force center. The stack of individual parts is usually a good deal

less stable than it appears and its parts can easily fall. Wearing a hard

hat and toe reinforced shoes is advisable although such precaution

does not replace care because they do not provide much protection

from a falling steel plate. Consider also that the system is subjected to

loads which can amount to several hundred tons, which builds large

energy into the system that is released should a plate slip out of the

stack or the pile fail sud den ly, which can hurl the items around injuring

the bystanders. The stack must be retained by a cage protecting the

per son s p ositioned near the pile, an d/or all parts be secur ed by a wire

or rope that will catch them should they fall.

Other safety concerns rest with the arrangement of load on the

reaction platform. A good rule-of-thumb is not to build the platform

load higher than its width. The founda tion of the load ed platform must

be safe . Many testing failures are preceded by, eve n o riginate in, a

shift of a platform foundation.

When reaction is provided by anchors, make sure that should one

anchor fail, the others must not act as a suddenly released slingshot

sending beams and material swinging through the air.

If at all possible, all gages should be read from a distance to elimi

nate the need for going in under a test platform or close to a test set

up.

Use binoculars or a camera tele lens.

Itis a goo d approach to rope off the imm ediate test site and proclaim

the area off limit to everyone not actively participating in the perfor

mance of the test. This goes for uninvolved curious onlookers, as well

as for the involved ones such as the client and the owner. All persons

involved should be on alert for strange noise and movements of the

entire system. The person in charge should not become so absorbed

by the task of collecting data to forget oc casion ally to walk around and

visually inspect the test set-up for signs of distress concentrating on

the following questions:

1.

Is everything plumb? In all directions?

2 . Is there enou gh w eigh t on the frame?

3. Is the jack in line with the pile?

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4. Has th e dial ga ge ste m s moved side w ays indicating lateral insta

bility

5 . Are there any signs of leak of hydraulic oil from jack or pump?

If th e answ er to any of th e que stio ns is NO , you mu st tak e the tim e

to inves t igate what the cause is and you may have to abor t the tes t

and rectify the s ituation. Do not hesitate about being assertive. An

unsafe situation is not to be taken lightly and the need for safety must

not be underes t imated.

3.2 Sa fety to the Test

Also li t t le things that do not harm any person may jeopardize the

tes t. For ins tan ce, if th e reference beam is not prote cted from su ns hin e,

the movement readings may be wrong. Or, if a reference beam is

distu rbed (som eon e pu ts his foot up), th e data m ay be spoiled from the

dial ga ge s con nec ted to that bea m . Then, if the reference be am s h ave

been intercon nected , the dis tu rban ce of on e beam may offset the pos i

tion of also the second beam and all data may be lost. Note, stiffening

the reference beams by connecting them is a violation of the ASTM

recommenda t ions .

Also for rea so n s of safety to th e test, it is a goo d idea to ro pe off th e

test area and make it off-limit for everyone not directly involved.

3. 3 Points of W arning

Performing a s tatic pile loading tes t can be a risky pro ce ss . The t es t

arran gem en t must be des ig ned and buil t by per son s having e xp er ience

from this ty pe of wo rk. Below is offered a che cklist for refe ren ce to th e

danger points to cons ider before s tar t ing the tes t .

1.

Check tha t th e intende d m aximum load is sm aller than th e s truc

tural strength of the pile by a safe margin.

2 .

Check that the maximum test load is smaller than about 90

percent of the jack capacity.

3 . Check tha t the maximum te st load is ob tained at a jack pre ssu re

of about 80 percent of the maximum capacity of the pressure

gage (manometer) .

4 .

Check tha t the reaction load available is abo ut 2 0 per cen t larger

than the maximum test load.

5 . Check that the pac kag e betw een the pile head and the main tes t

be am (jack, load cell, swivel plate, and sp ac er p lates) are se cu re d

in a cage or otherwise prevented from falling to the ground

should they become loose dur ing the tes t .

6. Previous ly used tes t beams should be inspected to ensure that

their s trength has not been reduced by cutting or corrosion.

7. Cut all un ne ces sary tem po rary w elds within th e reaction system

before s tarting the test.

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Fig. 4.1 Illustration of th e

conceived failure load's

dependency on the

draughting scale

8. Do not allow weld ing or torch cutting clos e to tensio n stee l su ch

as high s trength threa d bars . The heat may w eake n th e s teel

and create a dangerous s i tuat ion.

9. Ensure tha t all pers on nel w ea rs a hard h at.

10. Note tha t hydraulic valves or co nn ecti on s mu st not be tigh ten ed

or otherwise adjusted while pressure is on. Jets of hydraulic

fluid can cause considerable injury; whipping hydraulic hoses

can kill.

4 .

INTERPRETATION OF FAILURE LOAD

For a pile which is stronger than the soil, a failure load by plunging is

reached when rapid movement occurs under sustained or s lightly in

creased load. However, this definition is inadequate, because plunging

requires large movements and the ultimate load reached is often less a

function of th e capa city of th e pile-soil system and m ore a function of th e

man-pump sys tem.

A co m m on definition of failure load ha s bee n th e load for w hich th e

pile head mo vem ent ex cee ds a certa in value, usual ly 10% of the diam eter

of th e pile. This definition do es not con side r the elastic sho rtenin g of th e

pile, w hich can be su bs tan tial for long p iles, wh ile it is neg ligible for s ho rt

piles. In reality, a movement limit relates only to the allowable movement

allowed by the superstructure to be supported by the pile, and not to the

capacity of the pile.

So m etim es, th e failure value is defined as load value at th e intersection

of two straight lines, approximating an initial pseudo-elastic portion of the

load-movement curve and a final pseudo-plastic portion. This definition

results in interprete d failure load s, wh ich dep en d g reatly on ju dg em en t

and, above all , on the scale of the graph. Change the scales and the

failure value changes also, as i l lustrated in the load-movement diagram

pre sen ted in Fig. 4 .1 . A loading test is influenced by man y o cc ur ren ce s,

but the draughting manner should not be one of these .

Without a proper def ini t ion, interpreta t ion becomes a meaningless

venture. To be useful, a definition of failure load must be based on some

mathematical rule and generate a repeatable value that is independent

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Fig. 4.3 Davisson's offset limit

method

Fig. 4.4 Chin-Kondner's method

initial variation, the plotted values fall on straight line. The inverse slope

of this line is the Chin failure load.

Generally sp eak ing, tw o points will determine a line and third point on

the sam e line confirms the line. However, bewa re of this statem ent wh en

using Chin's method .Itis very ea sy to arrive atafalse Chin value if app lied

too early in the test. Normally, the correct straight line does not start to

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Fig. 4.5 DeBeer's method

Fig. 4.6 Brinch-Hansen's 90%

criterion

materialize until the test load h as p as sed the Davisson limit. As a rule, the

Chin Failure load is about 20% to 40% greater than the Davisson limit.

When this is not a case, it is advisable to take a closer look at all the test

data.

The Chin method is applicable on both quick and slow tests, provided

con stant time increments are used. The ASTM standard metho d is

therefore usually not applicable. Also, the number of monitored values

are too few in the standard test ; the interesting dev elopm ent could well

appear between the seventh and eighth load increments and be lost.

Fig. 4.5 presents a method proposed by DeBeer (1967) and DeBeer

and Wallays (1972), where the load movement values are plotted in a

double logarithmic diagram. When the values fall on two approximately

straight lines, the intersection of these defines the failure value. DeBeer's

method was proposed for slow tests.

Fig. 4.6 illustrates a method proposed by BrinchHansen (1963), who

define s failure as the load that give s twice the m ovem ent of the pile head

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Fig. 4.7 Brinch-Hansen's method

(the 80% criterion)

as obtained for 90% for that load. This method, also known as the 90%-

criterion, has gained w idespr ead use in the Scan dinavian coun tries

(Swedish Pile Commission, 1970).

BrinchHansen (1963) also proposed an 80%-criterion defining the fail

ure load as the load that gives four times the movement of the pile head

as obtained for 80% of that load. The 80%-criterion failure load can be

estima ted by extrapolation from the load-movem ent curve directly, which

gives about 210 tons. The failure load according to the BrinchHansen

80-percent criterion can also be more accurately determined in a plot

which is very similar to that of the Chin-Kondner p lot. Fig. 4.7 sh ow s th is

plot for the test data from the example test, where the square root of

each movement value is divided with its corresponding load value and

the resulting value is plotted against the movement.

The following simple relations can be derived for computing the ulti

mate failure, Q

u

, according to the BrinchHansen 80%-criterion:

(4.1)

(4.2)

Where

Q

u

= failure load

A

u

= movement at failure

C

1

= slope of the straight line

C

2

= y-intercept of the straight line

When using the BrinchHansen 80%-criterion, it is important to check

that the point 0.80 Q

u

/0 .25 A

u

indeed lies on or near the mea sured load-

movement curve.

In the exa mp le c as e, Q

u

is 2 11 tons, which a gre es well with the value

determined from the load-movement curve, directly.

Notice that both the BrinchHansen's 80%-criterion and the Chin

method allow the later part of the curve to be plotted according to a

15


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Fig. 4.8 M azurkiewicz's meth od

mathematical relation, and, which is often very tempting, they make an

exact extrapolation of the curve possib le. That is, it is ea sy to fool

oneself and believe that the extrapolated part of the curve is as true as

the measured.

In Fig. 4.8, the method by Mazurkiewicz (1972) is illustrated. First, a

series of equally spaced lines parallel to the load axis are arbitrarily

ch os en and drawn to intersect with the load-movem ent curve. Then, from

each intersection, a line is drawn parallel to the movement axis, toward

and crossing the load axis. At the point of intersection with the load axis

of each such line, a 45 line is drawn to intersect with the line above.

Th ese intersections fall, approximately, on a straight line which ow n inter

section with the load axis defines the failure load. Mazurkiewicz' method

is also , understandably, called the m ethod of multiple intersec tions .

When drawing the line through the intersections, some disturbing free

dom of choice is usually found.

Fig. 4.9 illustrates a simple definition by Nordlund (1966) and Fuller

and Hoy (1970). The failure load is equal to the test load for where the

load-movement curve is sloping 0.05 inch per ton.

Fig. 4.9 also shows a development of the above definition proposed

by Butler and Hoy (1977) defining the failure load as the load at the

intersection of the tangent sloping 0.05 inch/ton, and the tangent to the

initial straight portion of the curve, or to a line that is parallel to the

rebound portion of the curve. As the latter portion is more or less parallel

to the elastic line (compare Fig. 4.3), Fellenius (1980) suggests that the

intersection be that of a tangent parallel to the elastic line, instead.

The Nordlund/Fuller and Hoy method penalizes the long pile, because

the elastic movements for a long pile are larger, as opposed to a short

pile; the slop e of 0. 0 5 inch/ton occu rs soo ner for a longer pile. The

Butler and Hoy dev elopm ent tak es the elastic deformations into accou nt,

substantially offsetting the length effect.

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Fig. 4.9 Nordlund/Fuller and

Hoy's method

Fig. 4.10 Vander Veen's method

Fig. 4.10 shows the construction of the failure load as proposed by

Vander Veen (1 953 ).Avalue of the failure load , Q

ult/

is ch osen and values

calcu lated from ln(1 - Q/Q

ult

), are plotted against the movement. When

the plot becomes a straight line, the correct Q

ult

has been chosen. The

Vander Veen method w as p roposed long before po cket calculators w ere

available. Without using tho se, h ow ever, its application is very time con

suming.

17


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Fig. 4.1 1 Com pilation of failure

criteria

In Fig. 4.11, the above determined nine values are plotted together.

As sho wn , the offset limit of 181 ton s is the low est and the Chin value of

235 tons is the highest. The other seven values are near the maximum

test load of 207 tons.

It is difficult to make a rational choice of the best criterion to use,

be ca use the preferred criterion de pe nd s heavily on one's pa st exp erien ce

and conception of what constitutes failure. One of the main reasons for

having a strict criterion is, after all, to enable a set of com patible refere nce

cases to be established. The author prefers to use, not one, but four of

the criteria. The preferred criteria are the Davisson limit load, the Brinch-

Hansen 80%-criterion, the Chin-Kondner failure load and the Butler and

Hoyfailure load with the p rop osed modification.Inca se of an engineering

report, the preference and experience of the receiver of the report may

result in the use of also other methods. Naturally, whatever one's pre

ferred mathematical criterion, the failure load or pile capacity value in

tend ed for use in design of a pile foundation must not be higher than the

maximum load applied to the pile in the test. A safety factor applied to

an extrapo lated capacity is not reliable.

The Davisson limit is chosen because it has the tremendous merit of

allowing the engineer, when proof testing a pile for a certain allowable

load, to d etermine

in advance

the maximum allowable movement for this

load with consideration of the length and size of the pile. Thus, as pro

posed by Fellenius (1975), contract specifications can be drawn up in

cluding an acceptance criterion for piles proof tested according to quick

testing methods. The specifications can simply call for a test to at least

twic e the desig n load, as u sual, and d eclare that at a test load equal to a

factor, F, tim es the d esign load, the m ovem ent shall be sm aller than th e

elastic column c om pressio n of the pile, plus 0 .1 5 inch, plus a value equal

to the diam eter divided by 1 2 0 . The factor

F

should be chosen according

to circumstances in each case. The usual range is 1.8 through 2.2.

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The BrinchHansen 80%-criterion is chosen because it usually gives a

Q

u

-value, which is close to what one subjectively accepts as the true

ultimate failure value. The value is smaller than the Chin value. However,

the criterion is more sensitive to inaccuracies of the test data than is the

Chin criterion.

The Chin-Kondner method is chosen, because i t a l lows a continuous

check on the test, if a plot is made as the test proceeds, and a prediction

of the maximum load that will be applied during the test. Sudden kinks

or s lope changes in the Chin line indicate that something is amiss with

either the pile or with the test arrangement (Chin, 1978). The Chin value

has the additional advantage of being less sensitive to imprecisions of

the load and movement values .

The Butler and Hoy method is chosen primarily because of i ts resem

blance to the offset-limit method. In some cases, a Davisson limit load

can be obtain ed withou t the in terpreter being willing to acc ep t intuitively

that the pile has reached failure. (In such cases, the Chin value will be

mu ch high er than th e D avisson limit). Further, as the Butler and Hoy slo pe

of 0.05 inch/ton is not approached unless fa i lure is imminent , absence

of a Butler and Hoy failure in addition to a high Chin value indicates that

the particular Davisson value is imprecise. The reasons for the latter

can be wrongly chosen values of pile elastic modulus or pile length, or

imprecise or erroneous values of load or movement.

5. FACTOR OF SAFETY AN D ACCEPTAN CE CRITERIA

5.1 Fac tor of Sa fety

The mo st com m on pu rpo se of a s tatic loading test is to d eter m ine

the capacity of a pile or that the pile has an at-least capacity. The

capa city is related to the desired safe load on the pile, th e allow able

load, by a factor of safety, wh ich is th e ratio betw ee n th e cap acit y

determined in the test and the allowable load. In so-called factored

des ign , a res is ta nc e factor is applied to the capa city and a load

facto r is app lied to th e load. In Euro pe, th e latter ap pr oa ch is called

par tia l factor of safe ty app roa ch and is today the dom inant app roac h.

The factor of safety is not a singular value applicable at all times.

The value to use depends on the desired freedom from danger, loss ,

and unacceptable consequence of failure, and on the level of knowl

edge and control of the aspects influencing the variation of capacity

at the s ite. Not least important are the method used to determine or

define the ultimate load from the test results and how representative

the test is for the s ite. For piled foundations, practice has developed a

range of factors to apply, as follows.

For exa m ple, in a testing pro gra m m e early in the d esig n w ork, using

piles which are not necessarily the same type, s ize, or length as will

be u sed for th e final projec t, th e safety factor ap plied could be 2 .5 . In

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23/47

the ca se of tes t ing dur ing a final des ign ph ase , wh en tes t ing th e und er

conditions more representative for the project, the safety factor could

be reduced to 2.2. Then, when testing is carried out on the actual pile

used for th e project an d installed by the actu al piling cont rac tor for

purpose of verifying the f inal design, the factor commonly applied is

2.0. Well into the project, when testing is carried out for proof testing

purpose and conditions are favorable, the factor may be further re

duced and become 1.8. Reduction of the safety factor may also be

w arran ted w hen limited variability is confirmed by m ea ns of com binin g

the design with detailed site investigation and control procedures of

high quality. One must also consider the number of tests performed

and the scatter of the test results between tests . Not to forget the

assurance gained by means of incorporat ing dynamic methods for

control l ing hammer performance and for capaci ty determinat ion

alongside the s ta t ic procedures .

The value of the factor of safety to apply depends, as mentioned, on

the m ethod used to determ ine i t. A conservat ive m ethod , such a s the

Davisson offset load, warrants a smaller factor than a method such as

the B rinchHansen 80% -criterion. It is goo d prac tice to apply more th an

one method for defining the capacity and to apply to each method its

own factor of safety letting the smallest allowable load govern the

design. As mentioned earlier , i t is not good practice to extrapolate the

test results to a capacity larger than the maximum test load and apply

a factor of safety to the extrapolated value. That is to say, a factor-of-

safety appro ach should not be used with capaci ty determ ined from the

Chin method.

In a design geared toward determining the load distr ibution along

a pile, the location of the neutral p lane , and t he s ettle m en t of th e piled

foundation, the factor of safety may not be the governing aspect. The

design may then be completed with a factor of safety that is larger

than the mentioned values , as well as , in some cases , smaller . The

more important the project, the more information that becomes avail

able, and the more detailed and representative the analysis of the pile

behaviorfor which a static test is only a part of the overall design

effortthe more weight the settlement analysis gets and the less im

portant the factor of safety becomes.

5.2 A cc ep ta nc e Criteria

Proof testing piles is carried out less for determining capacity (ulti

mate resis tance; failure load) and more for determining an at- least

capacity. The maximum test load is normally only twice the intended

allowable load. In older days, the acceptance criterion for the test w s

simply that the movement at the maximum load must not be larger

than a specified value, and that after unloading, the net movement

2 0


24/47

must not be larger than a specified value. Usually the testing method

w as the so-called sta nd ard ASTM me tho d. For sho rt piles , wh ich d em

onstrate small 'elastic ' compression for the applied load, this was nor

mally an economical and practical albeit somewhat l iberal criterion,

while for long piles , i t was often uneconomically conservative. Also,

the max imum -and-net-mo vement cr i terion cam e into pract ice w hen

loads were much smaller than the current loads and when most s truc

tures were less sensitive to differential movements. Apart from only

using one point on the curve neglecting the information provided by

the load-mov ement behavior of the tes ted pi le , the maximum -and-net-

mo vem ent c r ite rion inc ludes the misconcept ion tha t the m ovem ent

acceptable for the s tructure and the long-term movement of the pile

cap has anything directly to do with the load-movement behavior of

the tested single pile.

If m ov em ent of the pile cap is crit ical to the d esig n, the de sign m ust

include a proper settlement analysis of the pile group and the static

pile test may have to include instrumentation of the piles. If not, then

a simple factor of safety approach is sufficient as based on the shape

of the load-movement curve and the capacity determined from the

static test . From reasons of practical engineering and contractual as

pects , the acceptance cr i ter ion should be based on the combinat ion

that the offset limit and the failure load should not be reached before

test loads of, say, 1.8 through 2.2 and 2.0 through 2.5 times the allow

able load, respectively.

6. INSTRUM ENTATION OF THE PILE

6.1 Introduction

In the routine static loading test, measurements are taken at the pile

head only and it is impossible to estimate with any worthwhile accu

racy the mobilized toe resis tance from load-movement data obtained

at the pile hea d. That is , the pile-head load-m ovem ent d ata essentially

only tell the total cap acity of the pile giving very little to aid an interp re

tation of the load distribution in the test pile. Yet, in most tests, after

having determined the tota l capaci ty, one may be equal ly concerned

over wh at po rtion of the capa city is obta ined at the pile to e or over the

lower portion of the pile, where is the neutral plane located, what is

th e shaft re sis tan ce in a specific soil layer, etc . Th e co st s and efforts

involved in addressing these questions vary with the specific condi

tions and degree of accuracy required. However, already a minimal

and low-cost instrumentation effort may give a considerable boost to

the value of a static test.

In

brief

instrumentation of the pile refers to instrumentation

down

the pile which is extra to the routine instrumentation at the pile head

for pile head movement, applied load, and jack pressure.

2 1


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Instrumentation consists of a wide array of efforts from the simple

telltale rod inserted to the pile toe over installing a multi-telltale or an

electrical s train gage system all through to the incorporation of sepa

rate load cells . The topic is huge and the scope of these guidelines

precludes providing details of the various instrumentation systems.

Therefore, the views presented in this chapter are l imited to those

necessary to familiarize the reader with the different aspects involved

in instrumenting a pile. In an actual case, i t is necessary to make

reference to mo re com preh ensiv e texts , such as Geo technical Ins tru

m en tatio n for M onitoring Field Pe rfo rm an ce by Dunnicliff (19 88 ),

which gives extensive background to instrumentation of piles .

6.2 Telltale Instrumentation

The static loading test can be substantially enhanced by placing

telltale s in the pile. A telltale is a rod (or wire) wh ich lowe r en d is

connected to the pile, usually at the toe, but which stands free from

the pile along its overall length by m ea ns of a guide-pipe arra ng em en t.

By attach ing a dial ga ge at th e up per end of the rod and m easu ring

the change of dis tance between the rod top and the pi le head, the

sho rtenin g of the pile during th e test is m onitored . The telltale rod tells

a tale: that of the movement of i ts lower end and, therefore, of the

movement of the pile at the location of the lower telltale end in relation

to the pile head position. The absolute movement of the pile toe is

obtained as the measured pi le shor tening subtracted f rom the move

ment of the pile head.

With use of some foresight and planning, tell tales can be installed

rather easily and cheaply in all types of piles . Suggestions for s imple

te l l ta le arrangements are included in the ASTM D1143 s tandard with

reference to arrangement for telltale rods in pipe piles, steel H-piles,

and wood piles. Naturally, a telltale can also be installed in precast

prestressed concrete piles if they are equipped with a guide pipe cast

in the pile in the precast yard. Alternatively, outside placing of guide

pipes can be used. Instead of a stiff rod, a telltale can also consist of

a str etc he d wire . Telltales can b e installed singly or as m ultiple tell

ta le .

For details, see Dunnicliff (1988).

Fig. 6.1 pr es en ts an ex am ple of tes t results from a static loading tes t

on a pre cas t co nc rete pile. A guide-pipe for a tell tale had be en cas t in

the pile allowing a telltale to be inserted to the pile toe after the driving

to monitor the compression (shortening) of the pile.

The dif ference between the pi le head movement and the movement

of th e telltale end is th e sh orte nin g (or, in uplift tes tin g, th e leng then ing )

of the pile between the pile head and the location of telltale end.

The shortening value can be transferred to a value of s train over the

particular length of the pile by dividing the value with the length. By

2 2


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Fig. 6.1 The load-movement

diagram of a pile

equipped with a telltale

to the toe of the pile.

multiplying the strain with the modulus of elasticity (that is, applying

Hooke s Law), the average stress in the pile over the telltale length is

obtained. By multiplying the stress with the cross sectional area of the

pile, the average load in the pile is obtained.

In the case of a constant unit shaft resistance, the average load is

equal to the load in the middle of the pileor middle of the telltale

length. In the case of a linearly increasing unit shaft resistance, the

average load is equal

to

the load

at a

level located somewhere between

the midpoint and the upper third point. Obviously, knowledge of the

distribution of the shaft resistance is essential for the evaluation of the

load distribution.

The mathematical formula is as follows:

(6.1)

where

Q = average load

A = cross sectional pile area

= elastic modulus

L= shortening (lengthening) of the pile

L

= pile length

Having several telltales in a pile results in several values of average

load andanimprovement of the representativeness of the load distribu

tion evaluated from the measurements. Having two telltales results in

23


27/47

three ave rage value s of load; the third one being obtained from the

difference in compression measured over the distance between the

two telltale ends connected to the pile. Correspondingly, having three

telltales results in six load va lues , etc. There is a practical limit, b ec ause

from primarily practical co nsid era tion s of accura cy, it is not w orthw hile

to have telltale lengths and distances shorter than about 5 to 8 metre

(15 to 25 feet).

When using telltales, the accuracy of the compression measure

ments must be better than the accuracy usually accepted for move

ment measurements.

The nominal precision of measurements of movement using dial

gages is usually only 0.025 mm (0.001 inch). The actual accuracy of

the values is, of cour se, smaller than the precision. At best w hen using

mecha nical g ag es , the error is about 0.1 mm (0. 0 05 inch) or larger. On

special occasions, dial gages with a ten times finer reading precision

are used, the ten times finer gages will have a smaller error, but not a

ten times smaller.

It is necessary to have dial gages with stems that are long enough

to allow the telltale records to be taken during the entire test without

having to reset the gages or to shim them, because otherwise errors

are introduced which will destroy the value of the records.

A telltale rod must not be subje cted to force s alon g its length or be

let to snake and m ove about. Therefore, it is usually installed in a sle ev e

or a guide pipe. To minimize friction, the outside of the rod is well

greased and/or the annulus between the rod and the sleeve is filled

with lubricating oil. The exception being telltales which are made up

of very heavy duty pipes capable of standing free inside a pipe pile

and where low accuracy is accepted.

Theoretically, it would s ee m as if it d oes not matter if on e refe ren ces

the upper end of the telltale to the m easuring beam , in which ca se on e

m easures m ovemen t, or to the pile head, in which ca se one m easures

shortening. By simply subtracting the telltale measurement from the

pile head mo vem ent, on e o btains the other value. How ever, in practice,

one should always measure the shortening directly, that is, reference

the telltale to the pile head , beca us e shortening data u sed to d etermine

strain and stress, require an order of magnitude or better accuracy

than mo vem ent data do. And any reading, beitfrom the pile-head g ag e

or the telltale gage, is obtained with some inaccuracy. Having to take

the difference between two readings to get the shortening value, in

creases the inaccuracy in the shortening value as opposed to measur

ing it directly. Therefore, shortening, requiring the higher level of accu

racy, should be measured directly and telltale dial gages should be

installed to measure between the telltale upper end and the pile head.

An additional reason is that a tilting of the pile head will result in

greater error for a telltale measuring movement (reading against the

2 4


28/47

reference beam) as opposed to the te l l ta le reading compress ion di

rectly (referenced to the pile head).

Apart from the o bviou s tha t results of an analysis of tell tale m eas ure

ments depend foremos t on the accuracy of the measurements , the

analysis introduces the modulus of the pile material and the results

depend also on how accurate ly the modulus is known. Steel has a

constant modulus and steel piles are suitable for tell tale instrumenta

tion. In co ntras t, the m odu lus of co nc rete is not co ns tan t over th e s tr es s

range considered in a s tatic loading test. Therefore, tell tale measure

ments in concrete piles and concreted pipe piles are difficult to ana

lyze. As m en tion ed , strain eva luate d from telltale da ta is ob tain ed from

rea din gs of two telltales , w he rea s s train is ob taine d directly from strain

ga ge s . For th es e re aso ns , apar t from w hen te l l ta les are placed a t the

toe of a pile, tell tales in con cre te piles should not be u sed as th e

primary gage for determining load.

For evaluating load in th e teste d pile, th e accu racy of the m eas ure

ments must be very high. This means that the mechanical type dial

gages, even those with high precision gradation, are not suitable. Lin

ear voltag e dis pla cem en t tra ns du ce rs , LVDTs, are preferred.

In fact, w hen planning a s tatic loading tes t and con siderin g th e

inclusion of tell tales , it is recom m en de d tha t the telltales be limited to

one to the toe and one back-up placed, say 5 metre (15 feet) above

the toe. To obtain data which are useful for a detail analysis of load

distr ibution, the rest of the in strum entation for me asurin g strain should

be electrical s train gages.

The primary purpose of telltale instrumentation of a test pile is to

determine movement and in par t icular the movement of the pi le toe .

For any pile, where the elastic shortening or lengthening of the pile is

difficult to calculate from pile material data and geometry with suffi

cient accuracy when determining the movement of the pile toe, a tell

tale to the pile toe should be installed. This means that most tests on

piles of embedment length exceeding about 15 m (50 ft) will benefit

from having a telltale installed to the to e of th e pile.Asingle toe telltale

is easily installed and its co sts a re insignificant in relation to th e ov erall

costs of the test, as well as to the benefits derived from the mea

surement .

6.3 Strain Ga ge Instrumentation

To determine load at a point in a pile with some accuracy, necessi

ta tes s tra in gages . Stra in gages can be e lectr ical res is tance gages or

vibrating wire gages. In very cursory principle, the s train gage reacts

by chan ging i ts res is tan ce or f requency in respo ns e to a shor tenin g or

lengthening and the response is picked up by a read-out ins trument

and calibrated to s train. As mentioned earlier , the s train can be trans-

2 5


29/47

ferred to s tress and/or to load. Dunnicliff (1988) presents a number of

aspects re la ted to us ing s tra in gage ins trumented pi les .

As opposed to telltale data, load values obtained by means of s train

g ag es are not av erag e lo ads over a length of a pile, but th e load acting

at the location of the gages. Furthermore, the s train gage will provide

strain data which are an order of magnitude or better than obtainable

with the best tell tale system. However, the accuracy of a s train-gage

dete rm ined load value still relies on the accu racy of the elastic m odu lus

of the pile material which is used with the strain data. In other words,

the s train gage uses the pile as a part of the load determination.

Most s train-gages have a tendency to drif t and, therefore, s train

gages may not be very accurate for measurements s tre tching over a

longer period of time.

It is not possible to have a telltale in a pile before it is installed in

the grou nd . Therefore, tell tales are zer oe d to the con dition s of s train

and load existing in th e pile at th e time of th e ins tallation of the telltale .

In contrast, some strain gages can be installed before driving a

prem anufactured pile and be theoret ical ly zero ed to the s tre ss and

strain con diti on s in th e pile im m ediately after th e driving or even befo re

it was driven. However, the driving stresses usually cause the zero

value to drif t , and normally a new reference reading under zero condi

tions m ust be taken before every static loading. For this reas on , the

load changes induced in a pile between its installation and a test are

normally lost and the data interpreted without consideration of such

effects . Chapter 8 discusses methods of overcoming these difficulties .

Stra in gage ins trumentat ion cos ts more than te l l ta le ins trumenta

tion, s train gages must be installed by well trained technicians, and

they are sens i t ive to mechanical damage and mois ture . While a toe

telltale should be incorporated in almost every static test , s train gages

belong to special projects with specific questions to address in the

test.

6.4 Load Cells

So m etim es , it is nec essa ry to very accurate ly determ ine th e load not

just at the pile head but also down the pile. In particular when long

term stability is desired, special load cells must be designed to fit

the pile that are insensitive to time effects , moisture changes, and

properties of the pile material. Such load cells are expensive and not

readily available, but they do exist. Details on them, however, lie out

side the scope of this publication.

7. DETERMINATION OF ELASTIC MO DULUS FOR USE WITH

STRAIN DATA

7.1 Ba sic Principles of Stre ss Stra in A na lysis

In a s tatic pile loading tes t wh ere th e pile is instrum ente d with s train

g ag es or tell tales , the g ag es serv e to de term ine the axial s train induc ed

2 6


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in the pile by the applied load. The strain data are used to evaluate the

load distribution in the pile according to Hooke's Law, that is, the

stress-strain relation expressed by Eq. 7.1

(7.1)

where

s t ress

E = m odu lus of pile m aterial

L = ch an ge of length (telltale length)

L = leng th (telltale length)

= strain

Fig. 7.1 shows a typical stress-strain diagram of data from an instru

mented loading test on a pi le with constant elast ic modulus. The l ine

with da ta points that is curved near the origin and be co m es l inear

tow ard higher strains, the upp er l ine, indicates m ea su re d data. The

line which is straight from the origin, the lower line, is the theoretical

elastic line for a column with equal properties to that of the pile. The

difference be tw ee n the lines is, of co ur se , du e to shaft re sist an ce act

ing on the pile in the loading test.

All shaft resis tanc e has bee n overc om e in the test , wh en the m ea

su red curve be co m es paral lel to the theore t ical . W hen evaluating

th e res ults from a loading test, finding this point is de sira ble , altho ug h,

in practice, its location is often difficult to determine.

However, by plott ing the tan ge nt m odu lus of the m ea su re d curve,

the point bec om es easi ly discernible. The tan ge nt m odulus is the slope

of the curve and it is plotted as the increment of load divided by the

increm ent of strain plotted aga inst the strain. The tan ge nt m odulus plot

of the stress-strain lines is shown in Fig. 7.1B. The tangent modulus,

or, more correctly termed, the chord modulus initially reduces with

increasing strain to become constant at a certain amount of strain.

This occurs when al l the shaft resistance has been overcome and the

constant value is equal to the pile modulus.

Often, the exact modulus of the test pile is not known. Then, the

tan ge nt m odulus plot be co m es a valuable aid in determ ining the modu

lus,

which then is used in the calculations to determine the distribution

of the load in the pile.

7.2 Actual Te st Re sults

Fig. 7.2 presents actual test results from a static loading test on a

steel pi le. The pile w as e quipp ed with two tel l tales, on e u ppe r and on e

to the toe of the pile. The figure s ho w s the app lied load at th e pile hea d

plotted a ga inst m ea sur ed strain (i.e., sho rten ing d ivided by telltale

2 7


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Fig. 7.1 Typical data from an instrumented static pile

loading test on a pile with a constant

modulus. A. Stress-strain diagram of the pile

head (upper curve) and of the corresponding

free standing column. B. Plot of tangent

modulus against strain. (After Fellenius,

1989).

length) for the upper and lower telltales and for the d ifference be tw een

the telltales, i.e., the strain along the bottom portion of the pile.

It is very difficult to obtain anything quantitative from the diagram

in Fig. 7.2A . H owever, w hen studying the diagram in Fig. 7.2B sho w ing

the tangent modulus plot, it can easily be determined that the

curve

for the upper telltale indicates that a constant modulus (a horizontal,

straight line portion) develops at a value of 0.3 millistrain, which oc-

2 8


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Fig. 7.2 Load-strain and modulus diagrams. A. Load-

strain diagram for tw o telltale lengths and for

the difference between the two telltale

lengths. B. Tangent modulus diagram for the

two telltale lengths and for the difference

between the two telltale lengths. (After

Fellenius, 1989)

curred wh en the applied load wa s 1,0 70 KN (1 2 0 tons). For the lower

telltale, a constant modulus is indicated for a strain of 0.8 millistrain

occurring when the applied load was 2,450KN(275 tons). Finally, the

curve for the telltale difference (bottom portion of the pile) indicates a

constant modulus at a strain of 0.5 millistrain at the applied load of

2 ,4 9 0 KN (2 80 tons).

The analysis of the tangent moduli for a range of applied load of

2 ,5 1 8 KN to 2 ,6 7 0 KN (2 8 3 to 3 0 0 tons) indicates a modulus for the

upper, lower, and bottom portion telltale lengths, of 2.776, 2.785, and

2.847 MN/strain (312, 313, and 320 ton/millistrain). The agreement

between the upper and lower telltale values is excellent. It is not sur

prising that the value for the lower portion is slightly off as any inaccu-

2 9


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racy in the te l l ta le readings would be exaggerated when taking the

difference of them.

Thu s, th e evaluation indic ates tha t the tan ge nt m odu lus of th e pile

cro ss section is equal to 2 .78 M N/strain (31 2 ton/m illis train). By in

serting this value into the conventional relation LOAD = AREA times

MODULUS tim es STRAIN with the cro ss section al area equal to 12 8.5

cm

2

(19.9 in

2

) , an ela stic m odu lus of 2 1 4 GPa (3 1 ,0 0 0 ksi) is ob

ta ined, which compares well with the usual ly assumed value of 210

GPa, when considering the accuracy of, in particular, the values of

cross sectional area of the pile and of the guidepipes.

The analysis becomes a little bit more difficult when evaluating

strain data from other than steel piles , i .e . , concrete piles or concrete-

filled pipe piles. Contrary to common

belief

a concre te column does

not exhibit a linear stress-strain relation when loaded. That is, the

Young's modulus of concrete reduces with the applied load. Fig. 7.3A

illustrates an assumed stress-strain curve of a concrete column (lower

line) having a s tres s dep en de nt mo dulus . It has bee n a ssu m ed that th e

line is a sec on d de gr ee curve and th at the final s lop e of th e line is 30 %

of the initial slop e. This redu ction of the s lop e, th at is, th e m od ulu s, is

extre m e, and ha s been cho sen for reas on s of instructional clarity. (An

example of an actual case will be given later).

The upper curve in Fig. 7.3A, the line with the data points , shows

the same column taken as a pile subjected to shaft resis tance. As in

the ca se of the pile with the co ns tan t m odu lus il lustrated in Fig. 7 .1 ,

as soon as all the shaft resis tance has been overcome, the two lines

are parallel. Due to the curving of the lines, it is very difficult to tell

when this occurs , however .

In Fig. 7.3B, the tangent modulus of the column line is plotted

against the strain (solid line). Because the stress-strain relation for the

column has been assumed to fol low a second degree equat ion, the

tangent modulus is a s traight l ine, and, as the modulus is not constant

but reducin g, the line s lo pe s dow nw ard with increasing strain. The line

with the d at a poin ts is the tang en t mo dulus line for the pile. It be

comes parallel with that of the column after the shaft resis tance has

been overcome. As shown, i t plots s lightly below the column line.

Extrapolating the pile modulus line to the y-axis and integrating it,

wou ld result in a resto red colum n cu rve located marginally below

the true column stress-strain curve in Fig. 7.3A.

7.3 Mathematical Relations

Mathematically, the lines and curves are expressed, as follows:

The equation for the tangent modulus line is :

(7.2)

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Fig. 7.3 Typical data from an instrumented static pile

loading test on a concrete pile with a

modulus reducing with increasing stre ss. A.

Stress-strain diagram of the pile head (upper

curve) and of the corresponding free standing

column. B. Plot of tangent modulus against

strain. (After Fellenius, 1989).

where

E

t

= the tangent modulus

= induced stress

= induced strain

A = slope of the tangent modulus line

B = Y-intercept (initial tangent modulus)

Integrating the tangent modulus line results in the following equation

3 1


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Fig. 7.4 Load-strain and m odulus

diagrams from a static

loading test on a

prestressed concrete

pile.

A. Load-strain

diagram for two telltale

lengths and for the

difference between the

tw o telltale len gths. B.

Tangent modulus

diagram for the two

telltale lengths and for

the difference between

the two lengths.

(After

Fellenius,

1989)

for the column stress-strain relation:

(7.3)

And the stress in the pile for an induced strain:

= E

s

(7.4)

where

E

s

= the secant modulus

and

(7.5)

7.4 Example from a Pile w ith a Non -Constant M odulus

The tangent modulus method of evaluation applied to pi les of non-

co ns tan t ela stic m odu lus is illustrated in Fig. 7.4 by th e resu lts from

a s tat ic loading tes t on a precas t pres t ressed concrete pi le equipped

with several telltales. Data from two telltales have been chosen for the

illustration: Telltale 7 at a depth in the pile of 3 8. 6 m (12 6. 64 feet) a nd

Tell tale 9 at a depth of 50.2 m (164.62 feet) , where the maximum

movements relat ive to the pi le head measured for the tel l tale points

w ere 11 .3 mm (0.44 inch) and 24 .9 m m (0.98 inch), respectively. Th ese

telltales were chosen for reasons of ensuring that all or most of the

shaft resistance over the tel l tale lengths had been overcome at the

maximum load, which is not the case for the lowest telltale lengths.

3 2


36/47

Fig. 7.4A shows the applied loads plotted against the measured

strains over the two telltale leng ths and over the difference be tw een

the two telltales. It is obvious that the lines are curved. An immediate

question wh en seein g such curves is: are they curved bec au se the

shaft resistance is not yet fully overcome, or because the modulus is

reducing with increasing load, or both?

The answ er to the question is given in Fig. 7.4B , show ing the ta ng ent

modulus plot of the data. The tangent modulus lines are becoming

straight at larger strains, which, indeed, indicates a second degree

curve for the stress-strain relation for where shaft resistance is not a

factor.

The tangent modulus lines are not only used to evaluate at what

applied load the shaft resistance along the pile was fully mobilized, but

also for determining the s ecan t m odulus of the pile material (need ed for

calculation of the load in the pile according to Eqs. 6.1 and 7.1).

Linear regression of the data points making up the straight portion

of the three lines may be used to provide the equation of the modulus

lines,

that is, to determ ine the con stan ts A and B in Eq. 7.2. Reg ression

of the modulus line for Telltale 7 results in that the constants A and B

are equal to2.14 KN/millistrain and 4.877 KN/millistrain, respec

tively (in English units: 0.240 ton/millistrain and 548.2 ton/millis-

train, respectively). The linear regression correlation coefficient is

0 . 9 9 8 0 .

Applying Eqs. 1 through 4 , results in the follow ing va lue s of initial

and final tangent moduli, and final secant modulus:

Initial E

t

= 37.8 Pa(5,480 ksi)

Final E

t

= 16 .2 GPa (2 ,3 4 0 ksi)

and Seca nt E

s

= 2 7 .0 GPa (3 ,9 0 0 ksi)

Inserting the values of A and B into Eq. 2 gives the average load in

the pile over the length of a telltale as a function of induced strain:

Q = - 1 . 0 7

2

+ 4880 (KN)

In English units the relation becomes:

Q = - 0 . 1 2

2

+ 5 4 8 (tons)

Naturally, the tan gent mo dulus m ethod is not restricted to the analy

sis of telltale data, but are as easily applicable to strain gage data. (In

fact, a considerable improvement of the accuracy of the load determi

nation is obtained by using strain gages directly in lieu of telltales).

One of the most immediately noticed benefits of the tangent modu

lus method is that inaccuracies in the data become readily apparent.

For example illustrating this aspect, see Fellenius (1989).

By means of the tangent modulus analysis, strain measurements

can be analyzed to determine accuracy, to establish at what applied

3 3


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load th e shaft r esis tan ce is fully mobilized, and to e valu ate w ha t value

of the secant modulus to use for determining the load distr ibution in

th e pile in th e data redu ction effort following the tan ge nt m odu lus

analys is . A data reductio n shou ld co nsid er factors s uch a s th e residual

strain in the pile, as well as variation between individual gages.

8 . INTERPRETATION A ND EVALUATION OF INSTRUMENTATION

DATA

8.1 Ba sic A na lysis of Telltale Data

When analyzing data from a telltale instrumented loading test, the

toe resis tance of the pile can be estimated from the values of average

load calculated according to Eq. 6.1 from the telltale measured short

ening of the pile. Building on the assumption of constant unit shaft

resistance acting along the full length of the pile (the telltale length),

the following relations can be derived (Fellenius 1980):

(6.1)

R

t

= 2 Q

a v e

- Q

h

(8.1)

R

s

= Q

h

- R

t

(8.2)

Where

Q

ave

= av era ge load in th e pile

A = cross sectional area of the pile

AL = m eas ure d c om pres sion of th e pile

L = pile or telltale leng th

R

t

= toe res is tance

Q

h

= load applied to the pile head

R

s

= shaft re sis ta nce

The data for analysis should be chosen from when the applied load,

Q

h

, is closest to the failure load obtained from an analysis of the pile

head load-movement data (Chapter 4).

Instead of assuming constant unit shaft resis tance, i t is assumed

tha t the unit shaft resis tan ce inc rea ses linearly ( triangular distribution),

the relation (Eq. 8.1) for the toe resis tance becomes:

Rt = 3 0

a v e

- 2 0

h

(8.3)

Where

R

t

= toe res is tan ce

Qave = average load in the pile

Q

h

= load applied to the pile head

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Fig. 8.1 Load-movement diagram for shaft and toe

resistances assuming that the unit shaft

resistance is either constant (rectangular)

or linearly increasing (triangular).

By inserting test data into the equations, the toe and shaft resist

ances can be placed in between the two extremes of unit shaft resis

tanceconstant and linearly increasing, respectively. Fig. 8.2 shows

a plot of the resulting ranges of resistance for the example given in

Fig. 8.1 .

8.2 Leonards and Lowell's Method of Analysis of Telltale Data

Leonards and Lovell (1978) presented an analysis method for de

termining the load distribution in a pile instrumented with one telltale,

where only the relative distribution of unit shaft resistance needs to

be known. Alternatively, the ranges of the relative distribution are

known and an upper and lower boundary type analysis is performed.

The shaft resistance does not need to be uniform, but can be of any

irregular distribution. The Leonards-Lovell method of analysis builds

on a few basic definitions as illustrated in Fig. 8.2 for a pile of a

length,

L,

subjected to a load

at

the pile head,

Q

h

.

The applied load has

mobilized a shaft resistance,R

s

and a toe resistance, R

t

The middle

35


39/47

Fig. 8.2 Basic concep ts of the Leonards-Lovell

method.

diagram shows the distribution of load in the pile, Q

z

, from the pile

head to the toe, and to the right is shown a diagram of the unit shaft

resistance, r

s

, acting along the pile.

If the pile was a free standing column, there would be no shaft

resistance and the toe resistance would be equal to the applied load.

In the test, the applied load ca us es a com pression of the pile, L,

which is measured by mea ns of a telltale to the pile toe . The com pres

sion can also be calculated by means of the following relation:

(8.4)

When the pile has no shaft resistance, that is, acts as a column, the

expression for the compression becomes:

(8.5)

Of course, the compression for a column can always be calculated,

wh ereas the compression for

a

pile is

a

function of an unknown amou nt

of shaft resistance. The comp ression is measured in the test, how ever,

and it is useful to define a ratio between the measured compression

of the pile and the compression of an equivalent column, as follows:

(8.6)

The compression of the pile obtained from shaft resistance only is

L

s

. It cannot be measured, but it can be calculated, as follows:

(8.7)

3 6


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The corresponding compress ion for a column subjected to a load

equal to the shaft resis tance cannot be measured, either, but i t also

can be calculated, as follows:

(8.8)

A ra tio betw een the two calcula ted shaf t-comp ress ion values is de

fined, as follows:

(8.9)

As shown by Leonards and Lovell (1978), the ratio is equal to the

relative distance from the top of the embedded portion of the pile to

the centroid of the area of the unit shaft resistance distribution (length

to centroid over em be dm en t length consid ered) . Thus , for a con s tan t

unit shaft resistance (rectangular distribution), C is equal to 0.5. For a

linearly increasing unit shaft resistance (triangular distribution), C is

equal to 0.67.

The ratio, C, is de term ine d from a know n relative distribution of sh aft

resi s tan ce ob tained from bore ho le data a nd oth er information. It is

not necessary to know actual values , only the general shape of the

distribution.

Also a third ratio is defined in th e L eonards-Lovell m eth od : th e ratio

between the toe res is tance, R

t

, and the applied load, Q

h

as follows:

(8.10)

Leonards and Lovell (1978) show that the alpha-ratio can also be a

function of C and C, as follow s:

(8.11)

The Leonards-Lovell method consists of determining the appropriate

C-ratio from the soil profile data and the C'-ratio from the calculated

column compress ion and the measured actual compress ion. Then, the

portion of the applied load that reaches the pile toe as toe resis tance

is obtained from Eq. 8.11.

8. 3 Exam ple of a Leonards and Loveil's A na lysis

A static loading tes t has bee n performed on a s teel pile and th e

applied load closest to the ultimate load is 230 ton. At this load, the

m eas ure d c om pres sion of th e pile w as 1.061 inch. The length of the

pile (tell-tale length, rather) is 90 feet and the cross sectional area is

3 7


41/47

14 in

2

. What values of shaft and toe resistances were mobilized in the

test?

Inserting the data in Eq. 8.5, a value of the column compression is

obtained, as follows:

Then, Eq. 8.6 gives the C -ratio:

Assume that the shaft resistance is constant along the pile, which

m ea ns that the C-ratio is equal to 0.5 . Then, Eq. 8.1 1 giv es the -ratio,

as follows:

and Eq. 8.10 gives the toe resistance:

R

t

= a Q

h

= 0 .73 4 2 3 0 = 169 tons

and the shaft resistance is:

R

s

= Q

h

- R

t

= 2 3 0 - 169 = 61 tons

A linearly increasing shaft resistance would have given a C-ratio

equal to 0.667, instead, and the a-ratio, as follows:

and Eq. 8.10 the toe resistance:

R

t

=

Q

h

= 0 .601 2 3 0 = 13 8 tons

and the shaft resistance is:

R

s

= Q

h

- R

t

= 2 3 0 - 138 = 92 tons

Thus,

depending on whether the unit shaft resistance is constant or

increases linearly, the mobilized toe resistance is 169 or 138 tons,

respectively.

The example data could just as well have been analyzed using the

simple relations expressed in Eq. 6.1 and Eqs. 8.1 through 8.3. How

ever, not if the distribution of shaft resistance had been in a soil for

which more complicated distributions had been valid.

Furthermore, the simple relations do not lend themselves toward

analyzing the data from several telltales in a pile, but the Leonards-

3 8


42/47

Lovell method does . For ins tance, to analyze the compress ion mea

sured between two points in a pile as obtained from taking the differ

ence between two te l l ta les measurements , the C-rat io for the about

trapezoidal distr ibution of shaft resis tance between the points is easily

estimated and the analysis rapidly performed.

Having more than two telltales in a pile, will provide a means for

matching load distr ibutions calculated from compressions over differ

ent lengths. Obviously, while the calculations are s imple, having more

than tw o telltales in a pile will then ne ce ssi tate carrying out the calcula

t ions by means of a computer . Matching the resul ts means t rying out

which C-ratio that will give the same load distribution along the pile

for all compression data over all telltale lengths (Lee and Fellenius,

1989) .

9. INFLUENCE OF RESIDUAL CO MP RESSION

9.1 Residual Co m pression in a Leonards-Lovell A na lysis

Residual com pres sion is com pres sion indu ced in th e pile from re-

consolidation of the soil around the pile after the installation, or com

pression induced in the pile due to negative skin fr iction occurring

before the com m en cem en t of the tes t . In mo st analyses , res idual com

pression is ignored by assuming that all tell tale readings show zero at

the s tar t of the tes t and only consider ing the compress ions imposed

and measured during the test. However, as this can introduce large

errors into the evaluation of load transfer, the analysis should be ex

tended to include residual compression or, at least, to investigate the

consequence of a potent ia l res idual compress ion.

The significance of the effect of residual compression can be dem

onstra ted by adding a small value to the measured shor tening of the

pile before p roce edin g with the analys is . For the exa m ple pr ese nte d

above, a residual compression of a mere 0.1 inch included in the

analysis, results in the following:

The ne w measu red com press ion bec om es 1 .061 + 0 . 10 0 =

1.161 and the ne w C'-value be com es:

The C-values , 0.500 and 0.667, are unchanged and, therefore , th

Statik Load

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