Upload
khairul-umam
View
267
Download
0
Embed Size (px)
Citation preview
8/9/2019 Statik Load
1/47
GUIDELINES
for the
INTERPRET TION ND
N LYSIS
of the
ST TIC LO DING TEST
Continuing Education Short Course Text
DEEP FOUND TIONS INSTITUTE
8/9/2019 Statik Load
2/47
8/9/2019 Statik Load
3/47
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
ii
8/9/2019 Statik Load
4/47
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.
1
8/9/2019 Statik Load
5/47
8/9/2019 Statik Load
6/47
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-
3
8/9/2019 Statik Load
7/47
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
4
8/9/2019 Statik Load
8/47
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
5
8/9/2019 Statik Load
9/47
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
6
8/9/2019 Statik Load
10/47
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.
7
8/9/2019 Statik Load
11/47
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
8
8/9/2019 Statik Load
12/47
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?
9
8/9/2019 Statik Load
13/47
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.
10
8/9/2019 Statik Load
14/47
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
11
8/9/2019 Statik Load
15/47
8/9/2019 Statik Load
16/47
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
13
8/9/2019 Statik Load
17/47
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
14
8/9/2019 Statik Load
18/47
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
8/9/2019 Statik Load
19/47
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.
16
8/9/2019 Statik Load
20/47
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
8/9/2019 Statik Load
21/47
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.
18
8/9/2019 Statik Load
22/47
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
19
8/9/2019 Statik Load
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
25/47
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
8/9/2019 Statik Load
26/47
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
30/47
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
8/9/2019 Statik Load
31/47
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
8/9/2019 Statik Load
32/47
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
8/9/2019 Statik Load
33/47
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)
3 0
8/9/2019 Statik Load
34/47
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
8/9/2019 Statik Load
35/47
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
37/47
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
3 4
8/9/2019 Statik Load
38/47
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
40/47
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
8/9/2019 Statik Load
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
8/9/2019 Statik Load
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