Assessment of flexible airfield pavements using Heavy Weight Deflectometers

371 Pages·2017·8.59 MB·English

by  Michaël Broutin

Assessment of flexible airfield pavements using Heavy
Weight Deflectometers. Development of a FEM
dynamical time-domain analysis for the backcalculation
of structural properties.
Michaël Broutin
To cite this version:
Michaël Broutin. Assessment of flexible airfield pavements using Heavy Weight Deflectometers. De-
velopment of a FEM dynamical time-domain analysis for the backcalculation of structural proper-
ties.. Mechanics of materials [physics.class-ph]. Ecole des Ponts ParisTech, 2010. English. ￿NNT:
2010ENPC1008￿. ￿pastel-00565757￿
HAL Id: pastel-00565757
https://pastel.archives-ouvertes.fr/pastel-00565757
Submitted on 14 Feb 2011
HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est
archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents
entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non,
lished or not. The documents may come from émanant des établissements d’enseignement et de
teaching and research institutions in France or recherche français ou étrangers, des laboratoires
abroad, or from public or private research centers. publics ou privés.
A Dissertation

by

Michaël BROUTIN

Submitted to the Ecole Nationale des Ponts et Chaussées
in partial fulfillment of the requirements for the Degree of

DOCTOR OF PHILOSOPHY

Major subject: Structures and Materials

Assessment of flexible airfield pavements using
Heavy Weight Deflectometers

Development of a FEM dynamical time-domain analysis for the
backcalculation of structural properties

Defended on June 11th, 2010 at the LCPC Paris
before a jury composed of:

Committee members: H. DI BENEDETTO (reviewer)

F. PFEIFFER (reviewer)
P. ARGOUL
D.BRILL
P. LERAT
J.M. PIAU
J.F. SEMBLAT

Supervisor : M. FREMOND
2
3

THESE

présentée pour l’obtention du diplôme de
Docteur de l’Ecole Nationale des Ponts et Chaussées

Spécialité : structures et matériaux

par

Michaël BROUTIN

Evaluation des chaussées souples aéroportuaires à
l’aide du déflectomètre à masse tombante (HWD)

Développement d’une méthode d’analyse dynamique temporelle par
éléments finis pour le calcul inverse des propriétés structurelles

Soutenue le 11 juin 2010 au LCPC Paris
devant un jury composé de :

Rapporteurs : H. DI BENEDETTO
F. PFEIFFER

Examinateurs : P. ARGOUL

D.BRILL
P. LERAT

J.M. PIAU
J.F. SEMBLAT

Directeur de thèse : M. FREMOND
4
5

Acknowledgements

First, I would like to express my gratitude to Prof. Michel Frémond for having accepted to be
my supervisor, and to the STAC management for having authorized this work.

I also would like to extend my deepest thanks to Prof. Hervé Di Benedetto, Prof. Friedrich
Pfeiffer, Dr. Pierre Argoul, Dr. David Brill, Dr. Patrick Lerat, Dr. Jean-Michel Piau and Prof.
Jean-François Semblat for their participation in this dissertation committee. I especially want
to thank again Prof. Hervé Di Benedetto and Prof. Friedrich Pfeiffer for having accepted to
review the document.

Then I would like to thank all the people who have helped and inspired me during this
doctoral study. I cannot enumerate all of them here. Amongst these people, I would like to
warmly thank my friends and colleagues Cécile Caron, Jean-Claude Deffieux, Vincent
Souque, Régis Bost (STAC), Dominique Guédon (LRPCT), Jean-Michel Simonin, Jean-
Michel Piau and Jean-Maurice Balay (LCPC):

Thank you Cécile for our numerous collaborations, your infectious enthusiasm and unfailing
good-humour for which you only know the secret. Thank you for having supported and
encouraged me in crucial moments. And finally thank you for having worked on the English
language of the memoir.

Thank you Jean-Claude for your everyday help, your outspokenness, and for the original
experiments we launched together, and for everything you have taught to me since my arrival
at the STAC, in all scientific domains.

You both will leave the STAC by the end of the year. I will miss our scientific lively debates,
as well as all the numerous good moments spent together.

Thank you Vince for your valuable operational help, for never having hesitated to wake up at
4 a.m. each time I had the good idea to launch experiments.

Thank you Regis for having been present each time I encountered difficulties, whatever it
dealt with (the programming, the memoir making up, …). This work would not have been
finished on time without your valuable help. Especially thanks for having stood me at my
worst moments and for having succeeded in motivating me each time it was necessary,
especially in the last weeks.

Thank you Domi for the good moments spent during the construction of the STAC’s test
facility, and for everything you did on the project. Unfortunately, even though initially
planned for that purpose, the instrumentation has not been tested in the frame of the thesis due
to cumulative delays induced by construction unconformities. However, this test facility will
be the main tool for further improvements of the method developed.
6
Thank you Jean-Michel (Simonin) for your advices, especially at thesis early times, and for
the overview about pavement testing you provided me.

Thank you Jean-Michel (Piau) for the scientific discussions we have had, your valuable help
about several points, and for the time you dedicated to me in the last weeks, to debate about
the sticking points of the work.

Last but not least, thank you Jean for everything you did for me during the whole thesis
period, for having lavishly found time to help me, whatever the hour of the day or even the
day of the week, for the numerous receptions you priviledged me in Nantes. Thank you for all
fundamental questions you raised (some of them having resulted in perpetual debates still
unsolved…), for having allowed me taking some stand back when necessary, and having
clarified this work with your valuable advices.

Finally I would like not to forget the colleagues which did not directly work with me, but
whose friendship and good-humour indirectly contribute to the work accomplishment. Once
more I cannot quote everyone. Thank you to you Anne-Marie, France, Aude, Alain, Jean-
Yves without forgetting the old STACiens Cédric, Arnaud and Robert.
7

Executive summary

The Heavy Weight Deflectometer (HWD) is viewed worldwide as the most appropriate
device to assess the bearing capacity of airport pavements. Its principle consists in applying a
transient impulsive load simulating the weight effect of an aircraft rolling wheel, onto a
stationary load plate placed over the pavement, through a buffer system, and studying the
vertical displacements of the pavement surface (deflections) induced by this dynamic loading.
The latter are continuously measured during the test by means of geophones placed under the
plate and at predetermined locations at the pavement surface. These deflection measurements
are used to determine the structural properties of the pavement layers (and optionally other
parameters like layer thicknesses but these studies are marginal), by means of a so-called
“backcalculation” numerical procedure which consists in 1- choosing a mechanical model for
the pavement and 2-identifying the parameters of the model for which theoretical computed
deflections fit the experimental data set. Then, forward calculations can be performed to
estimate the bearing capacity or the remaining life of the studied structure.

This thesis deals with the assessment of flexible pavement. Geophones are in this case placed
beneath the load plate and radially.

Usual processing methods are based on static multilayered elastic models. The only structural
properties to be backcalculated are the stiffnesses (Young’s moduli) of the different layers.
The backcalculations are performed from the pseudo-static deflection bowls. These bowls are
reconstituted from the deflection peak values measured by each geophone. As emphasized by
several authors, these methods have shown limitations. Actually they use only a few part of
available information (peak values), and the static modelling is far from the reality of the test.

The objective of the thesis is to develop an advanced method for the assessment of flexible
pavements using HWD tests data. The aim is to achieve a better representation of the
observed physical phenomena during dynamic loading and to consider whole available
information. Computation of time-related deflections is proposed, relying on a dynamical
modelling of applied dynamic loading, and ensuing pavement response.

The main body of the document presents three parts.

The first part describes preliminary studies.

First, the reliability of HWD data for dynamical analysis has been studied, based on the
adjunction of external instrumentation to the HWD sensor system, and on crossed tests with
other HWD on known test facilities. Then, an experimental repeatability study has been
performed. Finally, good linearity of the deformations with applied loading has been
demonstrated using experiments conducted on several pavements. Moreover, the occurrence
of damping phenomena in the pavement has been emphasized according to gauge signals
measured in an instrumented tested pavement. The crossed comparison between theory and
practice has also allowed defining a precise experimental protocol for operational test
campaigns.
8
The second part focuses on modelling. The mechanical model is presented first, which allows
calculation of displacements and strains in the pavement for a given parameters data set.
Then, the developed backcalculation procedure is described, based on this mechanical model.
Finally a forward calculation procedure is proposed, which relies 1- on strain calculations
using information derived from the backcalculation phase, and 2- on material performances.

A time-domain modelling has been developed. It relies on a 2D axisymmetric finite element
dynamical model implemented in the Cesar-LCPC FEM software. According to the
preliminary studies, a multilayered isotropic elastic model with damping has been chosen.
Load plate modelling is included in the mesh. The model takes into account the applied
dynamical load. Three alternative versions of the model have been studied, which differ by
the applied dynamical load modelling. First model considers the time-related load imparted to
the pavement surface through the plate. In the second model, rubber buffers and the falling
mass are included in the mesh and velocity of the dropped mass at impact is imposed at their
top. Third alternative version is based on the shock theory. Behaviour laws take into account
inertia of materials and structural damping. In Cesar, the only available damping modelling is
so far a Rayleigh damping, globally defined at the structure scale. Thus parameters of the
model are layer thicknesses (including bedrock depth), material bulk densities and stiffnesses,
and structural damping, for the first two alternative models. The third one includes additional
parameters which describe the shock laws. The model allows a computation of the theoretical
induced time-related surface deflections, for a given set of parameters. A careful study has
been carried out to optimize the temporal and spatial discretizations according to the dynamic
problem considered.

The backcalculation problem amounts to find a set of parameters resulting in a satisfactory
fitting between theoretical and experimental deflection signals. Usual assumption that layers
are fully bonded under heavy dynamic load is drawn. Moreover, bulk density ranges are
generally quite precisely defined, and layer thicknesses are well-known thanks to ground
penetrating radar investigations. In the thesis framework, bedrock depth is independently
determined via an advanced existing method using the cut-off frequency of the pavement. The
assumption is made that precisions on densities, layer thicknesses and bedrock depth are
sufficient so that these parameters can be a priori set. Hence the only parameters to be
backcalculated are the layer stiffnesses and the structural damping ratio. A thorough study has
been performed 1- to investigate the relative influence of the different parameters on the
deflection time-histories, 2- to discuss a posteriori the relevance of arbitrarily setting some of
them, and 3- to establish a criterion to assess if the resulting set of backcalculated parameters
is appropriate or not. This study phase relies on the choice of an objective function and a
linked target error value. A numerical parametric sensitivity analysis has been performed
combined with the results of the aforementioned repeatability tests. This study has allowed 1-
proving that the uncertainties on imposed parameters has no significant impact on the
backcalculated results, so that the a priori hypothesis made is reasonable, and 2- selecting
consistent error values with regard to the uncertainties on experimental data and imposed
parameters. According to this study, an automated convergence algorithm has been developed
for numerical resolution of the backcalculation procedure. This work is based on previous
studies led in the frame of several successive supervised training courses where several
algorithms have been tested. The retained algorithm consists in a Gauss Newton algorithm
including a regularization process. Letting n be the number of parameters to be
backcalculated, n+1 direct calculations are needed at each algorithm epoch. Its convergence is
highlighted on a simulated data set relative to a given pavement structure. It is also shown that
robustness of the algorithm is better for dynamic calculations than in the pseudo-static case
9
which has also been considered. Such a result can be explained by the high overdetermination
of the problem when the whole time-histories are considered instead of the peak values only.
Conversely, the disadvantage of the dynamic method is that each FEM forward calculation is
time consuming, especially in the absence of specific method for calculating the gradients of
Gauss Newton algorithm.
A numerical tool has been developed which allows automating the finite elements mesh
creation and both backcalculation and forward calculation phases: the so-called
PREDIWARE (Pavement Rational Evaluation using Deflections Induced by Falling Weights,
for Airfield and Road Engineers) software.

A forward calculation phase is proposed as an illustration of the possible practical use of the
backcalculation results. It is performed using the same mechanical model as in the
backcalculation phase, taking into account the previously backcalculated parameters. It allows
determining the strains generated by a single wheel or a given aircraft landing gear at critical
levels in the structure. Thanks to the knowledge of the mechanical material performances
(laboratory tests being used to establish the allowable strains versus the number of coverages),
and climatic data for temperature corrections, it is then possible to evaluate, amongst other
parameters, the conventional bearing capacity of the pavement (in terms of single wheel or
considering the whole landing gear assembly), or, for a given traffic mix, the remaining life of
the pavement, and if necessary the thickness of a flexible overlay.

The third and last part is dedicated to experimental validation of the proposed numerical
method. Several full-scale validations of both backcalculation method and strains
determination have been conducted. They consist in test surveys run on 3 well-known
pavements, one of them being instrumented. The validation relies on the comparison between
backcalculated and laboratory-determined material properties, and on the comparison between
theoretically expected strains and gauge measurements, when available.
The method to determine the bedrock depth has been tested on two test facilities, the first one
lying over a shallow bedrock (3 m), and the second one over a 12 m deep substratum
(theoretical infinite half-space). Results are satisfactory in both cases.
Dynamical backcalculations performed for all test surveys have shown that 1- Good fittings
can be found, corresponding to low values of the objective function, with regard to
repeatability and sensitivity studies conclusions. 2- However, errors obtained remain much
higher than those reached on simulated data. This means that the model is not completely
reliable for describing the physical test and can be improved.
Experiment performed on the instrumented pavement has shown a very satisfactory
correlation of calculated strains with gage measurements. Nevertheless only one gage profile
has been tested, so that this result needs to be confirmed by other experiments.
Comparison between backcalculated and laboratory-determined material properties has
emphasized that 1- the dynamical backcalculation provides better results than the pseudo-
static one for deep layer moduli determination. Backcalculated values in the dynamical
method are close to expected values, 2- numerical moduli found for surface asphalt layers are
significantly different from laboratory results, in both dynamic and pseudo-static methods,
and 3- damping modelling is not good approximation of the reality. Actually, the contrast
between unbound and asphalt materials imply a great difference between mean damping
ratios of the different layers, as confirmed in the study by laboratory tests. Moreover,
Rayleigh modelling is not representative of the real frequency-dependence of the asphalt
materials.