2019
Passive Pressure on Skewed Bridge Abutments
Pression passive sur des culées de pont asymétriques
Jessee S.
Terracon Consultants, Oklahoma City, Oklahoma, USA
Rollins K.
Brigham Young University, Provo, Utah, USA
ABSTRACT: The passive force-deflection relationship for abutment walls is important for bridges subjected to thermal expansion
and seismic forces. Although a number of tests have been performed to investigate these relationships for non-skewed abutments, no
tests have been performed for skewed abutments. To determine the influence of skew angle on the development of passive force, lab
tests were performed on a wall with skew angles of 0º, 15º, 30º, and 45º. The wall was 1.26 m wide and 0.61 m high and the backfill
consisted of dense compacted sand. As the skew angle increased, the passive force decreased substantially with a reduction of 50% at
a skew of 30º. An adjustment factor was developed to account for the reduced capacity as a function of skew angle. The horizontal
displacement necessary to develop the peak passive force was typically about 2.5 to 3.5% of the wall height, H, and the residual
passive force typically dropped by 40% at a deflection of 4 to 6% of H. For the no-skew case, the shape of the failure plane closely
resembled that predicted by the Rankine theory but was much shorter than that predicted by the log-spiral approach. Nevertheless, the
log-spiral method accurately predicted the measured force while the Rankine method grossly under predicted the force.
RÉSUMÉ: La relation force- déformation passive des murs en retour est importante pour les ponts soumis à la dilatation thermique et
des forces sismiques. Bien qu'un certain nombre de tests aient été réalisés afin d'étudier ces relations pour les murs non-biais, aucun
test n'a été effectué pour les murs biais. Pour déterminer l'influence de l'angle du biais sur le développement de la force passive, des
tests de laboratoire ont été effectués sur un mur ayant des angles de 0 º, 15 º, 30 º et 45 º. Le mur a une largeur de 1.26 m et 0.61 m de
hauteur, le remblai se compose de sable compacté. Lorsque l'angle du biais augmente, la force passive diminue considérablement avec
une réduction de 50% pour un biais de 30º. Un facteur d'ajustement a été mis au point pour tenir compte de la réduction de capacité
en fonction de l'angle du biais. Le déplacement horizontal nécessaire pour développer la force maximale passive est généralement
d'environ de 2.5 à 3.5% de la hauteur du mur H, et la force résiduelle passive chute généralement de 40% pour un biais de 4 à 6% de
H. Pour les cas non-biais, la forme du plan de rupture est proche de celle prévue par la théorie de Rankine, mais beaucoup plus courte
que celle prédite par la méthode de la spirale logarithmique. Néanmoins, la méthode de la spirale logarithmique prédit avec précision
la force mesurée alors que la méthode de Rankine sous-évalue largement la force.
KEYWORDS: Passive force, Passive Pressure, Skewed abutments, Earth pressure, Dense sand, Plane Strain, Log-Spiral.
1 INTRODUCTION
The passive force-deflection relationship for abutment walls is
important for bridges subjected to thermal expansion and
seismic forces. Although a number of tests have been performed
to investigate these relationships for non-skewed abutments
(Maroney 1995, Duncan and Mokwa 2001, Rollins and Cole
2006, Rollins and Sparks 2002, Lemnitzer et al 2009), no tests
have been performed to investigate these relationships for
skewed abutments. Performance of skewed bridges during the
2010 M8.8 Chilean earthquake suggests that this may be an
issue of concern as several such bridges were observed to have
rotated about a vertical axis, becoming unseated in their acute
corners (EERI, 2010).
While current design codes (AASHTO 2011) consider that
the ultimate passive force will be the same for a skewed
abutment as for a non-skewed abutment, numerical analyses
performed by Shamsabadi et al. (2006) indicate that the passive
force will decrease substantially as the skew angle increases.
Reduced passive force on skewed abutments would be
particularly important for bridges subject to seismic forces or
integral abutments subject to thermal expansion. To better
determine the influence of skew angle on the development of
passive force, a series of large size laboratory tests were
performed on a wall that was 1.26 m (4.1 ft) wide and 0.61 m (2
ft) high. A dense sand was compacted behind the wall to
simulate a bridge approach fill. Passive force-deflection curves
were measured for skew angles of 0º, 15º, 30º, and 45º. Vertical
columns of red soil were embedded into the backfill sand so that
the failure surface could be located at the completion of the
testing. This paper describes the test program, the test results,
and the implications for design practice based on analysis of the
test results.
1 BACKGROUND
The distribution of forces at the interface between a skewed
bridge and the adjacent backfill soil is illustrated in Fig. 1 as
originally outlined by Burke (1994). The longitudinal force
(P
L
) can be induced by thermal expansion or seismic forces.
For static or simplified pseudo-static analyses, the components
of the longitudinal force normal and transverse to the abutment
must be resisted by the passive force (P
p
) normal to the
abutment backwall and the shear resistance (P
R
) on the
backwall. Summing forces normal to the abutment produces the
equation
P
p
= P
L
cos
(1)
where
is the skew angle of the backwall.
