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An evaluation of influence factors that affect pressures in backfilled trenches
Une évaluation de facteurs d'influence qui affectent les pressions dans des tranchées remblayées
Li L., Aubertin M., El Mkadmi N., Jahanbakhshzadeh A.
Polytechnique Montreal, Canada
ABSTRACT: Infrastructure rehabilitation and development drives part of the economy of many countries. This line of engineering
works includes many projects with conduits in trenches, which are commonly used to house service infrastructures such as cables,
sewers, and pipes. The design of these subsurface facilities requires a correct evaluation of the loads imposed by the fill material on
the buried conduits. The analysis of these stresses necessitates an assessment of the interaction between the backfill and abutment
walls. In practice, conduit design is usually based on an overly simplified arching solution proposed by Marston. A number of
influencing factors are not taken into account with this approach. In this paper, the authors use numerical simulations to investigate
the effect of key influencing factors on the vertical stress distribution in a backfilled trench. The results show how the vertical stresses
may change with the filling sequence, trench width, walls inclination, and backfill properties.
RÉSUMÉ : La réhabilitation et le développement des infrastructures conduisent une partie importante de l'économie de plusieurs
pays. Cette ligne des travaux d'ingénierie comprend beaucoup de projets avec des conduites dans tranchées qui sont couramment
utilisées pour loger des infrastructures de service tels que des câbles, des égouts et des tuyaux. La conception de ces aménagements
nécessite une évaluation correcte des charges imposées par le matériau en remblai sur les conduites enfouies. Dans la pratique, la
conception de ces conduites est souvent basée sur une solution sur-simplifiée proposée par Marston. Plusieurs facteurs d'influence ont
été négligés dans cette approche. Dans cet article, on utilise des modélisations numériques pour investiguer l'effet des facteurs clés sur
la distribution des contraintes verticales dans une tranchée remblayée. Les résultats montrent comment les contraintes verticales
peuvent changer en fonction de la séquence de remblayage, de la largeur d'une tranchée, de l'angle d'inclinaison des murs, et des
propriétés des remblais.
KEYWORDS: Trenches; Backfill; Stress distribution; Arching; Numerical modeling.
1 INTRODUCTION
The rehabilitation and development of municipal and industrial
infrastructures are driving the economy of many countries.
These works include projects with conduits in trenches, which
are commonly used to deliver gas, water and other services. The
design of these subsurface facilities requires the evaluation of
the loads imposed by the fill on the buried conduits. The
analysis of these stresses necessitates an assessment of the
interaction between the backfill and abutment walls. Due to the
stiffness contrast between the relatively soft fill material and
abutment walls, the former usually tends to settle more in the
trenches, while the latter holds the fill in place due to frictional
stresses along the interfaces. Part of the overburden weight is
then transferred to the stiffer walls. This load transfer, known as
"arching effect" (Janssen 1895), is a common phenomenon in
geotechnical engineering when a particulate material is placed
in silos and bins (Blight 2006), behind retaining walls (Goel and
Patra 2008), in mining stopes (Li et al. 2005; Li and Aubertin
2008, 2009a,b,c, 2010; Ting et al. 2011; Thompson et al. 2012),
and in relatively shallow and narrow trenches (Whidden 2009).
The design methods for conduits buried in trenches are often
based on a stress solution proposed by Marston (1930), who
made use of Janssen (1895) arching theory. The validity of
Marston’s solution is limited by several simplifications,
including the fact that it has been developed for vertical walls.
In practice, trenches with inclined walls are often used, in order
to reduce the risk of soil sliding. A common practice for
estimating the loads on buried conduits is then to use Marston’s
solution with the trench width at the top level (crown) of the
conduit (Handy and Spangler 2007). The load obtained from
this approach corresponds to that of a vertical opening with the
width at the crown of the conduit. This approach can however
lead to overestimate the magnitude of the stress transfer to the
walls, and to underestimate the pressure on the buried conduit.
The calculated load is then non-conservative (Handy and
Spangler 2007). Efforts have thus been devoted to modifying
the basic Marston’s solution (e.g., Li et al. 2012a,b). Other
limitations of this solution include neglecting the effect of
backfill cohesion, dilation, and stiffness, and also of the filling
sequence.
This paper presents results from a numerical investigation on
influence factors that may affect the vertical stress distribution
in trenches with inclined walls.
2 MODELING WITH FLAC
Analytical solutions are practical for engineers, but numerical
modeling constitutes a more powerful tool to handle complex
problems. In this investigation, the commercialized software
FLAC (Fast Lagrangian Analysis of Continua; Itasca 2002) was
used; this code is well adapted to help solve geotechnical
problems with sequential excavation and/or backfilling.
Figure 1 shows a trench having walls inclined at an angle
,
a width
L
b
at its base, filled to a thickness
H
b
. The backfill
obeys an elasto-plastic law with the Coulomb criterion. Its
response is controlled by the values of
E
b
(Young’s modulus),
b
(Poisson’s ratio), γ
b
(unit weight),
'
b
(internal friction angle),
c
' (cohesion), and
'
b
(dilation angle). The modeled trench is
filled to a height of 10 m. The walls inclination
is varied from
90° to 20° (with respect to the horizontal). The material
surrounding the backfill is homogeneous, isotropic, and linearly
