1695
1 INTRODUCTION
The construction of highway embankments often intersects with
the alignments of existing utility lines. Since additional
embankment loads are not accounted for in the initial design of
these buried structures, embankment construction may result in
overstressing and damage of the existing utility lines.
Geogrids are flexible, synthetic meshes that are used for slope
stabilization, highway pavement reinforcement, earth retention
and sub-grade improvement. One of the main purposes of
geogrids used in soil reinforcement is to provide confinement
and reinforcement to the soil medium. Love (1984) and Hass et
al. (1988) indicated that interlocking between geogrid and the
aggregate leads to an increased confinement within the granular
base course. Due to this enhancement in confinement, lateral
spreading of the particles is minimized and the stiffness and
strength moduli of the base course increase.
Geogrids improve the structural integrity of reinforced soil
foundations by confining the soil and distributing applied forces
which result in an increased load distribution angle.
Geogrids have been widely used to improve weak foundation
soils for the construction of access roads and highways. More
recently, they have been used in standard flexible pavement
sections to reinforce the base course to support vehicular traffic
during the life of the pavement structure (base-course
reinforcement). When used as base-course reinforcement,
geogrids provide significant structural benefits that are very
attractive to transportation authorities, which include an
improved pavement life and/or equivalent performance with a
reduced structural section.
Geogrid bridging is an effective construction technique that can
be used where bridging an area of very weak subgrade soils is
necessary. In this technique a layer or more of geogrid works as
a bridge that transfers overlaying stresses and distribute them to
larger areas away from the zone of weakness.
Current versions of Canadian Highway Bridge Design Code
(CHBDC 2006) and the AASHTO LRFD bridge design
specifications (AASHTO 2007) do not include any clauses
related to the performance of geogrids bridging. Only
recommendations regarding the culvert installations are related
to the ones installed using positive projection method (PPM).
Design recommendations given in these codes for PPM include
the assumptions of uniform earth pressure on top of the culvert
crown and the uniform pressure on the bottom slab of culvert
that is equal to the sum of the crown pressure and the pressure
due to dead load of culvert. In order to gain a better
understanding of the stress reduction that may be achieved by
geogrid bridging, Ontario Ministry of Transportation (MTO)
constructed an instrumented full scale test embankment over a
bridged trench in order to study various stress reduction
measures.
The main objectives of this study are to evaluate the stress
reductions achieved by the use of geogrid bridging installation
Field Performance of Geogrid Bridges for Stress Reduction on Buried Utilities
Performance in-situ des pontages en géogrille pour réduire les contraintes dans les infrastructures
souterraines
El Naggar H.
Assistant Professor, the University of New Brunswick, Fredericton, NB
Turan A.
Foundation Engineer, Ministry of Transportation, Ontario, Canada
ABSTRACT: The construction of highway embankments in urban areas often interferes with existing underground facilities such as
sewer lines and other buried conduits. In many instances, the extra loads imposed by embankment construction on buried conduits
would be unacceptably high. The severe consequences of overstressing an underground utility conduit include damage and
interruption of services for both the utility and highway. This paper present results of a full scale instrumented test embankment
constructed by Ontario Ministry of Transportation to study the effects of embankment construction on the underground utilities. The
test embankment comprised four sections which facilitated the evaluation of four different configurations including the positive
projection installation, induced trench installation and two at-grade geogrid reinforcing bridging with different spans. The numerical
models of the test embankment are developed using two dimensional finite element analyses. This paper presents the results of stress
measurements inside the trench protected using at-grade geogrid bridge arrangement as well as the results of numerical model that
helped clarify mechanisms of stress reduction. The material presented is considered to be of interest to researches and engineers.
RÉSUMÉ : La construction de remblais dans les zones urbaines existantes interfère souvent avec des installations souterraines comme
les égouts et autres canalisations enterrées. Dans de nombreux cas, les charges supplémentaires imposées par la construction du
remblai sur les conduites enterrées sont trop élevées. Les conséquences graves d’une surcharge sur une conduite souterraine des
services publics comprennent à la fois les dommages et l'interruption des services. Cet article présente les résultats d'un remblai
d’essai instrumenté à grande échelle, construit par le Ministère des Transports de l’Ontario pour étudier les effets des étapes de la
construction sur les réseaux souterrains. Le remblai d'essai comprend quatre sections, ce qui a facilité l'évaluation de quatre méthodes
différentes d’installation de conduites enterrées : projection positive, tranchée induite et deux autres avec pontage par géogrille de
renforcement avec des portées différentes. Par ailleurs, des modèles numériques par éléments finis en deux dimensions sont
développés pour simuler le comportement du remblai d’essai. Les mesures expérimentales et numériques obtenues dans cette étude
sont présentées et analysées. Les résultats obtenus sont d’un grand intérêt pour les ingénieurs praticiens.
KEYWORDS: Geogrid bridge, Test embankment, Underground utilities, Stress reduction, 2D FEM.
