1241
Deformation Performance and Stability Control of Multi-stage Embankments in
Ireland
Performance en déformation et contrôle de stabilité de remblais construits par étapes en Irlande
Buggy F.J.
Roughan & O’Donovan, Dublin, Ireland
ABSTRACT: The Limerick Tunnel project includes approximately 10 km of approach roads, most of which are constructed on
embankments up to 10m high within the River Shannon flood plain. The ground conditions consist of very soft organic silts up to 13m
deep. A combination of vertical drains and basal reinforcement plus 2 to 2.5m of surcharge was generally adopted as ground
improvement. The embankments were carefully built at controlled rates in multiple stages with continuous monitoring of performance
by means of piezometers, inclinometers, settlement plates and survey monuments. The paper describes the observational approach
used to control embankment stability primarily by means of monitoring filling rates, pore pressures and deformation ratio of lateral
toe displacement to vertical crest settlement. Performance data relating to a failure of a 3m high embankment during the first stage of
loading is included. Comparison of critical filling rates prior to minimum stability as indicated by maximum deformation ratio is
presented for the Limerick Tunnel project as well as other case histories of multi-stage embankments in Ireland and some critical
conclusions are drawn.
RÉSUMÉ : Le projet du Tunnel de Limerick comprend environ 10 km de voies d’accès, la plupart desquelles sont construites sur des
remblais jusqu'à 10 m de hauteur dans la plaine d’inondation de la Rivière Shannon. Les conditions de sols consistent en une vase
organique très molle qui va jusqu'à 13 m de profondeur. Une solution combinant des drains verticaux avec un renforcement de la base
du remblai, plus une surcharge de 2 à 2.5 m de hauteur a généralement été utilisée pour améliorer le sol. Les remblais étaient
soigneusement construits en plusieurs étapes avec une surveillance continuelle du comportement par des piézomètres, des
inclinomètres, des plaques de règlements et des repères topographiques. Le document décrit la méthode observationnelle utilisée pour
contrôler la stabilité du remblai, principalement par la surveillance des vitesses de remblaiement, des pressions interstitielles, et du
ratio de déformation latérale en pied de remblai rapporté au tassement en crête. Les données du comportement d’une rupture d’un
remblai de 3m de hauteur lors d’une première étape de chargement sont incluses. La comparaison des vitesses de remblaiement
critiques juste avant d’atteindre une stabilité minimum telle qu’indiquée par le ratio de déformation maximale est présentée pour le
Tunnel de Limerick, ainsi que pour d’autres études de cas de remblais construits par étapes en Irlande, et quelques conclusions
importantes sont données.
KEYWORDS: Ground Improvement; Surcharge; PVD; Multi-stage Embankments; Performance Monitoring; Observational Design.
1 INTRODUCTION.
1.1
Project Description
The Limerick Tunnel project is located to the south and west of
Limerick City in SW Ireland and provides a dual carriageway
bypass of the city via an immersed tube tunnel beneath the
River Shannon. The route passes through flat, low lying alluvial
flood plains of the River Shannon and its tributaries. Much of
the 10 km long roadway is on embankment to maintain the road
above potential flood levels and for crossings of existing creeks,
roads and railways. The flood plain of the River Shannon is
underlain by extensive deposits of very soft to soft alluvium
comprising mainly organic silt to depths typically from 3 to
13m deep.
The embankments were carefully built at controlled rates in
multiple stages with continuous monitoring of performance by
means of piezometers, inclinometers, settlement plates and
survey monuments. This paper describes the observational
approach used to control embankment stability primarily by
means of monitoring filling rates, pore pressures and
deformation ratio of lateral toe displacement to vertical crest
settlement. Principle methods adopted for earthworks along the
project include one of more of the following ground
improvement solutions:
Full or partial excavation and replacement;
Prefabricated Vertical Drainage (PVD);
Geosynthetic Basal Reinforcement;
Multi-Stage Construction Techniques; and
Surcharging.
A more detailed description of the design, construction and
performance of embankments along the project is contained in
Buggy & Curran (2011).
1.2
Site Characterization and Alluvium Properties
A brief summary of the engineering properties of the soft
alluvium follows but a much more extensive description is
given in Buggy & Peters (2007). The uppermost 1m,
approximately, of alluvial material is a firm to stiff desiccated
“crust” overlying very soft to soft, grey, silt with organic
material or lightly overconsolidated grey silt with abundant
organic material. The stratum occasionally contains bands of
more sandy material or shell fragments but is generally free of
distinct laminations and partings. Classification test data for the
alluvium are summarized as follows:
Natural moisture content - 40 to 120 % in organic silt and
150 to 300 % in peaty layers;
Liquid Limit - 40 to 150 % in organic silt and 150 to 300 %
in peaty layers;
Plasticity Index – 30 to 75 %;
Organic content (loss on ignition) typically 2 to 10 % but up
to 34% in peaty layers;
Undrained strength ratio c
u
/ p
o
’ varies 0.36 CAUC triaxial
tests; 0.3 DSS test; 0.2 CAUE triaxial tests; 0.3 average
assumed in design; and
Coefficient of consolidation C
v
= 0.5 to 4 m
2
/yr (lab tests)
and 0.8 to 1.5 m
2
/yr (field back calculated).
The alluvium sediments are underlain by deposits of
predominantly fine grained glacial tills with occasional coarse
grained layers and limestone bedrock.
