2039
Deep excavation in Irish glacial deposits
Excavation profonde des dépôts glaciaires Irlandais
Long M., O’Leary F.
Department of Civil, Structural and Environmental Engineering, University College Dublin (UCD), Ireland
Ryan M., Looby M.
Byrne Looby Partners, Dublin, Ireland
ABSTRACT: A good number of deep excavations have been recently completed in Irish glacial deposits. These have included
propped walls up to 12 m deep. Experience elsewhere in the world was used to design and construct these walls. However case
history data has shown that the behaviour of these walls is very rigid and much stiffer than comparable systems worldwide. It appears
this behaviour is due to the inherent natural strength and stiffness of the soil and the slow dissipation of excavation induced depressed
pore pressures or suctions. There appears to be scope for developing more efficient designs and in particular for greater use of
cantilever walls and less stiff systems with smaller piles and reducing propping requirements. For temporary works, the use of
undrained parameters in serviceability limit state calculations together with implementation of the observational approach on site
could be considered for future schemes.
RÉSUMÉ : Un bon nombre d’excavations profondes ont été récemment réalisées dans les dépôts glaciaires Irlandais. Elles ont inclus
des murs de soutènement jusqu'à 12 mètre de profondeur. La conception de ces murs a été réalisée grâce aux méthodes utilisées dans
les autres pays du monde. Toutefois, les analyses a posteriori ont montré que les caractéristiques de ces murs étaient extrêmement
rigides et bien plus rigide que d’autres systèmes comparables dans le monde. Il semble que ces caractéristiques sont dues à la
résistance naturelle et à la rigidité du sol ainsi qu’à la lente dissipation des pressions interstitielles déprimées ou aux succions
induites par la réalisation des excavations. Il semble y avoir des possibilités de développer des méthodes de conceptions plus réalistes
et moins conservatives permettant en particulier une utilisation accrue des murs cantilevers et des systèmes moins rigides avec de
petits pieux de fondation et en réduisant les besoins de butonnage ou d’ancrage. Pour les travaux temporaires, l'utilisation de
paramètres non drainés dans les calculs des états limite de service ainsi que la mise en œuvre de l'approche observationnelle sur le
chantier, pourraient être envisagées pour de futurs projets.
KEYWORDS: deep excavations; retaining walls; glacial depositis; ground movements
1 INTRODUCTION.
Over the previous 15 years, economic growth in Ireland has led
to an increase in the use of underground space, with some
development now including four underground levels. Valuable
full scale data has been obtained from a good number of these
projects. The purpose of this paper is to provide an overview of
aspects relating to the design, construction and behaviour of
retaining walls in Irish glacial deposits.
This paper will exclude projects located in the two main
cities of Dublin and Cork. Case histories from these two
locations have already been well documented (Long et al.,
2012a; Long et al., 2012b; Long et al., 2012c; Long et al.,
2013).
Specifically the paper will:
Briefly review the background geology and ground
conditions,
Outline the presently used design approach,
Review in detail the general behavior of walls in Irish
glacial deposits by reference to two case histories namely:
lightly supported “regular” wall for 6 m excavation in
Limerick,
6 m excavation in Middleton, Co. Cork where a
cantilever solution was used,
Present an overall summary of the behavior of retaining
walls in Irish glacial deposits compared to similar support
systems worldwide,
Provide some overall conclusions and recommendations for
the design and construction of future similar schemes.
2 BACKGROUND GEOLOGY
Broadly speaking the coastline of Ireland is formed of strong
older igneous and metaphorphic rocks such as sandstones,
granites and quartzite. The central basin of Ireland is formed of
Carboniferous limestone. However the engineering geology of
Ireland is dominated by the mantle of 10,000 to 15,000 year old
glacial tills and glacial sands and gravels which cover much of
the country. These deposits generally have a high gravel, cobble
and boulder content and are usually medium dense to dense or
stiff to very stiff in consistency. They form a thin layer (but
usually greater than 3 m in thickness) draped over and
conforming to the underlying bedrock topography. Later
alluvial activity has covered the glacial deposits with soft clays,
silts and peats in some low lying areas, in estuaries and along
river flood plains (Edwards and Warren, 1985).
3 THE SITES
The location of the study sites is shown on Figure 1 and some
details of the projects under consideration are given on Table 1.
The sites are located throughout Ireland but mostly in the most
populated areas to the east and south-east.
At each of the chosen sites inclinometers had been used to
measure the lateral pile retaining wall movements. Details given
on Table 1 include the depth of excavation (H), the support
details including the spacing of the structural piles and the
maximum measured pile deflection (
h
max).
