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Advanced testing and modelling delivers cost effective piled raft foundation solution
Essais avancés et modélisation délivre une solution économique empilés fondation sur radier
Bourne-Webb P.
Instituto Superior Técnico, Lisbon, Portugal
Cunningham M.
Shell Global Solutions Intl. BV, Den Haag, Netherlands
Card G.
GB Card & Partners, London, United Kingdom
ABSTRACT: A piled raft solution was proposed as an alternative to a conventional fully piled foundation for a new shopping
development in Cambridge, UK. This paper demonstrates how the use of precedent knowledge, appropriately targeted investigation
and modelling can provide cost effective and resource efficient foundation solutions.
RÉSUMÉ : Une solution radeau empilé a été proposé comme une alternative à une base conventionnelle entièrement empilés pour un
développement nouveau point de vente à Cambridge, Royaume-Uni. Cet article montre comment l'utilisation des connaissances
préalables, l'enquête ciblée et modélisation peut fournir rentables et efficaces des ressources des solutions de fondation.
KEYWORDS: Gault Clay, piled raft foundation, nonlinear stiffness, site characterisation.
1 INTRODUCTION.
1.1
Piled rafts and settlement reducing piles
There is a wide literature on the various methods of analysis and
use of raft foundations generally (Cooke 1986, Price & Wardle
1986, Poulos 2001, Reul & Randolph 2003) and the use of piles
as settlement reducers (Burland 1995, Love 2003).
In essence, where the soil underlying a structure is
sufficiently stiff it is often the case that the use of a plain or
piled raft solution will lead to economies when compared to the
costs associated with a fully piled foundation system.
Effectively, load from the superstructure is first distributed
through a plain raft to the subsoil and if the analysis predicts
settlement in excess of that deemed to be acceptable, settlement
reducing piles can be introduced at strategic points in order to
stiffen the support to the raft and bring the expected settlements
down to an acceptable level.
However, this solution is not often examined due to the
sophisticated nature of the soil-structure interaction that needs
to be analysed – the work involved in delivering the solution is
too ‘complicated’ and the structural engineer prefers the ease,
risk transfer and robustness of a fully piled option.
1.2
Gault Clay and geological setting
The Gault Clay is an over-consolidated clay that was laid down
towards the end of the Lower Cretaceous period (c. 100 Ma),
and its engineering behaviour lies between that of a soil and
weak rock (Marsh & Greenwood 1995). In Cambridge, UK the
Upper Gault sub-division predominates
In the Cambridge area, the strata over-lying the Gault, most
significantly the Chalk of the Upper Cretaceous, have been
largely removed by erosion, and the area was also subjected to a
number of glaciations.
The removal of an estimated 150 m of overburden (Samuels
1975), in addition to the ice cover, has subjected the clay to
significant stress relief with associated intense fissuring and
softening and in addition, the stratum has experienced moderate
levels of tectonic activity.
In engineering terms, the Upper Gault is a high plasticity
clay (LL ~ 70 - 80%; PI ~ 45 - 55%) with moisture contents
close to its Plastic Limit and a significant calcite content (30 –
40%).
At the site, the soil profile (Table 1), especially the
distribution of the superficial soils and the upper surface of the
Gault Clay, has been modified by historic construction that had
reduced the original ground level.
Groundwater was found to be perched within the superficial
soils at a level between 0.5 m and 1 m above the surface of the
clay (noting that the surface of the clay undulated significantly),
and water levels in the Lower Greensand were found to be in
hydraulic continuity (Nash et al. 1996).
Table 1. Site specific soil profile
Top of
layer: m OD Thickness: m Soil description
0.3 to 3.3
Made Ground
0.3 to 1.7
Brickearth
+5.8 to +11
0.1 to 2.4
Sand & Gravel
0 to 3.6
Gault Clay (weathered)
+3.2 to +8.5
>34.7
Gault Clay
-30 approx.
Not proven
Lower Greensand
1.3
Development at adjacent sites, Grand Arcade and
foundation options
Located in central Cambridge (Fig. 1), the Grand Arcade site is
immediately adjacent to the Lion Yard shopping centre and the
Crown Plaza Hotel (Lings et al. 1991, Ng & Nash 1995). The
development covers a total area of about 1.4 hectares.
The excellent field and laboratory research work undertaken
in relation to the deep basement excavation at the latter site,
informed the decisions made during the development of the
ground model for this project.
In addition to these relatively modern buildings, other
structures of significance to the project were the Post Office
(PO) and Telephone Exchange (BT) building(s) in the northeast
corner of the site, and retained facades along the eastern
boundary, facing St. Andrews Street; all of which had to be
protected from damage during the works.
Comment des essais avancés, associés à la modélisati
, permettent d’obtenir une olution
éconoomique de fondation sur pieux et radier
