1863
The role of fibre optic instrumentation in the re-use of deep foundations
Rôle d'une instrumentation en fibre optique pour la réutilisation de fondations profondes
Bell A.
Cementation Skanska
Soga K., Ouyang Y., Yan J.
University of Cambridge
Wang F.
Tongji University
ABSTRACT: The re-use of existing foundations, in particular piled foundations has increased in recent years due to the significant
environmental and commercial benefits. However, there has been limited progress in assessing the condition of such piles by
considering the effect of initial use and the impact from the subsequent demolition process which often requires a detailed study. This
paper will provide details of a recent project in London that successfully reused all exiting piles beneath the site and optical fibre
sensors were instrumented to the existing foundations in order to monitor the behavior of piles during the demolition of the existing
building. The use of optical fibre instrumentation is believed to be the first of such an approach in observing the behavior of reuse
piles during demolition in the UK and as urban environments become more congested particularly below ground, the approach
discussed in this paper will become increasingly valuable. The monitoring data is presented and discussed in detail and the role of
using these sets of data in assessing the reuse strategy is also highlighted in this paper.
RÉSUMÉ : La réutilisation de fondations existantes, en particulier de fondations sur pieux, a augmenté ces dernières années en raison
des avantages environnementaux et commerciaux significatifs. Cependant
, l’état des pieux suite à leur première utilisation et le
processus de démolition sont souvent négligés. Ce document présente un projet récent à Londres où les pieux ont été instrumentés
avec des capteurs en fibre optique, avant la compagne de démolition. Les fibres optiques permettent de mesurer des déplacements le
long des pieux lors de la démolition. L'utilisation d’une telle instrumentation
est une première au Royaume-Uni. Les données de la
compagne de surveillance sont présentées et discutées en détail. Une stratégie de réutilisation des fondations sur pieux est également
proposée dans ce document.
KEYWORDS: Re-use foundations, Optical Fibre Sensors (OFS), Brillouin back-scattering
1 INTRODUCTION
Foundation re-use can generate significant environmental and
commercial benefits, and is becoming a popular engineering
option, particularly in congested urban environments. Due to the
many practical constraints, most redevelopments need to be
constructed on the existing foundations together with a new pile
system; therefore it is crucial to understand the geotechnical
behaviour of reused piles and their compatibility with the new
structure. This is often difficult without removal of significant
parts of the substructure. Previous researches (Leung et al.,
2011; Begaj-Qerimi&McNamara, 2010) have shown that pile
behavior may change with time, due to consolidation and
ageing, residual stress at the pile base and increased soil
stiffness; hence reused piles are often stiffer than new piles. On
the other hand, the building demolition process could
potentially introduce ground heaving and the physical unloading
of the reused piles can also generate tension cracks. These
differences in pile responses need to be properly assessed in the
design of a new pile system.
A recent project in London provided the opportunity to
further develop the understanding of foundation reuse by
installing fibre optic sensors in both existing piles and a
borehole to observe the impact of the demolition process on the
changes in piles behavior and ground response. The site is
located at 6 Bevis Marks and near to Liverpool Street, London,
UK, and it was proposed to reuse all existing foundation piles
and the majority of the basement substructure on this project.
This approach produced significant commercial and
environmental benefits.
The existing piled foundations are large diameter under-ream
piles, and there was a concern that these piles would be
damaged during the demolition process. Such damage is usually
caused either by the forces generated by the removal of
significant load as the building is demolished, the tensile forces
within the piles and surrounding soil, or physical damage
caused by demolition of the substructure, pile caps and pile
breaking down. This can lead to the reuse being questioned and
ultimately being discounted.
This paper explains how optical fibre instrumentation was
used to monitor pile and ground response under demolition and
will present the data captured by the fibre optic instrumentation
during the demolition process. It will show how the use of such
sophisticated instrumentation was fundamental to the successful
reuse of the existing piles on this project.
2 SITE DESCRIPTION
2.1 Existing site and proposed redevelopment
The existing building at 6 Bevis Marks was constructed in the
early 1980’s
and comprised eight superstructure floors and two
basements. The existing foundations system includes (i) piles
located inside the basement, which is approximately 7.0m
below pavement level, and (ii) piles constructed in the Bevis
Marks pavement, which is approximately 3.5m below pavement
