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Comparison of monitoring techniques for measuring deformations in an excavation
Comparaison de techniques d’auscultation pour la mesure de déformations dans une excavation
De Vos L., Van Alboom G., Haelterman K.
Geotechnics Division, Flemish Government, Ghent, Belgium
Maekelberg W.
TUC RAIL, Brussels, Belgium
ABSTRACT: Active monitoring is often suggested as a method to decrease the required safety coefficients in the design stage of a
construction. In order to apply active monitoring precise, reliable and interpretable measurements of the actual behaviour of the
structure and soil-structure interaction are required. To obtain this data, accurate and robust monitoring tools should be available at an
acceptable cost. An online monitoring test set-up was realized in a railway-infrastructure project site in Anderlecht (Belgium). The
braced excavation consists of a nailed jet grout wall with HEB profiles, installed immediately next to a railway track. Both advanced
and traditional monitoring equipment is installed to measure the deformation of the jet grout wall, deformations behind the jet grout
wall (on the railway tracks) and forces in the nails. The present paper focuses on the results of the measurements in and behind the jet
grout wall and on the comparison between the different techniques.
RÉSUMÉ : L’auscultation active est souvent suggérée comme une méthode permettant de réduire le coefficient de sécurité du
dimensionnement d’un ouvrage. Afin d’appliquer une auscultation active, des mesures précises, fiables et interprétables du
comportement réel des ouvrages et de l’interaction sol-structure sont requises. Afin d’obtenir ces données, des outils d’auscultation
précis et robustes doivent être disponibles à un coût acceptable. Un essai de surveillance en ligne a été réalisé sur le site d’un projet
d’infrastructure ferroviaire à Anderlecht (Belgique). L’excavation consiste en un mur d’étanchéité cloué, avec des profils HEB,
installé à proximité immédiate de la voie ferrée. Des technologies aussi bien avancées que traditionnelles ont été utilisées pour
mesurer les déformations du mur d’étanchéité, les déformations derrière le mur (sur les voies ferrées) et les forces dans les clous. Le
présent article vise à comparer les résultats des mesures dans et derrière le mur et à comparer les différents techniques de mesure.
KEYWORDS: active monitoring, advanced monitoring techniques, monitoring test site.
MOTS-CLÉS: auscultation active, outils d’auscultation avancés, site d’essai d’auscultation
1 INTRODUCTION.
The Geotechnics Division of the Flemish Government (GEO)
realised an online monitoring test set-up to extend the
experience with new monitoring techniques and to make a step
forward in the application of active monitoring on construction
sites. The project was partially funded by the Agency for
Innovation by Science and Technology (IWT), allowing 3 firms
to develop and perform online monitoring for an excavation.
Verification measurements were made by several parties, using
both more traditional as well as new monitoring techniques.
1.1
Main objective
The main goal of the project is to evaluate different monitoring
results and suitability of proposed monitoring schemes for
application in interactive design. This implies that accuracy,
installation possibilities, reliability and cost are important
aspects to be considered.
The monitoring scheme consists of measuring and logging:
• deformation of a vertical wall (x,y,z)
• maximum bending moment in a vertical wall
• deformation of the soil (z) behind a vertical wall
• anchorage forces in nails
1.2
Site description and applied equipment
The monitoring site is located in Belgium, Anderlecht
(Brussels), where an extra railway track will be constructed
alongside the existing tracks. For the foundation of the new
bridge, a nailed jet grout wall was installed next to the existing
railway. By doing so, the soil could be excavated vertically and
the foundation could be realised in an open construction pit. The
excavation depth is 12.5m starting from the railways. The jet
grout wall starts 4m below the railway level and has a total
length of 21m. HEB profiles with a length of 21m are inserted
in the jet grout wall. Five rows of nails are installed over the
excavated depth. Figure 1 shows pictures before excavation and
after excavation. The excavation is executed in different phases.
Each time 2m is excavated and consecutively, a row of nails is
installed. After installation of the nails, the contractor waits at
least 2 weeks before excavating the consecutive part. More
information on the site can be found in Van Alboom et al. 2012
and Verstraelen et al. 2013.
Figure 1. Picture of initial situation (left) and picture of the jet grout
wall after excavation (right).
1.2.1
Deformation of and bending moments in the jet grout
wall
To measure the deformation of the jet grout wall, both advanced
and traditional monitoring equipment is installed:
• Fiber Bragg Grating (fiber optics, FBGS)
• SAAF (In-place inclinometer, Inventec)
