Monitoring of the structural behavior of the Cheminée du Nil in Angoulême, a testimonial to the town’s industrial past

A listed structure since 2012 and a symbol of Angoulême’s industrial history, the tall Cheminée du Nil smokestack has been leaning for several years, the changing nature of which needed to be monitored. The city council launched a comprehensive assessment, which concluded that there was a need to implement safety measures while studies and then consolidation and restoration work were carried out.

The consolidation and restoration work on the Cheminée du Nil in Angoulême was entrusted to a consortium comprising the specialized construction firm Freyssinet Sud-Ouest, the structural engineering office ATES and the architectural practice BDF Architectures.

Upon its arrival at the building site in December 2018, Freyssinet Sud-Ouest entrusted the implementation of a structural instrumentation system to OSMOS, for the purpose of studying the smokestack’s mechanical behavior.

A structure under multiple stresses

With this type of slender, unprotected structure, the effect of the wind is by no means negligible. New sources of strain will soon be added to this, engendered by the construction work and requiring a real-time analysis of its dynamic behavior.

OSMOS has installed four Optical Strands at the base of the stack, to continuously measure structural deformation, in real time. A temperature sensor was also put in place, to determine the impact of temperature fluctuations on the structure’s behavior. In order to differentiate between the effects of the work and those of the wind, the monitoring system was supplemented by an air flow meter and a weather vane.

This set-up performs dynamic analyses based on the signatures of the recorded signals. Deformations are measured to check whether each dynamic event has led to any permanent or irreversible deformations characteristic of damage.

Initial results favorable to launch of the work

Since the monitoring system was installed in December 2018, data analyses have shown that the structure remained stable and that the planned work could be initiated.

The set-up recorded strong storms and a small earthquake, during which the smokestack vibrated, but fortunately without worsening its condition, as indicated by a comparison of its mechanical signatures before and after the events. Overall, the dynamic data demonstrated the solid resistance of the structure, with a return to normal behavior after each event. Despite the extraordinary intensity of the storms and the quake, the most extreme deformation amplitudes showed that these phenomena did not generate any damage to the structure. The monitoring will remain in place during the work.

Client testimonial:

Renaud Montes

Freyssinet Sud-Ouest, responsible for the Cheminée du Nil consolidation and restoration work

“We are fully satisfied with the OSMOS service. The monitoring has been of precious assistance, giving us a good understanding of the structure and its behavior.

Thanks to the information we obtained, we were able to reassure our client about its stability, particularly during the development phase of the consolidation studies conducted by heritage architect Aurélien Dufour.

Bringing OSMOS in on the project allowed us to adopt a stance of durability and promotion of our heritage.”