COSMUS: Real-time modelling and compensation of soil movements on underground sites

• Forschungsstelle: EU FRP
• Projektnummer: 96.0334
• Projekttitel Englisch: COSMUS: Real-time modelling and compensation of soil movements on underground sites

Schlüsselwörter (Englisch)

Fiber-optical sensors; optical demultiplexing; fiber Bragg gratings; piezoelectric actuation; optical demultiplexing; soil movement; grouting; finite element simulation

Alternative Projektnummern (Englisch)

EU project number: BRPR-CT96-0235

Forschungsprogramme (Englisch)

EU-programme: 4. Frame Research Programme - 2.1 Industrial and materials technologies

Kurzbeschreibung (Englisch)

See abstract

Partner und Internationale Organisationen (Englisch)

Soletanche (F), Tractebel DTR (B), Glötzl (D), CEA-LETI (F), Cambridge University (UK)

Abstract (Englisch)

A new method in civil engineering, the 'compensation grouting', has been investigated and developed to assess and control soil movements. Strains and inclinations of soil are measured by means of an optical sensor system based on the fiber inclinometers and fiber extensometers containg Fiber Bragg Gratings (FBG's). The output is an electrical signal transmitted to a peripheral device of a computer. FBG's reflect the light in a narrow frequency band. A strain applied to such a FBG fiber displaces this reflection band. This phenomenon is used to generate the optical output signal of the strain/inclination sensors. For the opto-electrical signal conversion of the read-out system the same phenomenon is applied in the converse direction. A reference FBG fiber is fixed into a stretching device allowing the application of a precise and reproducible strain. By scanning this reference over the maximal possible strain interval, the optical signals from the sensors can be tracked and followed. One reference is used for several sensors by means of a demultiplexing unit providing switching of optical channels. EPFL was engaged in the development of the fiber stretching device. The specifications given during the first project year were the following: Strains of up to 1 % should be achieved with a precision 1/1000 of the full range (or 10µe =10-5). In addition, a sinusoidal modulation of 10-4 should be superimposed. The required force for 1 % strain amounts to 8 N.
The original plans were to coat the fibers with a PZT thin film to actuate the fiber stretching. It was soon clear that the dc strain could not be achieved with a PZT film. Moreover, the precision of 10-5 was not achievable with PZT because of hysteresis phenomena. However, the ac modulation amplitude of 10-4 was in the range thin films. The major work was invested into the development of PZT thin films on glass fibers by reactive sputter deposition at 550 °C. Good results have been obtained in so far as:
1) Phase pure PZT was obtained,
2) Ferroelectric and piezoelectric properties were demonstrated.
The major problem was discovered when trying to stretch the PZT coated fibers: They all broke at strains of 0.4 % or even less. Most likely, the fibers degraded at the high temperatures in vacuum.
The project targets were thus changed in favor of the development of a bulk actuator with PZT ceramics with a built-in displacement sensor. It was found that the PZT bimorph disk is an ideal bending device for supplying the necessary excursion / force combination. A fiber holder was constructed to clamp the fiber on both ends of the FBG onto a passive frame on one end, and onto the PZT disk on the other end (overall dimensions: 8x8x2 cm). The device has a fundamental vibration at 600 Hz, and produces an excursion of 50 µm/150 V. This allows achieving the 1 % maximal strain with a clamping distance of 5 mm. A capacitive distance sensor was built into the device to sense the displacement. A read-out electronics was developed which converts the capacity change of 1.5 pF/µm to a linear voltage response of 1.5 µV/µe by means of a frequency-to-voltage conversion. The linearity is guaranteed by the capacitive measurement principle. The noise generated by the sensor electronics amounts to 40 mV at 1 Hz scanning frequency, equivalent to 27 µe and thus somewhat larger than the required precision of 10 µe. The required precision can be obtained at lower scanning speeds of 0.3 Hz.

Datenbankreferenzen (Englisch)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 96.0334