Retaining wall monitoring using a custom correlation tool in VDV

One of the significant difficulties in this case turned out to be that the length of the strain gauges used significantly varied with the temperature of the steel bars to which they were bonded. The VDV correlation tool turned out to be very useful in this case.

01

About project

One of the many structures supervised by the Public Service of Wallonia is a reinforced concrete wall, composed of several sections separated from each other by joints that retain soil on one side and overhangs a railway track below. During an inspection of this retaining wall, it was observed that a part of it was leaning towards the adjacent track.

 

VDV’s extensive functionalities have enabled us to analyze the data sent by our sensors in the best possible way.

— Sébastien Flawinne, Structural Health Monitoring Project Engineer
02

The challenge

The wall is approximately 6 meters in height and several dozen meters in length and composed of several sections separated by joints. The wall retains soil on one side and overhangs a railway track below. During a visual inspection of the retaining wall, it was observed that a part of the wall was leaning towards the adjacent track.

A system consisting of two steel beams was implemented to transfer the earth’s pressure to a stable part of the wall.

Tilting wall and retaining device.

After a few months of measurements, the unstable wall abutted against the beams, and seasonally varying stresses began to build up in the steel bars. Since the start of the contact (May 2019), the forces have been closely monitored to ensure that the restraint solution is not at risk.

One of the major difficulties in this monitoring comes from the fact that the strain gauges, which measure the efforts, are essential.

However, the length of the strain gauges (which give direct information on the stresses) varies greatly with the temperature of the steel bars, to which they are bonded. It is, therefore, crucial to remove the effect of thermal expansion from the bars. To do this, it is necessary to know the relationship between the measured temperature and the elongation of the bars.

Strain gauges
Another view of the strain gauges
03

The solution

In order to secure the situation, a system consisting of two steel beams was implemented to transfer the earth’s pressure to a stable part of the wall. In the initial configuration, the distance between the tilting wall and the retaining beam is about 5mm. The wall can therefore tilt freely by 5mm before coming to rest against the restraint. The retaining solution was placed in October 2018. Instrumentation with sensors has been put in place to monitor the evolution of the situation and the stresses in the beams and retaining bars in case of contact.

 

Implementation

The instrumentation is composed as follows:

  • Two displacement sensors to follow the movement of the tilting wall section in question in relation to the steel beam:
  • Four strain gauges, grouped in pairs and glued symmetrically to the steel bar M20, to monitor the forces transmitted by the support beam. The average of the 2 gauges placed diametrically on the bars gives the axial force in the bars.
  • Two inclinometers, one placed on the tilting wall and the other on the adjacent portion considered stable, on which the steel beams are fixed.
  • A temperature sensor, placed on one of the steel bars

Displacement sensors and inclinometers are used to check whether the wall continues to tilt over time and, in the event of contact with the metal beams, to check whether these are holding back the unstable part of the wall.

Tiltmeters
Displacement sensors

Strain gauges are used in the event of contact to ensure that the forces in the bars holding the wall do not become too great. If this is not the case, additional reinforcement measures should be taken.

Drawing of the retaining solution

Alarms for wall contact with the beams and alarms for excessive stresses in the bars have been provided. VDV manages these and automatically alerts the people concerned according to the level of stress reached (maintenance or rapid intervention group).

The dashboard used for the case

 

The VDV correlation tool

The correlation tool developed at our request is straightforward to use but very powerful. It allowed us to remove the temperature effect that was causing us problems. And by creating virtual variables enabled by VDV, we can concentrate on the only important parameter: the forces in the bars.

To be able to communicate optimally, the dashboard is the perfect tool. We also use the reporting tool with automatic sending frequency to ensure that the people involved are continuously involved.

When the wall had abutted against the beams and seasonally varying stresses began to build up in the steel bars, the situation has been closely monitored to ensure that the restraint solution is not at risk. With the length of the strain gauges varying greatly with the temperature of the steel bars, to which they are bonded, it is crucial to remove the effect of thermal expansion from the bars.

In order to be able to estimate the elongation accurately, we used the VDV correlation tool, which allows us to know the exact correlation between the measured temperature and the elongation of the bars. Of course, we made this correlation when the unstable wall was not yet in contact with the restraint (where only the temperature effect influenced the elongation of the gauges).

Correlation tool
Stresses in the bar since the start of monitoring

Once the correlation coefficient is known (different from the theoretical value because the average temperature in the bar is unknown), we can deduce this effect. We then create a virtual variable that only considers the forces in the bar and allows us to monitor the situation objectively.

 

 

04

The results

The results, the live sensor values, are displayed in a dashboard, shared with all the project participants (design office, agents of the public service concerned, municipality, etc.). The dashboard summarises the context, allows them to see the latest data live (real-time display), displays the values since the beginning of the measurements.

That way, all stakeholders can assess the situation accurately and in a very reactive way, making it possible to intervene as quickly as possible if necessary.