Cause and possible solutions

For several months, residents have reported unusual noise and vibrations in various locations on the metro network.

We have since been doing our utmost to find the source of this problem and provide a permanent and complete solution. Measures have been taken to assess the situation. Abnormal wear on some tracks and abnormal "ovalization" on some of the new metro trains’ wheels (“M7”) have been observed. All these factors are explained in more detail below.

The origin of the vibrations

In general, noise and vibrations are generated by:

• Wheels rolling on rails: metal-on-metal contact inevitably produces a certain level of noise and vibration. No matter what, there will always be an incompressible level of noise.
• Track defects or wear: breakage, wavy wear, etc.
• Wheel defects or wear: a “flat” wheel caused by emergency braking or a distorted wheel that loses its circularity or develops a bead. The deformation of these wheels generates vibrations felt by some residents.

The transmission of noise and vibrations to buildings can be influenced to a greater or lesser extent by the track substructure or the nature of the ground, but the fundamental factor is the track-wheel contact.

As soon as it was clear what was causing the problem, STIB-MIVB took action on several fronts to increase maintenance measures for the tracks and wheels.

Measures taken on the tracks

1. Reinforced grinding schedule

Normal maintenance of the network includes grinding the tracks with a “Speno” train to ensure that the tracks remain perfectly smooth.

Because of the abnormal track wear observed, the grinding schedule has been reinforced and targets the most worn sections. See below for the grinding schedule for each section.

2. Testing new equipment to slow the onset of wavy wear

In June 2023, with the support of a specialized consultancy, STIB-MIVB conducted trials with a new type of suspension (pad) placed under the tracks. This new type of suspension aims to reduce the wavy wear of the track running surface, which deteriorates the contact with the wheel and causes vibrations. It was deployed in June 2023 on line 1 towards the city centre between Gribaumont and Joséphine-Charlotte.

STIB-MIVB has received some results from these trials. At this stage, no significant improvement has been observed, neither in terms of vibrations nor structure-borne noise.

Structure-borne noise is caused by vibration waves spreading through the ground from the track infrastructure to the foundations of buildings. These vibrations then spread through the buildings, causing floors and walls to shake and act as “sound speakers”. A dull, rumbling noise is then produced as the metro trains pass by. This noise differs from airborne noise, which spreads through the air (e.g. road noise). Structure-borne noise has a low frequency component (below 250 Hz), whereas the frequency spectrum of airborne noise ranges from 20 to 20,000 Hz.

It still needs to be determined whether this type of device can slow down the onset of wavy wear, which would delay the occurrence of vibrational nuisances. At this point, STIB-MIVB does not have enough data to determine if the device has an impact on delaying wavy wear. Observations will continue over the coming weeks and months to determine whether this device is beneficial in the overall strategy to limit metro-generated nuisances.

Plan

Measures taken on the wheels

A manufacturing defect has been discovered on some of the wheels of the new M7 metro trains. The quality and strength of these wheels do not meet STIB-MIVB requirements: the wheel wears prematurely, becoming more oval in shape. These oval-shaped wheels generate vibrations. Several measures have been taken to address this problem:

1. A detection system at Maelbeek/Maalbeek

A system has been installed to track the vibrations generated by the wheels so that they can be sent for maintenance as quickly as possible. However, when the detection system reports a defective wheel, it is already creating vibrations that can be felt by local residents. Detection is therefore a posteriori.

2. Pit correction

When a defective wheel is detected, it is sent for maintenance to restore its round shape. However, due to the wheel's manufacturing defect, it quickly wears again. This process must therefore be carried out regularly, and is not a permanent solution.

3. Replacing defective wheels

Pit correction is not an ideal solution. We therefore decided to replace the most problematic wheels with ones that we had in stock. In total, all 16 wheels that STIB-MIVB had in stock were put to use.

The M7 metro manufacturer launched the production of new wheels to continue the replacement process. STIB-MIVB has already received a part of the order.

To ensure that the new wheels have no structural defects, 4 wheels were subjected to quality tests, which revealed that the wheels comply with STIB-MIVB requirements.

Following these positive results, we replaced 16 problematic wheels with new ones to test them in service (over a distance of 80,000 km). If these tests also prove conclusive, new wheels may be ordered. These replacement operations will take place over the course of 2024.

Lines

• 1

  Gare de l’Ouest - Stockel
• 2

  Simonis - Elisabeth
• 5

  Erasme - Herrmann-Debroux
• 6

  Roi Baudouin - Elisabeth