Bassett Convergence System

The Bassett convergence system is used to monitor transit tunnels for potential damage from nearby construction activities.

Operation

The BCS monitors movement of reference points that are mounted on the tunnel lining. The points are aligned to a plane that is normal to the axis of the tunnel.

A system of articulated arms links each reference point to the next, and forms a series of virtual triangles. A tilt sensor is mounted on each arm. Spatial displacement of the reference points moves the arms and results in changed tilt readings.

Tilt readings are recorded by a data logger and retrieved at scheduled intervals by a remote computer. The readings are then processed by BCSWin software and the resulting displacement data are displayed in graphic or tabular form. A BCSWin screen shot is shown below in reduced size.

The system can operate in near-real time. A complete set of tilt readings can be recorded in about five seconds, and the remote computer can update the graphic display in just a few seconds more.

Advantages

Low Profile Design: The BCS is designed specifically for tunnels. The system fits close to the tunnel wall and does not interfere with normal traffic. Arms can be shaped to provide extra clearance or to bypass obstructions.

No Optical Components: Unlike optical systems, the BCS has no line-of-sight requirements and can be installed around corners. In addition, the BCS is not affected by changes in the refractive index of air caused by passage of a train.

Robust and Reliable: The BCS easily accommodates normal tunnel traffic, vibration, temperature changes, and electro-magnetic emissions.

Near Real-time Data: The BCS can complete a survey of a tunnel section, process the readings, and generate displacement data in seconds. This is important for the protection of trains and passengers when nearby construction may damage the tunnel.