I SHUDDER to think of how Tesco, contracting engineers plus inspectors allowed those plans for a tunnel in Gerrards Cross to go ahead.

Trains, because of their weight, set up rhythmic vibrations that will crack open almost any structure, particularly should there be any joints in the rails under and near the tunnel to compound the problem.

Welded rails on that section do help to alleviate this particular difficulty. Beyond that problem tunnels must be lined with steel sheet, usually ships plate bent in an arc and then reinforcing matt welded to the steel to leave a gap (for the concrete) and the whole covered in more concrete of that necessary thickness to support any surface structures.

Safely is never cheap except when it is seen to work, like Victorian tunnels over a long period with the minimum amount of maintenance.

Wooden sleepers absorb some of the vibrations whereas concrete sleepers tend to shift the stress directed into the surroundings area of any given railway track.

In addition to all this there have been mild earthquakes earth tremors in this area more recently and because I live directly above a geological stress line I feel them from time to time whereas my neighbours don't.

There was yet another mild one a couple of weeks ago, believe this or not, which may have weakened the structure.

A Valentine, High Wycombe