From Professor J. B. Burland, FEng, FRS

Sir, It is hardly surprising that any proposed intervention to stabilise the Leaning Tower of Pisa will be controversial ("Experts wrangle over best angle", report, July 18). Yet historical studies of its continued movements, coupled with the most advanced analytical studies of its stability, show that the one certain outcome if nothing is done is that it will collapse.

The proposed method of stabilising the tower is not, as the report suggests, to "haul the tower back towards the upright with giant steel cables". It is the extraction of soil from beneath the northern (high) side of the foundations, thereby reducing the inclination in a gentle and controlled manner by an amount that is imperceptible to the eye.

The sole purpose of the cables (which are temporary) is to provide a positive method of holding the tower in the very unlikely event of unforeseen detrimental movements. Not to provide such a safeguard system would indeed be highly irresponsible.

The method of soil extraction satisfies all the architectural requirements of restoration in that the history and character of the building are fully respected. The method has been successfully used over the past four years to stabilise the Metropolitan Cathedral in Mexico City which has undergone differential settlements of over six feet.

The earliest known use of the method was by James Trubshaw in 1832 to stabilise the Tower of St Chad in Wybunbury, Cheshire. Professors James Beck and Piero Pierotti complain that the commission for the protection of the tower has acted in secrecy.

I recommend that they, and anyone else interested in the tower, read my paper published in the 1997 volume of the Journal of Architectural Conservation where the work of the commission, including this proposed intervention, is fully described and referenced.

Yours faithfully, JOHN BURLAND (Member, Pisa Commission; Professor of Soil Mechanics), Department of Civil Engineering, Imperial College of Science, Technology and Medicine, Imperial College Road, SW7 2BU.

July 19.