ST. PAUL'S IN DANGER

New Zealand Herald, Volume LXXI, Issue 21716, 3 February 1934, Page 2 (Supplement)

ST. PAUL'S IN DANGER

ENGINEERS' FRESH WARNING SAFETY MARGIN TOO SMALL SUGGESTED "SACRED AREA" St. Paul's Cathedral is in the gravest danger from tho effects of subsidence. A new warning is given in a report issued by two eminent engineers, Sir Alexander Gibb and Mr. Ralph Freeman, after their investigations lasting two and a-half years. " The margin of safety," they state, " is so small as to compel the most rigorous attention to any circumstance that could be a possible cause of reducing it still further." It is urged that immediate steps should bo taken to prevent further subsidence, to avoid traffic vibrations and to control building and similar operations in the suggested "sacred area" around the cathedral. An Act of Parliament is suggested to give the cathedral authorities the necessary power for control over the " sacred area." In the course of the inquiry the engineers excavated 23 test pits and examined records of 189 borings. The results arc thus summarised by tho NewsChronicle. The foundations of the cathedral are built on a stratum of potter's clay which occupies its site. This clay, resting as it does on an underlying bod of gravel and sand, would not bo considered in modern practice an altogether satisfactory foundation for the structure, and there

is no doubt that the loads arc greater than would now ho entertained. There arc evidences of extensive settlements during construction and some movement subsequently, confirming the view that the foundation pressures \i re high and the margin of safety so small as to compel the most rigorous attention to any circumstance that could be a possible cause of reducing it still further. " There is littlo doubt," says the report, " that interference with tho subsoil or tho subsoil water might easily upset the equilibrium that is now existing and cause further settlement of the building." Tho piers below the dome are undoubtedly more heavily loaded than any other part of the supporting structure, and it is above them that there is evidence of tho greatest degree of settlement. Pressure below these piers is calculated at approximately five tons per square foot, which " is larger than accepted practice of modern times would entertain as safe in similar ■conditions." Tho investigators state: "We are of opinion that no attempt should bo made to consolidate or replace the sand and gravel below the present foundations unless and until a continuation of the measurements taken from time to time in recent years indicates a significant degree of subsidence. The suggestion of a "sacred area" was made by Canon Alexander as long ago as 1913, and the engineers in their report put forward the view that in a described area " some control is essential, up to the point of prohibition in an extreme case, on both the design and method of construction of all underground works."