Surveying equipment that saved St Paul's

The Collections
Today at the Cathedral View More
Temporary closure of Stone and Golden Galleries
7:30am Morning Prayer
8:00am Eucharist
8:30am Doors open for sightseeing
12:30pm Eucharist
4:00pm Last entry for sightseeing
5:00pm Choral Evensong

Surveying equipment that saved St Paul's

‘Surveying the Fabrick’ – survey work at St Paul’s Cathedral in the 1920s, research by Jane Insley, November 2013. Edited for the website by Jagraj Gill.
 
The collections at St Paul’s Cathedral include some intriguing surveying instruments and equipment, most of which appears to date from the 1920s, and was supplied by E R Watts and Son, London. Some of the equipment would have been in general use during that period, but why was the kit in the cathedral? 
 
It rapidly became clear that periodically since the beginning of the build by Sir Christopher Wren, there had been questions about the safety of the enormous and grand structure that is St Paul’s. The combined weight of the Dome and the eight piers was at one point calculated to be 67, 500 tonnes. Externally since, threats have come from nearby building works; every time a new sewer, an underground train line, a skyscraper or a bridge across the Thames was suggested, the diggings represented a nibbling away at the hill that the Cathedral was standing on. The main concern had been focussed on the question of whether the masonry was still capable of holding up the structure. A Commission was formed in 1907, and careful measurements and monitoring took place, alongside a considerable amount of repair to the stonework. The stone itself was cracking and in some cases, in danger of falling away. The most dramatic moment came on Christmas Eve 1924, when the Dean and Chapter of the Cathedral were served with a Dangerous Structure Notice, and told that the Cathedral was to be closed immediately and to remain so until the eight piers had been completely rebuilt. 
 
In 1906, chief architect Mervyn Macartney became the Surveyor to the Fabrick. Sir Francis Fox, an architect with experience of stabilising historic buildings, most notably Winchester Cathedral, re-plumbed the Dome in 1913, and found it to be 5 ¾ in out of the perpendicular. Macartney began to publicise the vast amount of issues that he faced, particularly in terms of the Dome, with a paper for the Architectural Review in which he explained the works then in progress. The outbreak of the First World War did not stop the reparation of the Cathedral, although it did have to proceed under licence from the Ministry of Munitions. Macartney argued that:
 
‘The work is of National importance, and any stoppage now might entail serious consequences.’ 
 
The works and the concern continued after the war and at the beginning of the 1920's a series of studies were carried out endeavouring to establish the stability of the Dome. The main piers and bastions were built with a piled rubble core faced with wrought masonry. These had been patched and facing stones replaced where possible, using a variety of methods some of which were more effective than others. Although the rubble in-fill had settled, there would still be air spaces between the pieces, and the 20th century development of Portland cement and the method of grouting – filling the inside with a fluid cement mortar – was a technique considered to consolidate the piers. However, this had dangers. Yet another possible course of action, advocated by several, was to take the whole structure down and start again, but building solid supports. This was widely thought to be far too costly, time-consuming, and both difficult and hazardous to do. 
 
The 1921 St Paul’s Commission produced a first interim Report on 1 June 1922, having met thirteen times, and concentrated on the condition of the masonry in the main piers and the settlement of the foundations. Following this, it was clear that the system of reference points was installed, with the aim of taking three sets of observations at six monthly intervals, to establish any variations due to seasonal changes. Macartney recognised the importance of fundraising, and The Times launched another fund and published the name of contributors. By January it had received over £200,000. 
 
The Builder noted that St Paul’s Cathedral had been filmed by Pathe Freres Cinema Ltd in aid of the Restoration Fund, and all these professional journals engaged with the discussions about how the work should best proceed; through 1923 and 1924, St Paul’s featured almost regularly in drawings, paintings, and discussions about Wren. Interestingly, the engineering journals of the day were much less involved. November 1923 saw the publication of the Sir Christopher Wren Bicentenary memorial volume, promoted for the restoration Fund by the Architects Journal and Architectural Engineer. 
 
On Christmas Eve, 1924, the City of London served a Dangerous Structure Notice on the Dean and Chapter of the Cathedral, so that the cathedral should be closed immediately to all until the remedial works were over. The committee decided to publish its second interim report on 29 December, and details were also published in The Times. On 19 January, The Architect welcomed the fact that a Dangerous Structure Notice had been served, on the grounds that it drew public attention to the problems and the need for cash, and came at a timely moment as a tribute to Wren. The Architect noted that the public response had been magnificent, but that there was a disappointing rumour that the consolidation would be by grouting by liquid cement – they would have preferred a complete rebuild – and repeated this view a week later, as they were confident the money could be found to cover the costs. 
 
The final report of the 1921 Commission was issued on 14 February 1925. A combination solution was arrived at, involving grouting the piers, but also supporting them with tie bars set in boreholes which had been grouted first, and another chain to hoop the dome structure. Measurements would continue, and building work in the surrounding area would be monitored closely. Once the Commission had produced its final report, and the Dean and Chapter had approved the work to proceed, a Works Sub-Committee was set up to run the project. 
 
The Manual of Measurement recorded a series of observations as work progressed, and was deposited in the Archives after these researches started by the Clerk of Works. Not only did it describe the best way to measure levels at various heights in the cathedral, take plumbing (to link the levels and check for movement in the piers, measure the diameter of the Whispering Gallery in twelve different places, and to connect the exterior of the cathedral with the Post Office Datum point), but it also explained how to record the results and to carry out the calculations required as a result. The Manual was accompanied by an unfortunately incomplete set of photographs showing how the kit should be set up, and it described in detail the equipment and its use. With nine chapters in all, each was a self-contained guide to the procedure in question, so if a particular action was used in several activities it was described again in the full document. 
 
The first three chapters discussed the structure and the sub-soil, a history of earlier measurements and a list and instructions for the care of the equipment, amongst other things. Chapter 5 covered levelling, exploring the six main groups of measurements. Chapter 6 concerned plumbing, and explored how the structure was measured from a certain number of fixed points and how plumbing the Dome was a separate procedure. The final chapter dealt with sub-soil water flow. Work proceeded in accordance with the methods in the Manual from this point on. 
 
The measurements continued well into the 1980's, until the crack micrometers were supplanted by an electronic method that allowed for constant remote monitoring. 
 
 

Crack micrometer set: 8278

Level: 9559

Marker bolts & Key: 9557

Invar Tape: 9555

Leveling Staves: 9547

Tacheometer: 8274