New Surveys of the Crossing and Lantern, 2009–10
The lantern tower has not hitherto been the subject of a detailed structural, architectural or archaeological study, and until now no accurate drawings of it have existed. As part of a longterm project to compile a three-dimensional, digitally-based record of the entire structure of Westminster Abbey, the crossing and adjacent areas of the church were fully surveyed in 2009–10.176 From the plans, elevations and section drawings thereby produced, it is now possible to understand more fully the construction of the tower, and to measure precisely the amount of movement that has taken place in each of the main piers. The measurements generally confirm the observations made in 1941 (p. 83), that the north-west pier is more out-of-plumb than the others, and that the bowing of the shafts (measured on the diagonals) is 12.5 cm on the north-west pier, but is less than 10 cm on the other three.177 Demonstrably, there has been no significant change in the last sixty years, and since there is no sign of failure in Hawksmoor’s masonry repairs of the 1720s, it may be concluded that the crossing has been structurally stable for three hundred years, or more.
Wren attributed the bowing of the crossing piers to the failure of Henry III’s masons to build the lantern tower immediately, and hence to load the structure with sufficient dead-weight to prevent the lateral thrusts exerted by the arcades from distorting them (p. 31). Some commentators, including Sir Gilbert Scott, have wondered whether the tall, slender piers of the crossing would ever have been capable of supporting a substantial tower and spire. Did Henry of Reyns over-reach himself and attempt to erect a structure that was inherently unstable and would have failed anyway, even if it had been completed?
Despite misgivings by some architectural historians, one has only to look at the great thirteenth-century churches of France to appreciate that lofty crossings could and often did support masonry towers, lanterns and spires of breathtaking height and complexity. Monumental towers had been built in western Europe for upwards of three centuries, and all without the benefit of engineering calculations. The approach was purely empirical, and a large body of practical knowledge had been amassed. Thus when designing the crossing at Westminster, Henry of Reyns would have been able to draw on centuries of experience of French masons. With the benefit of modern structural engineering knowledge, it is now possible to analyze the existing fabric and to determine how much load the crossing piers (in their original unbowed state) could have borne without failing. These calculations have recently been made, with surprising results.178
One of the areas of critical concern from a structural engineering point of view is the nature of the foundations upon which Henry III’s crossing was built: the strength of the substructure is of no less importance than that of the superstructure. Remarkably, very little information is currently available on the subject of the Abbey’s foundations, but in September 2009, Time Team conducted an archaeological excavation outside the nave, to the west of the north transept, and this revealed that the mid-thirteenth-century transept wall and its buttresses stand on a massive, stepped raft built of mortared masonry. The full depth was not ascertained during the 2009 investigation, but from observations made in the nineteenth century it is believed to be in the region of 3.0 m.179 It seems likely that the buttressed outer walls and the arcades of the transept and nave aisles stand on huge rafts of solid masonry, and do not have separate strip-foundations. This was expressly confirmed in the case of the eastern aisle of the south transept by an excavation which was carried out in 1938 to investigate a grave in the centre of the southernmost bay. The grave, which was 1.12 m deep, had been cut entirely into solid foundation masonry.180 Various boreholes and deep excavations in the near vicinity of the Abbey have yielded data on the depth and nature of the natural deposits of gravel and clay that form the bedrock of the site. These data provide further evidence to assist the structural calculations, which have confirmed that the foundations are strong enough to support an additional 1,200 tons of masonry at the crossing.181
Next, the structure of the crossing piers themselves was investigated. For one-third of their height, these are freestanding columns cut from massive blocks of Purbeck marble, a material which can withstand enormous weight under compression, without the stone crushing. For the remaining two-thirds of their height the crossing piers are integrated with the masonry of the triforium and clearstorey. [8] Consequently, the effective cross-section of masonry comprises a mixture of Purbeck marble and Caen stone: the latter has less ability to resist crushing under compression than the former. Since the structural strength of the crossing is only as good as its weakest component, that had to be identified and engineering calculations applied on it. The weakest point lies at the interface between the freestanding column and the beginning of the composite masonry of the triforium, i.e. immediately above the level of the main arcade capitals. Again, engineering calculations demonstrated that the tower, as originally built, could have taken an additional 1,200 tons of loading, over and above the existing masonry. Consequently, prior to the bowing of the crossing piers, there would have been sufficient structural support for Henry III to have crowned the Abbey with a great lantern tower. By the same token, there would have been no problem with supporting the tower and spire that Wren envisaged.
As part of a general survey of the structure and condition of the floors throughout the Abbey, ground-penetrating radar (GPR) is being used to map the sub-surface topography.182 The results obtained in the crossing and transepts have proved most interesting, revealing the complex honeycomb of the underfloor vaults and shafts constructed to receive post-medieval burials. While both transepts have been used intensively for burial, it is surprising to find that the crossing is apparently devoid of tombs, with a single exception at the centre. The radar scans indicate that the chambers are between 1.0 m and 1.5 m in depth, and have not therefore interfered with the foundation structure to any significant degree. Unfortunately, it has not been possible to detect the lateral extent of the foundations of Henry III’s work within the abbey church. In particular we cannot be sure, without archaeological investigation beneath the floor, whether the four crossing piers rise from individual foundations, or are linked together by continuous strip-foundations. The arcades that adjoin the crossing on all sides almost certainly have continuous foundation rafts under their piers, and these may also encompass the crossing itself, although that remains to be demonstrated.