Sir Thomas Drew on fractures in the nave piers of Truro Cathedral
source The Architectural Review volume 1 Nov 1896 - May 1897, page 3
In his recent Report to the Building Committee upon the fractures that have developed in the nave piers of Truro Cathedral, Sir Thomas Drew appears to be fully sensible of the delicate nature of his task. To the man of narrow mind and illiberal instincts there is no greater joy than to find fault with the work of others, and it is a pleasure to read Sir Thomas Drew's chivalrous references to the architect of Truro Cathedral. In the forefront of his Report, Sir Thomas expresses his admiration of the late Mr. Pearson as the greatest ecclesiastical architect of his time and says that he has no adverse criticism for the designer of the piers that failed.
While fully sympathising with the generous feelings that evidently prompted these remarks, we are unable to avoid the hard logic of facts. It is perfectly obvious that if no faults had originally existed the piers would not have failed, and the improved form of reconstruction now advocated would not have been necessary. The best of us are liable to error, and those are truly wise who seek knowledge from past mistakes. Therefore, it seems well that careful search should be made for the true causes of the mishap to which we now refer. We are unable to agree with the criticism expressed in the earlier pages of the Report, upon modern constructive science, for the methods now followed are based upon correct appreciation of the proper uses and limitations of stone masonry.
It sounds very well to speak of "mobile construction, restrained by countervailing forces, of balance, counterpoise, and thrusts," as exemplified by the buildings of medieval architects, but such conditions are scarcely consistent with true stability, and must occasion strains that ought not to be caused in a material so brittle as stone. The strength of stone masonry really depends upon the strength of the stone, the size of the blocks, the accuracy of the dressing, and the strength of the mortar, and we are therefore surprised to learn from the report that the cause of disaster should have been attributed to the modern specification as to mortar and bedding, and to the accurate and conscientious way the terms of the specification were observed. No doubt, as the Report says, beds of stone in old work were usually laid in comparatively thick swimming beds of mortar produced by an unscientific mixing of lime and sand by rule-of-thumb methods, while in modern practice bed-joints are of properly made mortar no more than ¼ in thick. But we must remember that in former times stones were more roughly dressed, and so required more mortar to keep projecting points apart.
The stones at Truro were truly wrought, and when worked into their seats the bed-joints were little more than a bare ¼ in. thick of superior mortar. It appears to be considered that this treatment involved a risk of undistributed pressure upon the lowest bed. Of course, there are always risks in this world, but why inequality of pressure should be suggested by accurately levelled and truly faced work, we cannot quite understand. As for the thickness of the beds, ⅛ in. is considered to be sufficient by competent authorities, when stone has been faced almost to a plane, for pressure is then distributed nearly uniformly, and the mortar serves chiefly to transmit pressure to small depressions, In fact, if the joints were very smoothly finished, mortar would be, practically, unnecessary for the distribution of pressure; but such extreme smoothness would create a risk of displacement by the sliding of one stone on another. Again, if the cushion of mortar between two blocks of stone were of insufficient strength, it might be squeezed out laterally, so tending to cause failure of the stone by tension. For these reasons, we are unable to agree with the first hypothesis in the Report.
We now come to a proposition which is entirely reasonable, and in complete accordance with modern practice. After pointing out that the bed under the plinth of each pier is practically a dampcourse of asphalte ½in. thick, between the irregular surface of the foundation pier and the smooth under-face of the plinth course The Report says that asphalte was an unfortunate insertion as the bedding joint on which stress was concentrated. The accuracy of this conclusion will be abundantly evident after inspection of figure 1, which represents the sections of the piers as built.
The following undesirable features will also be observed, to which attention is not specially directed in the report:
(1) the irregular composition of the rubble masonry is by no means suitable for the uniform reception of pressure;
(2) if the section given in the Report may be accepted as correct, the stones do not break joint properly—notably in the centre of the foundation pier—and no headers are evident; and,
(3) instead of using one large stone for the base of the column, two thin stones, measuring 11 ⅞ in. and 9 ⅞ in. respectively, are employed for the plinth and torus.
Adding to these points the unsuitable nature of a plastic substance like asphalte for the equalisation of pressure, there was every reason for anticipating fracture of the foundation-bed, followed by failure of the base stones due to tension. Further, as we gather from the Report that moisture exists in the subsoil, it is not impossible that failure may have been influenced to some extent by subsidence tending to rupture the foundation at the weak point mentioned above. Next, we observe a suggestion that the building contractor—whose conscientious work was previously held to be partly responsible for the disaster—may possibly have further contributed to failure by "a certain modern energy and impatience under a certain modern contract" by imposing weights rather too soon on the substructure. The complimentary way these remarks are made in the Report does every credit to the author, but we fancy the builder would be quite willing to forego praise of the kind under the circumstances. Finally, it is said that pointing up the bed joints under the weak spur-bases, as the work proceeded, manifestly contributed to fractures; a theory that is certainly feasible.
As to the measures that ought to be taken for reparation of the injury, we are in accord with Sir Thomas on almost every point. He proposes that fractured stones should be replaced, and that the bases should consist of one stone each, 21-in. deep. The rubble foundation, he thinks, should be lowered to permit the formation of a concrete cope as a seat for the new base stone, and to act as a damp-course in place of the asphalte removed. An inspection of figure 2 will enable our readers to gather the effect of these alterations, but it will be noticed that lead is suggested as an alternative to mortar for the beds of the courses. This option is one which, we trust, will not be accepted, for the same reason that dictates the rejection of asphalte. As the strength of the Corsham Down Bath stone used in the construction of Truro Cathedral has been questioned, it is satisfactory to note that the direct compressive stress per square foot in the piers is well within the safe limit, and there seems no reason for doubting the judgment of Mr. Pearson in selecting this material.
Generally, it is reassuring to learn that the magnitude of the fractures should not be over-rated, and that they need cause no anxiety as to the stability of the cathedral.
Since the above remarks were written, we have learned that the Cathedral Committee do not propose to take any immediate steps, but will wait to see, we suppose, whether any further movement takes place. We think they would be wiser to carry out Sir Thomas Drew's suggestions without delay. If anything worse develops a rather heavy responsibility will rest on them.