Engineering Wonders of the World
Volume III

GREAT UNDERPINNING ACHIEVEMENTS. 319 better than poor red bricks, loosely jointed together. Precarious as such under- pinning must ever be, jerry work of this kind made the task of the engineers doubly difficult. In the circumstances it became necessary to place a continuous sheathing of steel joisting under the area of each base, so as to tie the loose mass of woodwork together, and dis- tribute equally the weight upon the needle girders. This was a very tedious operation, as only a small part of the base could be dealt with at one time. As soon as this portion of the labour was completed, the ST. MARY WOOLNOTH—CRYPT, SHOWING VIEW OF OLD FOUNDATIONS OF COLUMNS AND ARCHES SUPPORTING CHURCH FLOOR. south wall, on the King William Street side, was pierced at intervals of Work under about ßve feet. Strong needle the South . . . Wall girders were inserted m the apertures so made, one end resting on the solid stone at the outside, the other being tied down to one of the main girders supporting the columns. Sufficient of the inside of the wall was then cut away to allow the girder (built before the needles were fixed) to be slid into position, and to permit also of a 14-inch. blue brick wall being made, carrying short lengths of bearing girders, which were wedged tight up to the needles. The object of this device was to reduce the overhang of the needles when the outer por- tion of the wall came to be cut away, as no reliance could be placed upon the old work. When the inside girder was fixed, steel wedges and packings were inserted between the top of the girder and the needles, the wedges being driven up tight to insure that the whole of the weight was carried by the girder and the blue-brick wall mentioned. The girder was designed to sustain perma- nently only half the wall, and it was there- fore assisted by timber packings below. After the wall had been pinned up above the girder, and everything was made solid by grouting, the task of fixing the outer girder became comparatively simple. The outer half of the wall below the needles was cut away, and the girder, meanwhile built, was moved into position. Thus the whole weight of the south wall was received by the two girders. The north wall on the Lombard Street side presented a much greater weight, and as the work of supporting it could not be under- taken from the outside, the method adopted on the south Underpinning .. i i za the North side was impossible. One mam ,,, ,, r Wall, girder was accordingly de- signed to carry the entire weight. But as it could not be placed far enough under the wall to be in a position to do this, suspended needles were attached to support the outer part of the wall, their tail ends being tied down to one of the girders for supporting the roof columns. Needle girders were fixed just below the church floor level, and under cover - of these the wall was cut away to allow the