Sections about 3 feet wide between the shores should then be excavated under the wall, new footing stones laid, and the space between the new and old footings filled with brick or stone work. Where the height between the new and old footings does not exceed 5 feet, granite posts, if available, offer special advantages for underpinning.
They should be from 12 to 18 inches wide on the face and of a thickness equal to that of the wall; they should be cut so as just to fit between the new and old work, and with top and bottom surfaces dressed square; they should be set in a full bed of Portland cement mortar, and the top joint also filled with mortar and brought to a bearing with steel wedges.
If granite posts are not available good flat stone or hard brick laid in cement mortar may be used instead, wedging up under the old wall with pieces of slate driven into the upper bed of cement, or with steel wedges. Under heavy walls the latter only should be used If the bottom of the old footings is of soft brickwork, pieces of hard flagging with a full bed of cement mortar, may be placed under them and the wedges driven under the flagging so as to bring the latter "hard up" under the old work. The portions of wall between these sections should then be underpinned in the same way and the shores moved along.
Where granite posts are used they may be placed 3 feet apart and the space between built up with flat rubble or hard brick, wedged up under the old wall with slate. inches apart, their ends resting on long beams placed parallel with the wall and supported by jackscrews. Very often an entire wall is supported in this way, several hundred jackscrews being required for the purpose.
If the soil under the old building is sufficiently firm, so that it will not cave or "run away," and there is working space beneath the lower floor, the ground may be leveled off, a platform of plank and timbers placed on top of it, and needles used for supporting the wall as shown in Fig. 54. Where needles are used all of the underpinning under the portion of wall supported may be put in at the same time.
The underpinning should be done as quickly as possible after the shores or needles are in place, so as not to require their support for a longer time than necessary. The needles or shores should, however, not be removed until the cement has had time to set.
98. Chicago Practice.
In building the modern tall office building in Chicago the foundations generally have to go below those of the adjacent buildings, and, the ground being compressible, new party wall foundations are almost invariably required. The consequence is that the old walls have to be supported while the new foundation is being put under them. This is usually done by means of steel needles placed from 12 to 24
In erecting buildings of skeleton construction it is often impracticable to remove the old wall, and the new building is supported by iron columns placed against the wall and resting on a new foundation put in under the old one. In building the New York Life Building in Chicago such was the case, and the adjacent wall was held up by jackscrews, as shown in Fig. 55, which were inserted to keep the wall in place during the settlement of the new work. As the new foundations settled the jacks were screwed up, so as to keep the old wall in its original position. In this case the jacks were left in place.
Where buildings have been built with a party wall, and one of the buildings is torn down, leaving the adjacent walls unsupported, they should be protected from falling by spreading braces or inclined shores, according to special conditions.
Where there is a building on the other side of the vacant lot, and within 40 or 50 feet, the walls of both buildings may be best supported by spreading braces, after the manner shown in Fig. 56.
If the distance between the buildings does not exceed 25 feet, the braces may be arranged as shown at A or B. If more than 25 feet, the braces must be trussed in a manner similar to that shown at C. [Iron or steel rods are preferable for the vertical ties, as they can be screwed up, and any sagging caused by shrinkage in the joints overcome.]
If the buildings are very high every other story should be braced. The ends of the braces or trusses must be supported vertically, so that they will not slip down. Where there are offsets in the wall these may serve for a vertical support; if there are no offsets, then the braces should be supported by vertical posts, starting from the foundation, or sockets might be cut in the wall and corbels let in and bolted through from the inside.
A truss should be placed opposite the fronts, and should be proportioned so as to resist the thrust from any arches there may be in the front. The braces should be about 8x8 or 10x10 inches in size, with 6x12 uprights against the wall, the ends of the braces being mortised into the uprights.
If there is no wall opposite the building to be braced, then inclined braces must be used, arranged in a similar manner to the shores shown in Fig. 53, only with a greater inclination. The ends of the braces should be brought to a bearing by oak wedges.