Of the Green Family from Harpole, Northamptonshire their Ancestors and Relatives

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Monday, 20 November 2017 12:25 AM GMT

Subsidence in Victorian Buildings - More

This article describes how Victorians built foundations for buildings on swampy soils. I hope methods have improved since then. "Foundations of Buildings in Swampy Soil" from Cassell's Technical Educator.

When the soil on which a building is to be erected is very soft or swampy for a considerable depth below the surface, which is the worst kind of foundation that can occur, it has been usual to secure the base of the walls by a wooden grating or platform of the nature described in the last lesson, but supported upon strong piles of a length sufficient to penetrate into harder soil, and acquire a fixture there, and which are driven until they will go no deeper. Such piles are usually shod with iron, and are driven at intervals of three or four feet apart. There ought to be at least two rows of piles under the foundation of every wall, as in the example shown in Fig. 531, so that every pair of piles support a transverse sleeper.

Less than two rows of piles would not give sufficient stability to the wall even of a common building. More than two rows, or three, at the utmost, can only be required for the foundation of very substantial masses of masonry, such as revetments, wharf walls etc.

The principle of the stability derived from a piled foundation is thus described by Sir Charles Pasley:- "By the laws of mechanics the force of percussion greatly exceeds that of simple pressure by dead weight; and in the pile-engines used in this country the ram (sometimes called the 'monkey') - that is, the weight by which the pile is driven - falls upon the head of it from a considerable height, which greatly increases the effect as compared with the old system of pile-driving still used abroad, in which the ram is not raised more than four or five feet. Now the momentum of ten or twelve hundredweight, a common weight for the ram of a pile-engine, falling from the height of fifteen or twenty to thirty feet, produce a greater effect than the dead weight of any common mass of building; hence, if a pile be driven by such an engine until it can go no further, there can be little risk of the foundation giving way afterwards in consequence of the mere pressure of the walls."

Foundations built on piles are, however, liable to failure, first from the piles losing their original position in consequence of the soft ground offering too little resistance against a lateral movement, so that the walls built on them may have a tendency to fall over.

In order to guard against this evil, the piles for walls are sometimes driven obliquely, so as to diverge at the bottom, and those especially for leaning revetments or for the abutments of bridges may be driven in such oblique directions as to oppose the greatest resistance to the unequal pressure, which acts chiefly on one side of the wall.

Secondly, a failure may arise from wooden piles rotting or being corroded by worms in process of time, the former of which evils is almost sure to occur in foundations alternately wet and dry. Fortunately, the kind of soil where piles are most required is that where water is found a little below the surface; and if the woodwork be kept at such a level as always to remain wet, there is little or no risk of it decaying by rot. The perfect state in which trees have been found after the lapse of many centuries is well known. A few years ago, on throwing two arches of Rochester Bridge into one, the piles of the intermediate pier, which must have been there for several centuries, were taken up perfectly sound.

Reverting to the system of wooden platforms in foundations, they are of course liable to decay in process of time under most circumstances.

Now, since the uniform thickness of such platforms cannot exceeed a few inches, this kind of failure taking place after the building has become completely consolidated, will not be attended with those injurious consequences which would arise from the decay of a piled foundation. Still, it is desirable to avoid using continuous planking, and to employ a couple of parallel longtitudinal timbers for the same purpose, laid at a sufficient interval apart, which, when surrounded by brickwork or masonry, are called chain timbers.

These are shown in Fig. 532, in which is shown a section of the walls of the buildings in Lancaster Place, adjoining Waterloo Bridge, London. In this illustration, which will be further described presently, two parallel chain timbers are seen in section. These, which are in thickness about equal to the height of four courses of brickwork, are laid upon transverse sleepers placed parallel to each other at certain intervals.

In this arrangement it will be evident that in the event of the woodwork decaying, the base of the wall will have a sufficient and solid footing on the natural ground or on the surface which has been prepared for it to rest on; for the intersecting cavities in the brickwork occasioned by the rotting of the timber are too small in themselves, and at too great intervals apart, to injure the stability of the brickwork.

It will be understood that the chain timbers which extend all round the building, and which therefore must be in several pieces, are firmly connected at the end by scarfing or otherwise, and they are pinned down to the sleepers on which they lie. A third chain timber is also used, not on the same level with the others, in which case it might weaken the wall too much, but in the middle of the thickness of the wall, two or three feet higher up, as, for instance, at the part where the uppermost offset of the footings terminates. This third chain timber is also shown in the example, and it will be evident that the three combined will produce nearly the same favourable effect in the first instance as a continued planking, but without being liable to any of its inconveniences.

The buildings referred to are founded in very bad soil, which proved on examination to have consisted originally of soft mud, but which was intersected in all directions by the remains of the foundations of old houses and wharves. Trenches having been cut to sufficient depth and width along the proposed line of the new foundations, fragments of the old walls were laid therein as regularly as possible, in the same manner as large flat stones might have been used. These were well sluiced with water, then mixed with grouted gravel ( Grout - A thin semi-liquid mortar, composed of quick-lime and fine sand and gravel, which is poured into the internal joints of masonry; it is particularly used where the work consists of large stones. The process is called "grouting."), and rammed down flat; the work being thus continued until the fragments alluded to were expended. Then the rough foundation was continued to the proposed footing of the brickwork with grouted gravel alone, and at that level transverse sleepers were used at intervals of about four feet apart (all under piers, none under the position of doors or windows), upon which were laid two longtitudinal chain timbers of the thickness of four courses, completely surrounded by the brick, but immediately under, and flush with what was intended to be the exposed surface of the principal wall of the building after the work should be raised to a certain height; and a few feet higher, opposite to the top of the uppermost offset of the brick footing, a third chain timber of the same scantling was bedded in the middle of the wall.Thus these three chain timbers combined cover a space of not less than 2 feet 3 inches wide, being equal to the thickness of a three-brick wall, and this arrangement is continued throughout the whole extent both of the external and cross or party wall of that row of buildings.

The foundations here described is everywhere 8 feet wide and of the same average depth. The brick footings have a base of 5 1/2 bricks, which, by four successive offsets on each side at intervals of three courses apart, diminishes at the height of 3 feet to 3 1/2 bricks, which is the thickness of the front and back walls of the lowest storeys of the buildings, which are vaulted and used as stables or stores. Over this the walls of the basement floor, and the inhabited part of those buildings, are 3 bricks thick, which dimension is gradually diminished by successive offsets to 1 1/2 bricks in the uppermost storey of the house.

The height from the top of the brick footing to the coping of the parapets in front and rear is not less than 88 feet, to which, adding the footings and foundation, it appears that there is a mass of brickwork, rough masonry, and grouting nearly 100 feet in height - that is, on the side nearest the river.

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