Subsidence in Victorian Buildings

Tuesday, 10 September 2002 03:44 PM BST

Contributed by: jami brooks

This article describes how Victorians built foundations for buildings. Have methods improved since then? Do modern buildings suffer less from subsidence? "Foundations of Buildings in Unfavourable Soil - Piled Foundations" from Cassell's Technical Educator.

It has before been observed that the soil upon which a building of any importance is to be erected should always be well examined before the work is commenced, which may be done by striking it to ascertain if it sounds hollow, and then by boring it in various parts with an earth borer. Small shafts, cut at certain intervals, may also answer the purpose of discovering the nature and depth of several strata when the ground is not of uniform consistency.

When the soil is partly good and partly bad, the latter may be cut deeper until good soil is obtained, if it can be found at moderate depth, and thus the footings may be built as in irregular ground - not on one continued level, but in steps or terraces.

When the soil is all the same quality, but looser in some parts than in others, ramming it well in the loose parts has been considered a very useful and, sometimes, a sufficient precaution; and this method of treating the soil is particularly recommended by some of the French engineers of the greatest eminence.

Even in buildings erected of bricks it has been common to commence the foundations with flat stones, placing the largest as regularly as possible, filling in the intervals with smaller ones, and ramming them down.

When the soil is of middling quality, not absolutely good, nor yet altogether unsound and treacherous, a grating or network of continued woodwork is often used as an expedient for ensuring the stability of an edifice.

Fig. 530 - In this case sleepers of about five or six inches square are laid transversely, at intervals of three or four feet apart, under the whole base of the intended wall. over these are fixed longitudinal planks forming a continued platform under the whole foundation of the walls of the building, as represented in the section shown in this example. The planks must be carefully pinned down upon the sleepers, and the intervals between the latter are usually filled with bricks or stones - not with earth.

As every heavy body (says Sir Charles Pasley), "if placed upon soft soil, has a tendency to sink more or less, which tendency is directly in proportion to its weight, but inversely as its base, the footing given to the walls of brick buildings has in such soil the effect of diminishing, or altogether rendering null, the said tendency, which might operate in a manner unfavourable to the stability of the edifice, if the walls had no such increae of base, but were of uniform thickness throughout, from the bottom upwards. In respect to the wooden platform which has been described, its use is of a temporary nature, as it merely serves to prevent the brickwork, if it sinks at all, from sinking unequally whilst the mortar is yet soft, or the work green, as it is technically termed. After a certain number of years, when the mortar and bricks shall have become completely consolidated into one compact mass, they are no longer liable to separate, even if the woodwork underneath could be removed."

It will perhaps be scarcely necessary to remark that the base of every brick wall of the same height and thickness, or, in other words, of the same weight, should be greater in direct proportion to the softness of the soil on which it is built; also that the projecting parts of the footing of a wall should have a certain depth to give them solidity, otherwise the greater weight upon the central part of the base might fracture the bottom of the wall, and thereby render the footing useless; for this reason it is desirable that the offsets, when they are several, should not follow in every succeeding course, but at greater intervals apart, as already mentioned. Moreover, it is advisable in erecting a building in middling soil upon a wooden platform, such as has been described, either to make all walls of equal thickness, and consequently of equal weight throughout; or, if that arrangement would not be convenient, to give the thicker wall the greater base in direct proportion to its weight.

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