This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
It is impossible to lay down general rules for the safe bearing-power of different kinds of ground. Each site must be judged by itself, but, speaking roughly, it may be said that alluvial soil, or quicksand, ought not to be loaded with more than about 10 cwts. per sq. ft.; soft clay (near the surface), from 10 to 15; moist clay, from 15 to 30; compact clay, nearly dry, from 30 to 50; dry compact clay of considerable thickness, from 50 to 100; loose sand, from 20 to 30; compact sand, or gravel and sand, from 40 to 60; ditto, dry and prevented from spreading, from 80 to 120, or even 150. The great point to be observed in designing foundation is so to arrange them that the pressure on the ground i- equal throughout Unequal loading (except on the firmest ground) gives rise to unequal settlement and probably cracks.
Fig. 19 - Plan of Subsoil-drains (the Subsoil-drains shown by dotted lines).
Fix, an - Inspection -chamber with Inlet (or Suubsoil - drain.
Foundations can be safely omitted only on exceptionally firm and uniform sites. Almost invariably some kind of foundation is required in order to distribute the weight of the superincumbent building over an area of ground sufficiently large to bear the weight without yielding. Foundations may be of timber, stone, brick, concrete, and even iron and steel.
Timber is seldom used nowadays for the foundations of buildings, except in the form of piles, and then only on soft ground of considerable depth.
Stone foundations are never used far from the quarries where the stone is obtained. In many localities, such as the flag-stone districts of East Lancashire and the West Killing of Yorkshire, they are still largely used,although even there they are being superseded by concrete. They may be obtained of any width up to 5 or 6, feet, and of any thickness up to about a foot. They arc usually of strong coarse rag-stone, and not at all uniform in thickness. Sometime- two or more courses are employed, the second course narrower than the first, and breaking joint with it. No mortar is used either on their beds or joints. The chief objection to these footings is their longitudinal incohesiveness. (See Fig. 22, p. 79) Brick footings are open to the same objection, and to the further one of transverse incohesiveness, especially where common-lime mortar is used in their construction. They consist of several courses of bricks, the lowest course being usually twice the breadth of the wall above, and the total height of the footings being not less than two-thirds of the breadth of the wall. They are narrowed as required by regular offsets (usually quarter-brick), as Shown in Fig. 21, which illustrates a brick and concrete foundation for an 18-inch wall. Five courses of bricks are here shown, having a total height of about 15 inches; four course- would meet the demands of most by-laws, but in the illustration the lowest projection is strengthened by being formed of two courses, an advantage where concrete is not used. On good ground, the brick footings twice the breadth of the wall above will a bed of concrete not less than 8 inches wider than the brick footings is usually laid under them; its thickness should he not less than one-fourth its breadth, but will vary according to the nature of the ground, the weight of the building, and the quality of the concrete.
Fig. 21.- Brick and Concrete Foundation for 18 inch Wall.
A. concrete 4 ft wide ; B, brick footings: C portion of footings built up to carry wall plate E ; D, stoneware ventilating damp' course 3 inches thick; E, wood wall - plate ; F,wood floor joint; G. floor -boards; H. concrete ground - layer be sufficiently firm, as shown in Fig. 25, p. 80. Where the ground is soft - whether natural or "made".
Brick footings as described above are made obligatory by the London Building Act, 1894, in all cases, except by special permission of the Council. Perhaps the by-law is on the whole a wise one, as concrete is so easily scamped, but there are many cases in which concrete alone would be more economical and more stable.
The ideal foundation, where great depth is unnecessary, is a solid bed of good concrete of uniform thickness, spread over the whole of the building-site and extending 2 or 3 feet beyond the walls on all sides. Its thickness must depend upon the nature of the ground and the weight of the building, but can seldom be less than 12 inches. Such a foundation is illustrated in Fig. 23, p. 79. The concrete foundation with thinner ground-layer, shown in tig. 18, page 56, is a cheaper modification of this. Over all a layer of asphalt should be spread, forming a damp-course both for floors and walls, and on it the walls may be built, and the basement floors, whether of wood blocks or concrete, may be laid.
Continuous concrete foundations and ground-layers cannot, however, always be adopted, as it is often necessary to carry some portion of the walls to a depth much below that of the basement floor.
The use of iron and steel in foundations can only be mentioned. Frequently the metal is in the form of steel rails embedded in concrete. Sometimes, as in the Ransome system, it consists of a series of twisted wires or rods embedded in the lowest part of the concrete to give transverse strength to the foundation. In either case, a stronger foundation can be obtained in less depth than when concrete alone is used.
Concrete for foundations was at one time generally made with a matrix of common lime, but nowadays hydraulic lime or Portland cement is almost invariably used. The latter is by far the stronger material, and good cement is more uniform in quality than good lime; hence the use of Portland cement is rapidly extending. There is, however, a great deal of rubbish sold as Portland cement - coarsely-ground and of a yellowish hue, - and this must be avoided. Plasterers, it may be said, regard it with especial favour; it is cheap, and great strength is not, they think, necessary in their work.
Portland cement should be finely-ground, sound, and of sufficient strength. The fineness of good cement is such that not more than 10 per cent will remain on a sieve containing 2500 meshes in a square inch; the best cement will leave a residue of less than 10 per cent on a sieve with 5625 meshes in a square inch. Soundness means that the cement does not contain free lime or other ingredient which might cause the softening or cracking of the concrete sonic time after it had .set. To ensure the slaking of the free lime, the cement should be spread in a perfectly dry place for a week or two before it is used, and occasionally turned over. The strength of cement is usually tested by its resistance to a pulling stress or tension; briquettes 1 inch or l ½ inch square are made of neat cement. or of cement mixed with 3 parts (by weight) of sand, and kept one day in air, then placed in water. The neat briquettes are usually tested at the end of 7 days, and should give an average tensile strength of 350 or 400 lbs. per sq. inch; and the briquettes of cement and sand should have a strength of about 120 lbs. per sq. inch at 7 days, and 200 lbs. at 28 days.
The hulk of concrete, however, consists of other materials, known as the aggregate, which may be gravel, broken stone or brick, coke-breeze, etc, and sand. For foundations the hardest materials should be used, such as gravel and hard broken stone. In London, Thames ballast is largely used, but a better (because cleaner) material is the broken and screened shingle which the Great Eastern Railway Company prepares at Lowestoft. The aggregate should be passed through a screen with 3-inch meshes, and should be free from clay, organic refuse, and other impurities.
Sand is almost invariably used in the making of concrete. A considerable quantity is present in Thames ballast, and no additional quantity is required when this is the aggregate employed. A certain amount of sand is made in breaking stone, quite sufficient, indeed, except when the stone is extremely hard; soft stone often yields too much sand, and some of it must be screened out if strong concrete is required. The volume of sand in concrete for foundations should not exceed one and a half or, at most, two volumes of the cement; if more than this be present in an aggregate, part should l)e screened out. Undoubtedly the best plan is to screen all the sand from the aggregate, then the cement, sand, and aggregate can each be accurately measured; but the builder will have to be closely watched or the plan will not be carried out. All land used for concrete should be clean, sharp, coarse, angular, and durable; pit-sand is usually much improved by washing. Street-scrapings are not sand in the estimation of an architect or magistrate, whatever they may be in that of a jerry-builder.
The proportions of cement, sand, and aggregate used in concrete for foundations vary from 1+1+4 to 1+2 + 8 or even 10. Good work may be ensured by a mixture of one part of cement, one and a half parts of sand, and five parts of broken stone or gravel. When such a mixture is specified, the builder will frequently argue that it is a 1 to 6 ½ mixture, and will measure the sand and stone together in a box 6 ½ times the volume of a bag of cement. This is advantageous to him, for the sand merely occupies the interstices in the broken stone, and his mixture really contains one part of cement, two or two and a half of sand, and six and a half of broken stone; or, to put it another way, the builder's concrete will contain one-fourth less cement than concrete made according to the specification. Perhaps for ordinary foundations - especially in small buildings where the constant supervision of a clerk-of-works cannot be afforded - the wisest method is to specify that the gravel or broken stone shall be measured with the sand naturally contained in it or made in breaking it, and that no more sand must be added.
The ingredients of concrete should always be accurately measured. A full bag of cement weighing 2 cwts. contains about 2 2/3 cub. ft., and boxes for the aggregate are often made some multiple of this: thus, the box to contain five parts of aggregate would contain 13 ⅓ cub. ft., and might measure 4' 7" x 2' 6" x 1' 4". It should be ascertained, however, that the bag does contain 2 cwts.; frequently a bag of cement weighs only 200 lbs., and half-bags are common.
The mixing of concrete is usually done by labourers on a platform of wood, but for large works a machine is employed. In hand-mixing, the ingredients should be turned over twice dry, sprinkled with clean fresh water while being turned over a third time, and finally turned over a fourth time. The mixture should at once be deposited in position, and moderately pounded. Foundations more than about 15 inches thick should l>e deposited in two or more layers, unless several gangs of men are employed in mixing and depositing the concrete.