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FOUNDATIONS

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Originally appearing in Volume V10, Page 741 of the 1911 Encyclopedia Britannica.
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FOUNDATIONS , in See also:

building. The See also:object of foundations is to distribute the See also:weight of a structure equally over the ground. In the construction of a building the weights are concentrated at given points on piers, columns, &c., and these foundations require to be spread so as to reduce the weight to an See also:average. In the preparation of a See also:foundation care must be taken to prevent the lateral See also:escape of the See also:soil or the See also:movement of a See also:bed upon sloping ground, and it is also necessary to provide against any damage by the See also:action of the See also:atmosphere. The soils met with in See also:ordinary practice, such as See also:rock, See also:gravel, See also:chalk, See also:clay and See also:sand, vary as to their capabilities of bearing weight. There is no See also:provision in any See also:English building acts as to the load that may be placed on any of these soils, but under the New See also:York Building See also:Code it is provided that, where no test of the sustaining See also:power of the soil is made, different soils, excluding mud, at the bottom of the footings shall be deemed to safely sustain the following loads to the superficial See also:foot: per sq. ft. Soft clay . . 1 ton. Ordinary soft clay and sand, together in layers, wet and springy . . 2 tons. See also:Loam, clay or See also:fine sand, See also:firm and dry . 3 tons.

Very firm coarse sand, stiff gravel or hard clay 4 tons. A comparison of the pressure exerted on an ordinary founda- tion by the walls of the several thicknesses and heights provided for by the See also:

London Building See also:Act of 1894, and a See also:corn- Load on parison of a few of the See also:principal authorities, will be tonnda- tlon found useful in helping us to arrive at a decision as to . what can safely be allowed. Take as an example a See also:wall of the warehouse class, 70 ft. high, whose See also:section at the See also:base for a height of 27 ft. is 22 bricks thick (or 222 in.), and for the same distance in height again is 2 bricks thick (or 18 in.), the See also:remainder to the See also:top being 12 bricks thick (or 14 in.). The weight of See also:brickwork per foot run of such a wall is 4.05 tons on any See also:area of 3.75 ft. super. of brickwork. According to the act the See also:concrete is to project 4 in. on each See also:side; we have then an additional area of •66 ft. super. to add, thus making the See also:total foundation area of each foot run of wall 4.41 ft. super. to take a weight of 4.05 tons or nearly a ton per foot super. (viz. .9 ton.) Another See also:factor must, however, be taken into See also:consideration, viz. the weight distributed from the loaded See also:floor and from the roof. In this See also:case there would be at least six floors, and the entire weight could hardly be taken at less than 6 tons, which would give a total weight of 10.05 tons on an area of 4.41 ft. super. or a load of 2.28 tons per foot super. This is on the See also:assumption that no extra weight has been thrown on the foundations by openings or piers, or by girders, &c., in which case, in addition to the See also:work being executed in See also:cement, the foundations should be increased in area. Piers always involve a See also:great increase of weight on the foundations, and in very many instances this increased weight, instead of being provided for by increasing the area of the foundations and so reducing the weight per foot super., is only partly met by the improper method of merely increasing the See also:depth of the concrete, while keeping the same See also:projection of concrete See also:round the footings as for the walls. As an example take an See also:iron See also:column to carry a safe load of 8o tons, See also:standing on a York See also:stone template, and in turn supported by a See also:brick See also:pier 221 in. square.

In this case we should have, after allowing for the projection of concrete on either side, an area of 4 ft. 5 in. square, or 19.6 ft. super., and this would give a pressure of 4.1 tons per foot on the foundations, or almost twice as much as in the previous example of a warehouse wall. Here, instead of increasing the depth of the concrete, it would be necessary to increase its width; if it were made 6 ft. square, we should have an area of 36 ft. super. to take the 8o tons, and thus the pressure would only be 2.2 tons per foot, and the cost of the foundation be much the same. If we compare a section of wall of the dwelling-See also:

house class, as prescribed by the London Building Act, we find that, taking a wall 50 ft. high and having a thickness at base of 222 in. as for the warehouse wall to which we have referred, we have a wall weighing 3.75 tons per foot super. on an area of 4.41 feet super., or .85 ton per foot without weight of floors and roof as against the •9 ton in the warehouse example. To this must be added the weight of, say, 5 floors and roof at a total of 3 tons per foot run of wall, and we then have an aggregate of 6.75 tons per foot run and 1.50 tons per foot super. as against 2.28 tons in the warehouse class. If we turn from the act to See also:text-books we find that See also:Colonel See also:Seddon in the Aide Memoir gives the load which ordinary foundations will See also:bear as a safe load per foot super. as follows: tons. Rock, moderately hard . . 9 Rock of strength of See also:good concrete 3 Rock, very soft . 1.8 Firm See also:earth . r to 12 Hard clay I to 12 Clean dry gravel and clean See also:sharp sand prevented from spreading sideways I to IZ Most of the work in London may be classed under one of the latter heads, and according to this table we have, when we erect walls in accordance with the building act, to overload our foundations. As to the possibility of spreading weights, we have as an example the See also:chimney at Adkin's See also:Soap See also:Works in See also:Birmingham,312 ft. high, so arranged that its pressure on the foundations is only 12 tons per foot super.; also the great St Rollox chimney at See also:Glasgow, which has a pressure of 1-1 tons; the weight of the Eiffel See also:Tower (7500 tons) is so spread over 4 bases, each 130 ft. square, that the pressure is only • 117 ton, or 21 cwt., per foot super. The Tower See also:Bridge has a load of r6 tons per foot on the See also:granite bed under the columns of towers, reduced by spreading to an actual pressure on the clay foundation of 4 tons. The piers under the See also:Holborn Viaduct have a load of 2; tons only, those of the Imperial See also:Institute 24 tons, and those of the destructor cells and chimney See also:shaft at Great See also:Yarmouth 4 tons 64 cwt. per foot super.

From these various examples it would appear that on See also:

sound clay or gravel foundation a load of from 24 to 4 tons may be employed with safety. One of the first and most important requirements in preparing drawings for a large building is to ascertain the nature of the sub-soil and strata at different levels over the proposed site, 7y9a1 so as to be able to arrange the footings accordingly at the See also:boring. various depths and to decide as to the various forms and methods to be employed. For this purpose trial holes or borings are sunk until a suitable bed or bottom is found, upon which the concrete foundation may safely be put. If no such solid bottom is found, as often happens near the See also:water side, See also:special foundations must be employed, such as See also:dock, gridiron, See also:cantilever and See also:pile foundations, &c., all of which will be described hereafter. As examples of the varying subsoils we may mention the following, in which will be noticed the great depths dug before getting through the made ground: At the See also:Bank of See also:England there were 22 ft. of made ground resting on 4 ft. of gravel. Some of the made ground was of See also:ancient date, and preserved See also:relics of See also:Roman occupation. In some parts the subsoils have been excavated for See also:ballast or gravel, as at See also:Kensington, or for brick earth, as at Highbury, and the pits filled in with rubbish. Rock, which forms an excellent and unchanging foundation in one situation, may prove a dangerous foundation in another. Thus chalk forms a good See also:limestone foundation in certain positions, but when it dips towards a slope or a cliff with an outcrop of the See also:gault or underlying clay, it is a very unsuitable foundation for any building, as the landslips in the Isle of See also:Wight and on the See also:Dorsetshire See also:coast bear See also:witness. A boring made in See also:Tallis See also:Street, near the See also:Thames See also:embankment, showed: (I) 18 in. ballast, dirty; (2) 6 in. See also:greensand, wet and dirty; (3) 2 ft. See also:peat clay; (4) 6 in. greensand; (5) 52 ft. peaty See also:bog; (6) 9 ft. See also:running sand; and (7) 4 ft. clean ballast, resting at a depth of 23 ft. below the ground See also:line upon See also:blue clay. A boring at Highbury New See also:Park gave: (I) 2 ft. made ground, (2) 18 ft. loam, (3) 9 ft. sand, (4) 4 ft. peat, and (5) 8 ft. gravel and sand. These examples show that while trial holes should always be made before designing a foundation, to ascertain the nature of the subsoil, care must be taken not to calculate upon uniformity.

Thus at the See also:

block 2 of the See also:admiralty See also:extension new buildings (London), one of the trial holes upon the See also:south-See also:west side of the old buildings showed the clay to be about 292 ft. below the See also:surface of the ground, while actual excavation proved the See also:dip of the clay to be such that in the See also:execution of the new building it became necessary to underpin the See also:north-west corner of the old building at the deepest See also:part 42 ft. below the ground. The foundations of block I of the new admiralty buildings are placed in a dock, built upon the London clay at a depth of 30 ft. in solid concrete 6 ft. thick. At the Hotel See also:Victoria, in See also:Northumberland See also:Avenue (London), the various subsoils are as follows: (I) 382 ft. made ground clay and gravel mixed, (2) 4 ft. gravel and sand, (3) 6 ft. rising sand; (4) 2 ft. fine ballast, and at a depth of 5o ft. blue clay. At the south end the clay was 43 ft. down and at the north end 37 ft. The front wall was constructed on a concrete bed 9 ft. wide. The site was surrounded by a similar wall of concrete about 6 ft. wide, forming a See also:species of boxes, and the whole was covered with a depth of 6 ft. of concrete upon which the walls were raised. The foundation for 53 See also:Parliament Street, where running sand was encountered, was constructed with See also:short piles, 7 or 8 ft. See also:long and 6 in. diam., pointed and placed as See also:close together as possible over the whole foundation, the tops were then sawn off level, and a concrete raft, 7 or 8 ft. thick, was built over the whole area. At the Institution of See also:Civil See also:Engineers, Great See also:George Street, See also:Westminster, the foundations to the two party walls upon each side of the building were carried down about 22 ft. below the See also:pavement level, that on the west side being 22 ft. deep and that on the See also:east side 24 ft. The London Building Act and the See also:model by-See also:laws prohibit the erection of buildings on sites that have been used as " shoots " for faecal See also:matter or See also:vegetable refuse, and in such cases the Constrncobjectionable material must be removed See also:prior to the See also:Con commencement of building operations, and the holes from which it was taken filled up with dry brick or other rubbish well rammed. Foundations are usually executed by excavators or navvies, and the tools and implements used are boning rods, level pegs; lines, spirit level, pickaxe, various shovels, See also:wheel-See also:barrow, rammer or punner, &c. In digging the ordinary trenches and excavations, should the ground be loose, planking and strutting have to be employed. This consists of rough boarding put along the sides of the trenches and wedged tight with waling pieces and struts; this work is done by navvies.

See also:

Figs. I and 2 show the See also:general forms of planking and strutting for the different soils. In very large works of excavation in soft soil a See also:steam digger is used for the bulk of the work. It consists of a large See also:steel bucket with a cutting edge; this is lowered by means of a See also:crane into the excavation, and on being withdrawn cuts off a portion of soil which is hoisted and deposited in carts for re- moval to any desired posi- tion within the See also:radius com- manded by the crane. The work of trimming the exca- vation to a See also:regular shape must always be done by , i See also:manual labour. rbvn to Concrete for filling into 1of4g°a41 the foundations is usually mixed by navvies; for large works it is sometimes mixed by machinery. In See also:order that the work of excavating and constructing the foundations may proceed in a water-logged site, pumps have to be employed, and where the inrush of water is great it is usual to sink a sump hole See also:lower than the depth required for the foundations, and to use a steam See also:pump kept going See also:day and See also:night. The foundation of a wall is required to be as follows in accordance with the London Building and See also:Amendment Acts: " The projection of the bottom of the footings of every wall on each side of the wall shall be at least equal to See also:half of the thickness of the wall at its base, unless an adjoining wall interferes, in which case the projection may be omitted where that wall adjoins, and the diminution of the footings of every wall shall be formed in regular offsets and the height from the bottom of such footing to the base of the wall shall be at least equal to two-thirds of the thickness of the wall at its base." (See BRICKWORK.) The base of a wall is the thickness above the footing; the footing is the brickwork built directly on the top of the concrete and diminishing in width in every course. Thus: " The projection of the bottom footing to be equal to one-half the thickness of wall on both sides " means that a 132-in. wall would require to have three courses of footings, the bottom one being 27 in. wide. " The height from the J^`' bottom of such footing to the base of the wall shall be at least equal to two-thirds the thickness of wall at its base " means that in the case of a 132-in. wall the height of footings would have to be 9 in., or three courses of brick-work, each measuring 3 in. The New York Building Code enters more fully into the require- ments for the foundation of walls as regards depth than that in use in London. Section 25, Part 5, requires that every building, except buildings erected upon solid rock, or upon wharves and piers on the water front, shall have foundations of brick, stone, iron or concrete laid not less then 4 ft. below the surface of the earth, on the solid ground or level surface of rock, or upon piles or ranging timbers when solid earth or rock is not found.

Piles intended to sustain a wall, pier or See also:

post, shall be spaced not more than 36 in. nor less than 20 in. on centres; they must be driven to a solid See also:hearing if practic- able, and their number must be sufficient to support the super- structure proposed. No pile shall be used of less dimensions than 5 in. at the small end and to in. at the See also:butt for short piles, or.piles 20 ft. or less in length. No pile shall be weighted with a load exceed- See also:ing 40,000 Ib. When a pile is not driven to refusal, its safe sustaining power shall be determined by the following See also:formula: twice the weight of the See also:hammer in tons multiplied by the height of the fall in feet divided by the least penetration of pile under the last See also:blow in inches plus one. There are also further requirements as to piles, &c., and the See also:commissioner of buildings must be notified when the piles are to be driven. The New York Code, Section 26, further goes on to say that foundation walls shall be constructed to include all walls and piers built below the curb level or nearest tier of beams to the curb, to serve as supports for the walls, piers, columns, girders, posts or beams. Foundation, walls shall be built of stone, brick, See also:Portland cement concrete, iron or steel. If built of See also:rubble stone or Portland cement concrete, they shall be at least 8 in. thicker than the wall above them to a depth of 12 ft. below the curb level, and for every additional 10 ft. or part thereof deeper, they shall be increased 4 in. in thickness. If built of brick, they shall be at least 4 in. thicker than the wall next above them to a depth of 12 ft. below the curb level, and for every additional to ft. or part thereof deeper, they shall be increased 4 in. in thickness. The footing or base course shall be of stone or concrete, or both, or of concrete and stepped up brickwork of sufficient thickness and area to bear safely the weight to be imposed thereon. If the footing or base course be of concrete, the concrete shall not be less than 12 in. thick; if of stone, the stones shall not be less than 2X3 ft. and at least 8 in. in thickness for walls, and not less than so in. in thickness if under piers, columns or posts. The footing or base course, whether formed of concrete or stone, shall be at least 12 in. wider than the bottom width of walls, and at least 12 in. wider on all sides than the bottom width of said piers, columns or posts.

If the superimposed load is such as to cause undue trans-See also:

verse See also:strain on a footing projecting 12 in., the thickness of such footing is to be increased so as to carry the load with safety. For small structures and for small piers sustaining See also:light loads the cornniissioner of buildings having See also:jurisdiction may, in his discretion, allow a reduction in the thickness and projection specified for footing or base courses. All base stones shall be bedded and laid crosswise, edge to edge. If stepped-up footing of brick is used in See also:place of stone above the concrete, the offsets if laid in single courses shall each not exceed 11 in., or, if laid in See also:double courses, then each shall not exceed 3 in. offsetting the first course of brickwork back one-half the thickness of the concrete base, so as properly to distribute the load to be imposed thereon. It will be seen by the foregoing that the See also:American acts are far more extensive than in London. The London Building Act mentions that the footings of a wall shall See also:rest upon the solid ground or concrete or upon other solid substructure. The building act amendment says: " The foundations of the walls of every house or building shall be formed of a bed of good concrete not less than 9 in. thick, and projecting at least 4 in. on each side of the lowest course of footings." Various Types of Foundations.—The most natural foundations for walls are those constructed where the walls are built directly upon the ground; this is only possible where the ground is very hard or consists of rock, and in either of these cases the ground is simply levelled and the building commenced. The next and most universally recognized method, which might safely be said to be adopted in 95 % of all See also:modern buildings, is the See also:system of placing a bed of concrete under the walls, digging trenches where the walls are to come until a solid bottom is reached, and in these laying the concrete. The London Building Act requires this concrete bed to be at least 4 in. wider than the bottom course of footings on each side of the wall, but it is generally made 6 in. wider on each side and in general circumstances the depth of the concrete is varied according to the weight placed upon it. Where a site is in close proximity to a See also:river or old water-course, &c., where deep basements are excavated, or where the ground lies See also:low, naturally water is met with, and where water is the ground is soft. It is here that special foundations are required. In certain cases it is necessary to use concrete legs or See also:stilts.

These are placed in such positions as to take the weights of the building, and sunk to depths of 40 ft. more or less as the case may Concrete require according to the nature of the ground; and on the tops of these stilts concrete See also:

arches or lintels are fie"' turned over (fig. 3). As an example of the See also:stilt principle, s Ita or mention may be made of some premises at See also:Stratford and a See also:church at South See also:Bermondsey, London, in which concrete piers were sunk at 12 ft. centres apart and 42 ft. square, in pot holes dug out of made ground; then concrete arches were formed over the intervening untrustworthy ground with a minimum thickness of 18 in. or the piers were connected by concrete lintels 3 ft. thick in which steel joists were embedded. At See also:Sion See also:College, Victoria Embankment, London, the foundations were formed with cement concrete stilts or piers 8 ft. square, and going down to the London clay; from the tops of these stilts brick arches were turned, spanning the spaces between the piers, and upon these arches the walls were built. Pile foundations, used in the case of soft ground, for small works, consist either of stout See also:scaffold poles or of timbers varying from 6 in. to 12 in. square according to requirements (fig. 4). The bottom ends of these timbers have an iron See also:shoe with a point, so as to be easily driven into the ground, and the tops of the timbers have an iron See also:band round, so that when the timbers are being driven in the band prevents them from splitting (fig. 5). The methods of See also:driving these piles are various. The usual See also:plan is to erect a tempor- ary structure, upon one side of which is a See also:guide path faced with See also:sheet-iron so as to give a smooth See also:face.

End of Article: FOUNDATIONS

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