1911 Encyclopædia Britannica/Foundations
FOUNDATIONS, in building. The object of foundations is to distribute the 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 average. In the preparation of a foundation care must be taken to prevent the lateral escape of the soil or the movement of a bed upon sloping ground, and it is also necessary to provide against any damage by the action of the atmosphere. The soils met with in ordinary practice, such as rock, gravel, chalk, clay and sand, vary as to their capabilities of bearing weight. There is no provision in any English building acts as to the load that may be placed on any of these soils, but under the New York Building Code it is provided that, where no test of the sustaining 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 foot:
|per sq. ft.|
|Soft clay||1 ton.|
|Ordinary soft clay and sand, together in layers, wet and springy||2 tons.|
|Loam, clay or fine sand, 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 foundation by the walls of the several thicknesses and heights provided for by the London Building Act of 1894, and a comparison of a few of the principal authorities, will be found useful in helping us to arrive at a decision as to Load on foundation. what can safely be allowed. Take as an example a wall of the warehouse class, 70 ft. high, whose section at the base for a height of 27 ft. is 21 bricks thick (or 221 in.), and for the same distance in height again is 2 bricks thick (or 18 in.), the remainder to the top being 11 bricks thick (or 14 in.). The weight of brickwork per foot run of such a wall is 4.05 tons on any area of 3.75 ft. super. of brickwork. According to the act the concrete is to project 4 in. on each side; we have then an additional area of .66 ft. super. to add, thus making the 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 factor must, however, be taken into consideration, viz. the weight distributed from the loaded floor and from the roof. In this 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 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 work being executed in cement, the foundations should be increased in area. Piers always involve a 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 depth of the concrete, while keeping the same projection of concrete round the footings as for the walls. As an example take an iron column to carry a safe load of 80 tons, standing on a York stone template, and in turn supported by a brick 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 80 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-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 221 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 text-books we find that Colonel Seddon in the Aide Memoir gives the load which ordinary foundations will bear as a safe load per foot super. as follows:
|Rock, moderately hard||9|
|Rock of strength of good concrete||3|
|Rock, very soft||1.8|
|Firm earth||1 to 11|
|Hard clay||1 to 11|
|Clean dry gravel and clean sharp sand prevented from spreading sideways||1 to 11|
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 chimney at Adkin’s Soap Works in Birmingham, 312 ft. high, so arranged that its pressure on the foundations is only 11 tons per foot super.; also the great St Rollox chimney at Glasgow, which has a pressure of 13 tons; the weight of the Eiffel 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 Bridge has a load of 16 tons per foot on the 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 Holborn Viaduct have a load of 21 tons only, those of the Imperial Institute 21 tons, and those of the destructor cells and chimney shaft at Great Yarmouth 4 tons 63 cwt. per foot super. From these various examples it would appear that on sound clay or gravel foundation a load of from 21 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 subsoil and strata at different levels over the proposed site, so as to be able to arrange the footings accordingly at the various depths and to decide as to the various forms and Trial borings. 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 water side, special foundations must be employed, such as dock, gridiron, cantilever and 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 Bank of England there were 22 ft. of made ground resting on 4 ft. of gravel. Some of the made ground was of ancient date, and preserved relics of Roman occupation. In some parts the subsoils have been excavated for ballast or gravel, as at 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 limestone foundation in certain positions, but when it dips towards a slope or a cliff with an outcrop of the gault or underlying clay, it is a very unsuitable foundation for any building, as the landslips in the Isle of Wight and on the Dorsetshire coast bear witness. A boring made in Tallis Street, near the Thames embankment, showed: (1) 18 in. ballast, dirty; (2) 6 in. greensand, wet and dirty; (3) 2 ft. peat clay; (4) 6 in. greensand; (5) 51 ft. peaty bog; (6) 9 ft. running sand; and (7) 4 ft. clean ballast, resting at a depth of 23 ft. below the ground line upon blue clay. A boring at Highbury New Park gave: (1) 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 block 2 of the admiralty extension new buildings (London), one of the trial holes upon the south-west side of the old buildings showed the clay to be about 291 ft. below the surface of the ground, while actual excavation proved the dip of the clay to be such that in the execution of the new building it became necessary to underpin the north-west corner of the old building at the deepest part 42 ft. below the ground. The foundations of block 1 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 Victoria, in Northumberland Avenue (London), the various subsoils are as follows: (1) 381 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 50 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 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 Parliament Street, where running sand was encountered, was constructed with short piles, 7 or 8 ft. long and 6 in. diam., pointed and placed as 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 Civil Engineers, Great George Street, Westminster, the foundations to the two party walls upon each side of the building were carried down about 22 ft. below the pavement level, that on the west side being 22 ft. deep and that on the east side 24 ft.
The London Building Act and the model by-laws prohibit the erection of buildings on sites that have been used as “shoots” for faecal matter or vegetable refuse, and in such cases the objectionable material must be removed prior to the commencement of building operations, and the holes Construction. 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, wheel-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. Figs. 1 and 2 show the general forms of planking and strutting for the different soils.
In very large works of excavation in soft soil a steam digger is used for the bulk of the work. It consists of a large steel bucket with a cutting edge; this is lowered by means of a crane into the excavation, and on being withdrawn cuts off a portion of soil which is hoisted and deposited in carts for removal to any desired position within the radius commanded by the crane. The work of trimming the excavation to a regular shape must always be done by manual labour.
Concrete for filling into the foundations is usually mixed by navvies; for large works it is sometimes mixed by machinery.
In 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 lower than the depth required for the foundations, and to use a steam pump kept going day and night.
The foundation of a wall is required to be as follows in accordance with the London Building and 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 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 131–in. wall would require to have three courses of footings, the bottom one being 27 in. wide. “The height from the 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 131–in. wall the height of footings would have to be 9 in., or three courses of brickwork, each measuring 3 in.
The New York Building Code enters more fully into the requirements 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 post, shall be spaced not more than 36 in. nor less than 20 in. on centres; they must be driven to a solid bearing if practicable, and their number must be sufficient to support the superstructure proposed. No pile shall be used of less dimensions than 5 in. at the small end and 10 in. at the butt for short piles, or piles 20 ft. or less in length. No pile shall be weighted with a load exceeding 40,000 ℔. When a pile is not driven to refusal, its safe sustaining power shall be determined by the following formula: twice the weight of the hammer in tons multiplied by the height of the fall in feet divided by the least penetration of pile under the last blow in inches plus one. There are also further requirements as to piles, &c., and the 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, Portland cement concrete, iron or steel. If built of 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 10 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 2 × 3 ft. and at least 8 in. in thickness for walls, and not less than 10 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 transverse 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 light loads the commissioner of buildings having 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 place of stone above the concrete, the offsets if laid in single courses shall each not exceed 11 in., or, if laid in 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 American acts are far more extensive than in London. The London Building Act mentions that the footings of a wall shall 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 modern buildings, is the 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 river or old water-course, &c., where deep basements are excavated, or where the ground lies 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 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 require according to the nature of the ground; and on the tops of these stilts concrete arches or lintels are Concrete piers, legs, or stilts. turned over (fig. 3). As an example of the stilt principle, mention may be made of some premises at Stratford and a church at South Bermondsey, London, in which concrete piers were sunk at 12 ft. centres apart and 41 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 Sion 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 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 shoe with a point, so as to be easily driven into the ground, and the tops of the timbers have Pile foundations. an iron band round, so that when the timbers are being driven in the band prevents them from splitting (fig. 5). The methods of driving these piles are various. The usual plan is to erect a temporary structure, upon one side of which is a guide path faced with sheet-iron so as to give a smooth face. Up and down this guide path a heavy iron weight, called a monkey, is worked; the monkey is hoisted to the top of the guide path by means of a crab worked by hand or steam, and when released descends with a good force, and so drives the piles into the ground. The monkey usually weighs from 2 cwt. to 10 cwt. and is allowed a drop of 15 to 40 ft.
Piles are driven all round under the walls at varying intervals or under piers where the weights of a building are to be concentrated. In the erection of the Chicago public library four Norway pine piles, each with an average diameter of 13 in., were driven to a depth of 521 ft. and loaded with a dead load of 50.7 tons per pile for a period of two weeks, and no settlement taking place 30 tons per pile was adopted as a safe load. The following are some examples of loads used in practice: passenger station, Harrison Street, Chicago, piles 50 ft. in length, each carrying 25 tons; elevator, Buffalo, N.Y., piles 20 ft. in length, weight 25 tons; Trinity church, Boston, 2 tons; Schiller building, Chicago, 55 tons per pile, but in this case the building settled considerably. All timber grillage and the tops of all piles should be kept below the lowest water level, and be capped with concrete or stone. In Boston it is obligatory to cap with blocks of granite.
Another form of foundation takes the shape of Portland cement concrete blocks, and is used chiefly for bridges and in marshy land, &c. In some cases cylinders of brickwork are built, and the centres are filled with blocks of concrete and grouted in. The Yarmouth destructor cells and chimney shaft Concrete piles. were built in this way; the cylinders were constructed of 9 in. brickwork built in Portland cement, the lower 4 ft. being encased in a wooden drum with cutting edge sunk into the gravel and sand at least 2 ft. The cylinders were sunk by the aid of a grab, the bottom being levelled and the concrete blocks laid by a diver. Use is also made of piles consisting of Portland cement concrete having steel rods embedded in it, and provided with iron shoes and head for driving (fig. 6).
|Fig. 5.||Fig. 6.||Fig. 7.||Fig. 8.|
Cast iron screw piles (fig. 7) used in very loose sandy soils, consist of large hollow cast iron columns with flat screw blades cast on the lower ends. The projection of this screw from the pile may vary from 9 in. to 18 in. with a pitch of from one-quarter to one-half of the projection, the blade making a little over one turn round the shaft. For most requirements a diameter of screw from 31 to 41 ft. will be found sufficient, a sandy foundation requiring the largest. The lower end of the tube is generally left open, the edge being bevelled and occasionally provided with teeth to assist in cutting into and penetrating the soil.
Another system of piling known as sheet piling (fig. 8), consists in driving piles into the ground at intervals, and between these, also driven into the ground, are timbers measuring 3 in. by 9 in., which form a wall to keep the soft earth up under the building. In this way the earth is prevented from spreading out and so causing the building to settle unevenly.
Another kind of foundation, known as plank foundation (fig. 9), consists of elm planks, about 9 in. by 3 in. laid across the trench and spiked together; on the top of these are laid similar planks but at right angles to the last, and upon the platform thus formed the wall is built. This method Plank foundations. is used in soft ground.
Caissons are usually employed by engineers for the construction of the foundations of bridge piers, but instances of their use in foundations for buildings are to be found in the American Surety and the Manhattan Life Insurance buildings, New York City. The latter building is 242 ft. high to the parapet, Caissons. and the dome and tower rise 108 ft. higher. The building is carried on 16 solid masonry piers, taken down 54 ft. below the street level to solid rock, and these piers support the 34 cast iron columns upon which the building is erected. The piers to each building were constructed by the pneumatic caisson process (see Caisson).
A good plan for foundations when the ground is loose and sandy is to build upon wells of brickwork, a method which has been successfully practised in Madras. The wells are made circular, about 3 ft. in diameter and one brick thick. The first course is laid and cemented together on the Well foundations. surface of the ground when it is dry, and the earth is excavated inside and round about it to allow it to sink. Then another is laid over it and again sunk. The well is thus built downwards. The brickwork is sunk bodily to a depth of 10 ft. or more, according to building to be erected upon it, and the interior is filled up with rubble work. All the public buildings at Madras were erected upon foundations of this kind. Well foundations were employed under the city hall, Kansas City, and the Stock Exchange, Chicago.
Coffer dams are wooden structures used to keep back the water whilst putting in foundations on the waterside, and are constructed with two rows of timbers, 12 in. square as piles spaced about 6 ft. apart, and filled in between with a double row of 2 in. or 3 in. boards, the space between the rows being Coffer dams. packed with clay puddle (fig. 10). The general rule for the thickness of a coffer dam is to make it equal to the depth of water. An interesting example of a coffer dam is that at the Keyham dock extension, where piles varied in length from 65 ft. to 85 ft. They were driven in a double row 5 ft. apart, and over 13,000 were used.
Dock foundations are constructed after the fashion of a large concrete tank, and are adapted to large sites where a difficulty arises as to the ingress of water. They are considered the best method of constructing a building on soft ground and of keeping a building dry (fig. 11). This type of Dock foundations. foundation was used at the new colonial office, Whitehall, London, and the new admiralty buildings at St James’s Park, London. A few buildings treated after the style of a dock, but in some instances without the enclosing walls, are the following: At the admiralty buildings already mentioned a concrete retaining wall completely surrounds the exterior below the ground, and is joined up to the underpinning work; the whole site being covered with concrete 6 ft. thick, a huge tank is formed of an average inside clear depth of 20 ft. in which the basements are built. The new “Old Bailey” buildings in Newgate Street, London, are constructed on a concrete table 5 ft. thick, as also are the Army and Navy Auxiliary Stores, Victoria Street. At Kennet’s Wharf, near Southwark Bridge, a concrete table, 8 ft. thick, was spread all over the site, with an extra thickness under the walls. Foundations formed similarly to dock foundations, but in addition having steel joists and rods inserted in the thickness of the concrete table, to tie the whole together, are known as gridiron foundations.
In the Hennebique concrete system, all beams, &c., are formed with small rods and then surrounded with concrete; it is designed for floors and for spreading the weight of a building over an extended foundation on soft ground.
Where a heavy wall is to be built against an old one and there is not sufficient room for the foundations, the plan is adopted of building pier foundations at some distance from the proposed new wall. On the top of these piers rest steel cantilevers over steel pin rockers upon cast Cantilever foundations. iron bedplates; the cantilevers are secured at one end to a column, while the other ends go through the full thickness of the new wall. Upon these last ends is placed a steel girder upon which the wall is built. This construction (fig. 12) has been used in America, and in the Ritz Hotel, Piccadilly, London.
Another form of cantilever foundations was employed in the case of some premises at Carr’s Lane, Birmingham, partly built over the Great Western railway tunnel (fig. 13). In this instance large piers were built below the ground at the side of the tunnel. From the tops of these piers large steel cantilevers were erected projecting over the crown of the tunnel, and on these steel girders were fixed and the building constructed upon them.
In modern Tunis, a section of which city is built on marshy ground, the subsoil is an oozy sediment, largely deposited by the sewage water from the ancient or Arab quarter of the city, which is situated on an adjacent hill. This semi-fluid mud has a depth of about Foundations in Tunis. 33 ft. To prepare the soil for supporting an ordinary house, pits from 8 ft. to 10 ft. square are excavated to a depth of about 10 ft., to the level of the ground water. A mixture is made of the excavated soil and powdered fat lime, procured from clinkers and unburnt stone from the lime-kilns, which soon crumbles to fine dust when exposed to the air. The mixture is thrown into pits in layers about 12 in. thick and rammed down for a very long time by specially trained labourers. A gang of 15 or 20 men will work at least 10 or 12 days ramming for the foundations of a moderate-sized house. An extremely hard bed is thus obtained, reaching to within 18 in. of the surface of the ground, and on this artificial bed the foundations of the building are laid. Although this method of construction is crude, it is stated that the practical results are superior to those obtained by using piles, concrete or other recognized methods, and in all cases the cost is much less, for labour is cheap.
A novel and interesting foundation was designed for a
signal station at Cape Henlopen, Delaware. This is built on
top of the highest sandhill at Cape Henlopen, so
that the observer may have an unobstructed
view; it rises about 80 ft. above the level of the
sea and is exposed to all Building
on sand.winds and weather, while it is absolutely required that it shall stand firmly planted in such a way that even a hurricane shall not shake it or make it tremble, since that would affect the sight of the telescope in the observatory. The usual mode of securing such a building is by means of a foundation of screw piles or of heavy timbers sunk into the sand; this method, however, has the disadvantage that if the wind shifts the sand away from around the foundation, it becomes undermined and its effect is destroyed. To avoid such an accident, recourse was had to the following design, which was considered to be cheap and at the same time to provide an effective anchorage. The building is entirely of wood; it has a cellar, above which are two rooms one above the other, and the whole is surmounted by the observatory proper. First, the ground sill is a square of 20 ft., made of yellow pine sticks mortised together and pinned with stout trunnels. The sill of the observatory is made likewise of heavy timbers, 12 ft. long. The two sills are joined together by four stout yellow pine corner posts, which in turn are mortised into both sills. The posts are 26 ft. in length. Five feet above the lower sill is the sill which supports the floor of the first room. Ten feet above this is the sill which supports the upper room. Both these sills again are mortised into the corner posts. The structure is sheathed outside with German siding, and inside with rough boards covered with felt, and again by tongued and grooved yellow pine boards. The observatory proper, octagonal in shape, is securely mortised into the top sill and covered with a corrugated iron roof conical in shape. The cellar is floored with 3 in. wood, and boarded all round on the inside of the posts. A pit was first dug in the sand about 6 ft. deep and fully 20 ft. wide on the bottom. The cellar sill was laid on this bottom, and the structure built upon it; thus the whole depth of cellar is sunk below the top of the hill or the level of the sand. The cellar was then filled up with sand and packed solid all round, consequently the building is anchored in its place by the load in the cellar, about 100 tons in weight.
|Fig. 13.—Cantilever Foundation over Railway Tunnel.|
The subject of foundations, being naturally of the first importance, is one that calls for most careful study. It is not of so much importance that the ground be hard or even rocky as that it be compact and of similar consistency throughout. It is not always that a site answers to this description, and the problem of what will be the best form of foundation to use in placing a building, more especially if that building be of large dimensions and consequently great weight, on a site of soft yielding soil, is one that is often most difficult of solution. The foregoing article indicates in a brief manner some of the obstacles the architect or engineer is required to surmount before his work can even be started on its way to completion.
Authorities.—The principal books for reference on this subject are: A Practical Treatise on Foundations, by W. M. Patron, C. E.; Building Construction and Superintendence, part i., by F. E. Kidder; Notes on Building Construction, vols. i. ii. and iii.; Aide Memoir, vol. ii., by Colonel Seddon, R.E.; Advanced Building Construction, by C. F. Mitchell; Modern House Construction, by G. L. Sutcliffe; Building Construction, by Professor Henry Adams; Practical Building Construction, by J. P. Allen. (J. Bt.)