This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by H. Y. Margary)
Before the foundation of any building can be designed the nature of the soil on the site must be inquired into. Soils are practically infinite in their variety, both with regard to their composition and their weight-resisting properties; but those more commonly met with are rock, chalk, clay, gravel, sand, silt, or quicksand, and various combinations of these.
Soils may be classified according to their behaviour under the influence of heavy loads, under the following heads: -
1. Practically Incompressible Soils.
2. Compressible Soils.
3. Soils which escape Laterally.
1. Practically Incompressible Soils include those which can only be worked by a pick or by blasting, such as rocks, stony earth, hard compact clay, etc., which enable building to be commenced directly upon them.
2. Compressible Soils comprise such soft soils as ordinary yellow and blue clay, common earth, marshy soils, etc., which, when built upon, require wide artificial rafts or "spread foundations " under the walls to distribute the pressure.
3. Soils which escape Laterally are such as sand, loose gravel, and silt, which would squeeze out from under heavy buildings unless some means were taken to confine or consolidate it to a given area.
Tables exhibiting the safe bearing powers of various kinds of soil have been compiled by various authorities, but these are of very little use in practice owing to the enormous variations in bearing powers of soils under varying conditions. A very satisfactory way of obtaining information concerning the nature of the soil upon which it is desired to build is to inquire among builders who have erected buildings on adjoining lots; but even this is not always satisfactory, as soils will vary within a comparatively short distance, particularly in the manner in which they overlie one another.
If no reliable information can be found concerning the soil, borings should be made at intervals across the site, and if the samples brought to the surface by this means indicate either a dangerous subsoil or a tilt in the strata, tests of the bearing capacity should be taken in the following manner: -
Excavate a trench and level the bottom at the depth at which it is proposed to form the foundation, and lay down four blocks of a definite area in square feet. On top of these build a strong platform and drive a stake into the ground alongside one edge, marking on it the top level of the platform. Gradually load up the platform with bricks, pig iron or other suitable material, and note when the platform begins to sink away from the mark on the stake. The load is measured and the ultimate resistance per square foot is determined and divided by a factor of safety varying from 10 to 25, according to the nature of the soil.
Before any important building is commenced the bearing capacity of the soil should be tested, and the larger the scale of the experiment the greater - as a rule - is the result thereof. A convenient size for the platform is 3 by 3 feet, or 9 square feet in area, as it corresponds with the bearing width of an ordinary wall, and is easily loaded and calculated. If the four angle blocks are made 6 by 6 inches, these have a total bearing surface of one square foot; and thus it comes about that the total load on the platform is the load per square foot borne by the soil.
The term "foundation" as applied to a wall having footings is, throughout this work, taken to mean the artificially formed support on which the footings of the wall rest. Where such foundations are used the term "foundation-bed" is employed to distinguish the surface upon which the foundation rests.
When designing foundations the following principles, which are applicable to all cases and all materials, should be strictly adhered to: -
1. They should be proportioned so as to ensure uniform settlement.
2. They should be perpendicular to the pressure brought to bear upon them.
3. They should be sufficiently large to resist that pressure.
4. They should be of some durable material.
1. Uniformity of Settlement. - No attempt need be made in designing footings to prevent settlement as this is not possible, but the footings should be so proportioned that the amount of settlement should be reduced to a mininum, and, what is more important, every precaution should be taken that the settlement may be uniform. To achieve this desirable object the causes of unequal settlement must be understood.
The most frequent cause of unequal settlement is due to the unequal loading of the foundations. For instance, if a tower is built on to a lower building its tendency is to settle to a greater extent than the less heavy portion of the building, and unsightly cracks appear on the face of the structure. To prevent this the foundation under the tower should be large enough to produce the same pressure per square inch as that under the lower walls. But even this is not always satisfactory, and a better plan in such a case is to avoid bonding the two portions of the building, preferably disconnecting them by means of a slip joint as shown in Fig. 53. Another frequent cause of unequal settlement is illustrated at A in Fig. 54, where it will be seen that the weight of the structure is brought down the piers upon the ends of the foundations, while the upward resistance of the foundation-bed causes the fracture here shown. The remedy for this is to isolate the foundation for each pier as shown at B in the same Figure. In town sites it is frequently impossible to build footings on both sides of walls next to walls of surrounding properties, when the centre of gravity of the wall falls away from the centre of resistance of the foundation-bed as in Fig. 55, with the result that the walls tend to tilt outwards, as shown by the dotted lines. This tendency cannot very well be prevented, but every precaution should be taken to prevent failure by anchoring the walls together and stepping the footing to a steep angle - never less than 60 degrees. An additional "heel" of concrete, as shown hatched in the illustration, is a great safeguard against any tendency to overturn and fracture. The modern method of overcoming this difficulty is by the use of cantilever foundations, as explained in Part II. of Volume IV.
Failure is often caused by foundations being placed upon soils with different bearing capacity, such as rock and soft soil. If the soft portion is small in extent it may be excavated and the fissure in the rock filled up solid with cement concrete, or it may be spanned by an arch (as Fig. 56), a girder, or a concrete lintel, as in Fig. 56; but if it is very large it is better to connect the portion of the structure by means of a slip joint similar to that shown in Fig. 53, and to proportion its foundation to the bearing capacity of the soft soil.
2. Direction of Pressure on Foundations. - The foundations of walls which are only influenced by vertical loads should be horizontal, while the foundation to walls subject to pressures from arches or retained earth should be placed perpendicularly to the resultant oblique pressure in order to prevent the foundations from sliding on their beds. This fact, though obvious, is one which is frequently overlooked in ordinary building works, though it is well recognised by engineers.
3. Size of Foundations. - The foundations should, of course, be of such a size that the load brought upon the foundation-bed per unit of its area may not be greater than what it will safely bear. It is also necessary to apply this principle to the footings of walls, which should be proportioned to the bearing capacity of the material of which the foundations are composed.
4. Durability of Foundations. - Both the foundations and the foundation-beds should be of some material which is of an unalterable nature; that is to say, such as will not decay or disintegrate on prolonged exposure to the atmosphere.
Frost is one of the greatest enemies of foundations, which should be sufficiently deep to be beyond its action, - about 3 feet is considered sufficient in England except for clay soils, which are frequently split up by fissures both during frost and in times of extreme heat and drought. A depth of 4 feet is, however, usually considered to be sufficient even with such a soil.