This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Nature of Soils. The nature of the soils usually met with on building sites are: rock, gravel, sand, clay, loamy earth, "made" ground and marsh (soft wet soil).
If the soil is hard and practically non-compressible, it is a good foundation and needs no treatment; otherwise it must be carefully prepared to resist the weight to be superimposed.
Two important laws must be observed: 1. All base-courses must be so proportioned as to produce exactly the same pressure per square inch on the soil under all parts of building where the soil is the same. Where in the same building we meet with different kinds of soils, the base-courses must be so proportioned as to produce the same relative pressure per square inch on the different soils, as will produce an equal settlement (compression) in each.
2. Whenever possible, the base-course should be so spread that its neutral axis will correspond with the neutral axis of the superimposed weight; otherwise there will be danger of the foundation walls settling unevenly and tipping the walls above, producing unsightly or even dangerous cracks.
In a church the gable wall is 1' 6" thick, and is loaded (including weight of all walls, floors and roofs coining on same) at the rate of 52 lbs. per square inch. The small piers are 12" x 12" and 5' high, and carry a floor space equal to 14' x 10'. What should be the size of bast-courses, it being assumed that the soil will safely stand a pressure of 30 lbs. per square inch?
If we were to consider the wall only, we should have the total pressure on the soil per running inch of wall, 18.52= 93G lbs.
Dividing this by 30 lbs., the safe pressure, we should need 936/30 =
31,2" or say 32" width of foundation, or we should step out each side of foundation wall an amount 32-18/2 = 7" each side.
Now the load on pier, assuming the floor at 100 lbs. per square foot, would be 14 X 10 X 100 = 14000 lbs. To this must be added the weight of the pier itself. There are 5 cubic feet of brickwork (weighing 112 pounds per foot) = 5.112 = 5G0 lbs., or, including base-course, a total load of say 15000 lbs. This is distributed over an average of 144 square inches; therefore pressure per square inch under pier.
- - = 104 or, say, 100 lbs. 144
We must therefore make the foundation under pier very much wider, in order to avoid unequal settlements. The safe pressure per square inch Ave assumed to be 30 lbs.; therefore the area required would be = 15000/30 = 500 square inches, or a square about 22" x 22".
We therefore shall have to step out each side of the pier an
22 - 12 amount - - = 5". 2
The safe compressions for different soils are given in Table V, but in most cases it is a matter for experienced judgment or else experiment.
It is usual to bore holes at intervals, considerably deeper than the walls are intended to go, at some spot where no pressure is to take place, thus enabling the architect to judge somewhat of the nature of the soil. If this is not sufficient, he takes a crowbar, and, running it down, his experienced touch should be able to tell whether the soil is solid or not. If this is not sufficient, a small boring-machine should be obtained, and samples of the soil, at different points of the lot, bottled for every one or two feet in depth. These can be taken to the office and examined at leisure. The boring should be continued if possible, until hard bottom is struck.
If the ground is soft, new made, or easily compressible, experiment as follows: Level the ground off, and lay down four blocks each, say 3" x 3"; on these lay a stout platform. Alongside of platform plant a stick, with top level of platform marked on same. Now pile weight onto platform gradually, and let same stand. As soon as platform begins to sink appreciably below the mark on stick, you have the practical ultimate resistance of the foundation; this divided by 36 gives the ultimate resistance of the foundation per square inch. One-tenth of this only should be considered as a safe load for a permanent building.
Drainage is essential to make a building healthy, but can hardly be gone into in these articles. Sometimes it is also necessary to keep the foundations from being undermined.
It is usual to lead off all surface or spring water by means of blind drains, built underground with stone, gravel, loose tile, agricultural" tile, half-tile, etc. To keep dampness out, walls are cemented and then asphalted, both on the outsides. If the wall is of brick, the cementing can be omitted. Damp-courses of slate or asphalt are built into walls horizontally, to keep dampness from rising by capillary attraction. Cellar bottoms are concreted and then asphalted; where there is pressure of water from underneath, such as springs, tide-water, etc., the asphalt has to be sufficiently weighted down to resist same, either with brick paving or concrete.
Drainage of toil.
Where there are water-courses they should be diverted from the foundations, but never dammed up. They can often be led into iron or other wells sunk for this purpose, and from there pumped into the building to be used to flush water-closets, or for manufacturing or other purposes. Clay, particularly in vertical or inclined layers, and sand are the foundations most dangerously affected by water as they are apt to be washed out.
Where a very wide base-course is required by the nature of the soil, it is usual to step out the wall above gradually; the angle of stepping should never be more acute than 60°, or, as shown in Figure 39. Care must also be taken that the stepped-out courses are sufficiently wide to project well in under each other and wall, to prevent same breaking through foundation, as indicated in Figure 40.