By Adding Sand. The simplest method of improving the bearing power of a compressible soil is to spread sand or gravel or broken stone over the bed of the foundation, and pound it into the soil, thus forming a comparatively compact stratum upon which to found the structure. This method is not very effective, since at best the effect of the blow can not extend very deep, while the heavy masses of the masonry make themselves felt at great depths.
A more efficient way is to make an excavation a little larger than the proposed structure and cover the bed of the foundation with a layer of sand or gravel. The sand should be deposited in successive layers, each of which should be thoroughly tamped before laying the next. The sand should be moist, so it will pack well. Sand, when used in this way, possesses the valuable property of assuming a new position of equilibrium and stability should the soil on which it is laid yield at any of its points; and not only does this take place along the base of the sand bed, but also along its edges or sides. The bed of sand must be thick enough to distribute the pressure on its upper surface over the entire base of the trench. Some authors attempt to determine the proper thickness of the layer by assuming that the pressure is uniformly distributed over an area bounded by planes extending downward from the lower edges of the wall at the natural angle of repose of the material used for filling; but the results of such computations are worthless, since the validity of this assumption is open to serious objections. The thickness to be employed is entirely a matter of judgment, or of experiment in each particular case.
The following example, cited by Trautwine, is interesting as showing the surprising effect of even a thin layer of sand or gravel: "Some portions of the circular brick aqueduct for supplying Boston with water gave a great deal of trouble when its trenches passed through running quicksands and other treacherous soils. Concrete was tried, but the wet quicksand mixed itself with it and killed it. Wooden cradles, etc., also failed; and the difficulty was overcome by simply depositing in the trenches about two feet in depth of strong gravel." By Driving Wood Piles. If the soil is very soft, it can be consolidated to a considerable depth by driving wood piles, for which purpose many small ones are preferable to fewer but larger ones. It is customary to employ piles about 6 feet long and about 6 inches in diameter, since this size can be driven with a hand maul or by drop ping a heavy block of wood with a tackle attached to any simple frame, or by a hand pile-driver (§ 751). They may be driven as close together as necessary, although 2 to 4 feet in the clear is usually sufficient.
In this connection it is necessary to remember that clay is com pressible, while sand is not, and that hence this method of consolidat ing soils is not applicable to sand, and is not very efficient in soils largely composed of it.
When the piles are driven primarily to compact the soil, it is customary to load them and also the soil between them, either by cutting the piles off near the surface and laying a tight platform of timber on top of them (see § 721), or by depositing a bed of con crete between and over the heads of the piles (see § 720).
If the soil is very soft or composed largely of sand, this method is ineffective; in which case long piles are driven as close together as is necessary, the supporting power being derived either from the resting of the piles upon a harder substratum or from the buoyancy due to immersion in the semiliquid soil. This method of securing a
foundation by driving long piles is very expensive, and is seldom resorted to for buildings, since it is generally more economical to increase the area of the foundation.
By Using Sand Piles. Experiments show that in compact ing the soil by driving wood piles, it is better to withdraw them and immediately fill the holes with sand, than to allow the wooden piles to remain. This advantage is independent of the question of the durability of the wood. When the wooden pile is driven, it com presses the soil an amount nearly or quite equal to the volume of the pile, and when the latter is withdrawn this consolidation remains, at least temporarily. If the hole is immediately filled with sand this compression is retained permanently, and the consolidation may be still further increased by ramming in the sand in thin layers, owing to the ability of the latter to transmit pressure laterally. And further, the sand pile will support a greater load than the wooden pile; for, since the sand acts like innumerable small arches reaching from one side of the hole to the other, more of the Toad is transmitted to the soil on the sides of the hole. To secure the best results, the sand should be fine, sharp, clean, and of uniform size.
By the Compressol System. This method consists in form ing a hole in compressible soil by dropping a heavy conical iron weight, or "perforator," from a considerable height, and then filling the hole with concrete upon which is to rest a column or beam which carries the superstructure. This is a comparatively new method of founding which has been employed to a considerable extent in Europe in the last few years. The perforator usually has a base of 21 to 3 feet, and weighs about 2 tons; and frequently has a fall of 20 to 30 feet. Holes have been sunk 50 feet deep by this process. The perforator compresses the soil laterally, and thereby greatly increases its water-tightness; and by dropping a little lime or clay into the hole before each fall of the perforator, it is usually possible to make the hole absolutely water-tight, since the lime or clay is plastered and compacted on the sides of the hole. Sometimes bowlders are rammed into the soil at the bottom of the hole, by dropping a pear shaped weight upon them, thus still further consolidating the soil. Finally the hole is filled with concrete, the thorough tamping of which enlarges the hole and still further consolidates the soil, the amount of concrete put into the hole frequently being three or four times the original volume of the hole.
This method of founding has a number of marked advantages. 1. No excavations are required, and therefore there is no danger of disturbing the equilibrium of the soil. 2. It eliminates all danger to men working below the surface of the ground. 3. It is compara tively cheap, since all the operations are performed by machinery. 4. It is quite rapid, since a hole from 25 to'30 feet deep can be sunk and filled in 3 or 4 hours. 5. It is possible to sink a hole as deep and as large as required by the desired bearing power.