In order to test a soil to find its compressive value, the bottom of the pit should be leveled for a considerable area, and stakes should be driven at short intervals in each direction. The elevations of the tops of all the stakes should be very accurately taken with a spirit level. For convenience, all stakes should be driven to the same level. A mast whose base has an area one foot square can support a plat form which may be loaded with several tons of building material, such as stone, brick, steel, etc. This load can be balanced with sufficient closeness so that some very light guys will maintain the unstable equilibrium of the platform. As the load on the platform is greatly increased, at some stage it will be noted that the mast and platform have begun to sink sightly, and also that the soil in a circle around the base of the mast has begun to rise.• This is indicated by the rising of the tops of the stakes. Even a very ordinary soil may require a load of five or six tons on a square foot before any yielding will be observable. One advantage of this method lies in the fact that the larger the area of the foundation, the greater will be the load per square foot which may be safely carried, and that the uncertainty of the result is on the safe side. A soil which might yield under a load concentrated on a mast one foot square, would probably be safe under that same unit-load on a continuous footing which was perhaps three feet wide; and if, in addition, a factor of safety of three or four was used, there would probably be no question as to the safety. a test need be applied only to an earthy soil. It would be practically impossible to produce a yielding by such a method on any kind of rock or even on a compacted gravel.
172. Bearing Power of Ordinary Soils. A distinction must be maintained between the crushing strength of a cube of rock or soil, and the bearing power of that soil when it lies as a mass of indefinite extent under some structure. A soil can fail only by being actually displaced by the load above it, or because it has been undermined, perhaps by a stream of water. A sample of rock which might crush with comparative ease when tested as a six-inch cube in a testing machine, will probably withstand as great a concentration of load as it is practicable to put upon it by any engineering structure. Even a gravel which would have absolutely no strength if it were attempted to place a cube of it in a testing machine, will be practically immovable when lying in a pit where it is confined laterally in all directions.
173. Rock. The ultimate crushing strength of stone varies greatly. The crushing strength is usually determined by making tests on small cubes. Tests made on prisms of a less height than width show a much greater strength than tests made on cubes of the same material, which shows that the bearing strength of rock on which foundations are built is much greater than the cubes of this stone. In Table I, Part I, the lowest value given for the crushing strength of a cube is 2,894 pounds per square inch, which is equal to 416,736 pounds per square foot. This shows that any
ordinary stone which is well imbedded will carry any load of masonry placed on it.
174. Sand and Gravel. Sand and gravel are often found to gether. Gravel alone, when of sufficient thickness, makes one of the firmest and best foundations. Pry sand or wet sand, when pre vented from spreading laterally, forms one of the best beds for foun dations; but it must be well protected from running water, as it is easily moved by scouring. Clean, dry sand will safely support a load of 3,000 to 8,000 pounds per square foot; and when compact and cemented, from 8,000 to 10,000 pounds per square foot. Ordinary gravel well 'bedded will safely bear a load of 6,000 to 8,000 pounds per square foot; and when well cemented, from 12,000 to 16,000 pounds per square foot.
175. Clay. There is great variation in clay soils, ranging from a very soft mass which will squeeze out in all directions when a very small pressure is applied, to shale or slate which will support a very heavy load. As the bearing capacity of ordinary clay is largely dependent upon its dryness, it is therefore very important that a clay soil should be well drained, and that a foundation laid on such a soil should be at a sufficient depth to be unaffected by the weather. If the clay cannot be easily drained, means should be taken to prevent the penetration of water. When the strata are not horizontal, great care must be taken to prevent the flow of the soil under pressure. When gravel or coarse sand is mixed with the clay, the bearing capac ity of the soil is greatly increased.
The bearing capacity of a soft clay is from 2,000 to 4,000 pounds per square foot; of a thick bed of medium dry clay, 4,000 to S,000 pounds per square foot, and for a thick bed of dry clay, 8,000 to 10,000 pounds per square foot.
176. Soft or Semi-Liquid Soils. The soils of this class include mud, silt, quicksand, etc., and it is necessary to remove them entirely or to reach a more solid stratum under the softer sail; or sometimes the soil can be consolidated by adding sand, stone, etc. The manner of improving such a soil will be discussed later. In the same way that water will bear up a boat, a semi-liquid soil will support, by the upward pressure, a heavy structure. For a soil of this kind, it is very difficult to give a safe hearing value; perhaps 500 to 1,500 pounds per square foot is as much as can be supported without too great a settlement.
177. Improving a Compressible Soil. The general method of doing this consists in making the soil more dense. This may be done by driving a large number of piles into the soil, especially if the piles will be always under the water line in that ground. Driving the piles compresses the soil; and if the piles are always under water, they, will be free from decay. If the soil is sufficiently firm so that the pile can be withdrawn and the hole will retain its form even tem porarily, it is better to draw the pile and then immediately fill the hole with sand, which is rammed into the hole as compactly as pos sible. This gives us a type of piling known as sand piles.