Foundations PDF

HEAVY REINFORCED CONCRETE, PRE-STRESSED CONCRETE AND STEEL CONSTRUCTION FOUNDATION SYSTEMS (Deep and Shallow Foundation) The foundation system transfers the lateral loads on the superstructure to the ground. The horizontal component of these lateral forces is transferred largely through a combinatio...

HEAVY REINFORCED CONCRETE, PRE-STRESSED CONCRETE AND STEEL CONSTRUCTION FOUNDATION SYSTEMS (Deep and Shallow Foundation) The foundation system transfers the lateral loads on the superstructure to the ground. The horizontal component of these lateral forces is transferred largely through a combination of soil friction on the bottom of footings and the development of passive soil pressure on the sides of footings and foundation walls. Foundation systems are classified into two broad categories --- shallow foundations and deep foundations. SHALLOW FOUNDATIONS Shallow or spread foundations are employed when stable soil of adequate bearing capacity occurs relatively near the ground surface. They are placed directly below the lowest part of a superstructure and transfer building loads directly to the supporting soil by vertical pressure. The types of shallow or spread footings are: INDIVIDUAL OR ISOLATED FOOTINGS – are spread footings supporting free standing columns and piers STRIP FOOTINGS – are the continuous spread footings of foundation walls. Stepped footings are strip footings that change levels to accommodate a sloping grade and maintain the required depth at all points around a building. COMBINED FOOTINGS Combined footings. supporting two or more columns. This type of footing is used where it is not possible to center the footing beneath its supported column as in the case of columns located at or very near the property line. In such case, the nearest interior column is selected and a combined footing constructed under both columns. The footing is so designed so that the center of gravity of the combined loads passes through the center of gravity of the footing area. Combined column footings are usually rectangular or trapezoidal in shape. Cantilevered footings. This type of footing may be used in place of a combined footing under the same conditions. In this type of construction, the footings of the exterior and interior columns are connected by a tie-beam or strap which is so extended to support the exterior column. The top of the beam or strap is usually placed level with the top of the footings. Continuous footings. These may be: 1. supporting a line of columns 2. supporting all of the columns by strips at right angles to each other They may be inverted slab or inverted tee continuous footings. Mat or Raft Foundations Mat foundations, like continuous footings are used on soil of low bearing power where there is a tendency towards unequal settlement due to unequal loading of soil. In this type of foundation all parts of the foundation are so tied together so that they will act as one and assist each other in keeping level and plumb. Mat foundations may be divided into the following general classes: STEEL GRILLAGE FOUNDATION When it is desired to avoid the deep excavation required for concrete and masonry footings, and when the load has to be distributed over a wide area of support, steel rails or beams are used to give the required moment of resistance with a minimum of depth. For steel-grillage foundations the foundation bed should first be covered with a layer of concrete not less than 6” in thickness and so mixed and compacted as to be nearly impervious to moisture as possible. The beams are placed on this layer, the upper surface brought to a line and the lower flanges carefully grouted so as to secure an even bearing. Subsequently, concrete should be placed between and around the beams so as to permanently protect them. The beam must not be spaced so near as to prevent the placing of concrete between them. The clear space between the flanges of the top layer of beams should not be less than 2” and should be somewhat more for the lower layers. DEEP FOUNDATIONS Deep foundations are employed when the soil underlying a shallow foundation is unstable or of inadequate soil bearing capacity. They extend down through unsuitable soil to transfer building loads to a more appropriate bearing stratum of rock or dense sand and gravel well below the superstructure. The types of deep foundations are pile and caisson foundations. 1. PILE FOUNDATIONS – A pile foundation is a system of end bearing or friction piles, pile caps, and tie beams for transferring building loads down to a suitable bearing stratum. Pile Cap 1. A slab or connecting beam which covers the heads of a group of piles, tying them together so that the structural load is distributed and they act as a single unit. 2. A metal cap which is placed, as temporary protection, over the head of a precast pile while it is being driven into the ground End bearing piles – depend principally on the bearing resistance of soil or rock beneath their feet for support. The surrounding soil mass provides a degree of lateral stability for the long compression member. Friction piles – depend principally on the frictional resistance of a surrounding earth mass for support. The skin friction developed between the sides of a pile and the soil into which the pile is driven is limited by the adhesion of soil to the pile sides and the shear strength of the surrounding soil mass. WOOD PILES Wood-pile Foundations. When it is required to build upon a compressible soil saturated with water and of considerable depth, the most practicable method of obtaining a solid and enduring foundation for buildings of moderate height is by driving wooden piles. Wooden piles are made from the trunks of trees and should be as straight as possible, and not less than 5” in diameter at small end for light buildings, or 8” for heavy buildings. The piles are driven by means of a drop-hammer or with a steam hammer, a succession of blows being given with a block of cast iron or steel called the hammer, which slides up and down; the uprights of the machine is placed over the pile-driver. The machine is placed over the pile so that the hammer descends fairly on its head, the piles being driven with the small end down. In driving wooden piles with a drop-hammer, the hammer is generally raised by steam power and is dropped either automatically or by hand. The weight of the hammers used for driving piles for building foundations is usually from 1,500 to 2,500 lb., and fall varies from 5 to 20 ft., the last blows being given with a short fall. Steam hammers are to a considerable extent taking the place of the ordinary drop- hammers as they will drive more piles in a day, and with less damage to the piles. The steam-hammer delivers quick, short blows, from 60 to 70 to the minute, and seems to jar the piles down, the short interval between the blows not giving time for the soil to settle around them. In driving piles care should be taken to keep them plumb, and when the penetration becomes small, the fall should be reduced to about 5 ft., the blows being given by rapid succession. Whenever a pile refuses to sink under several blows before reaching the average depth, it should be cut off and another pile driven beside it. When several piles have been driven to a depth of 20 ft. or more or refuse to sink more than ½ in. under 5 blows of a 1200 lb. hammer falling 15 ft., it is useless to try them further, as the additional blows result only in brooming and crushing the heads and points of the piles, and splitting and crushing the intermediate portions to an unknown extent. When the penetration is less than 6 in. at each blow the top of the pile should be protected from brooming by putting on an iron pile ring, about 1 in. less in diameter than the head of the pile, and from 2-1/2 to 3 in. wide by 5/8 in. thick. The head should be chamfered to fit the ring. Pile Ring – also called a drive band; a steel band which encircles the head of a timber pile to prevent it from splitting when being driven In driving in soft and silty soils, the piles drive better with a square point. When driven into compact soil, such as sand, gravel, or stiff clay, the point of the pile should be shod with iron or steel. This is usually in the form of a cast conical point about 5 in. in dia., secured by a long dowel with a ring around the end of the pile. Piles that are driven in or exposed to salt water should be thoroughly impregnated with creosote, dead oil or coal-tar, or some mineral poison to protect them from teredo or shipworm which will completely honeycomb an ordinary pile in three or four years. Piles should not be spaced less than 2 ft. on centers; usual spacing is from 2 to 3 ft. When long piles are driven closer than 2 ft. on centers, there is danger that they may force each other up from their solid bed on bearing stratum. Driving the piles close together also breaks up the ground and diminishes the bearing power. Maximum allowable load on wood piles is usually 20 tons. The top of the piles should be cut off at or below the low water mark, otherwise they will soon commence to decay. They should then be capped, either with concrete, or with timber or steel grillage. The usual practice is to use the reinforced-concrete cap, the method being to excavate 6 to 12” below the tops and one foot outside of the piles. Concrete is then placed around and above the piles. Approximately 3” above the top of the piles a layer or reinforcement running in both directions is placed. Caps are usually 18” or more in thickness. Heavy timber grillages may also be used for capping. These are bolted to the top of the piles and the concrete footings laid on top of it. The timbers for the grillages should be at least 10” x 10” in cross- section, and should have sufficient transverse strength to sustain the load from center to center of piles. They should be laid longitudinally on top of the piles and fastened to them by means of driftbolts. The advantages of timber grillage are that it can be easily laid and effectually holds the top of piles in place. It also tends to distribute the pressure evenly over the piles, as the transverse strength of the timber will help to carry the load over a single pile, which for some reason, may not have the same bearing capacity as the others. Where timber grillage is used, it should be kept entirely below the lowest recorded water line, as otherwise it will rot and allow the building to settle. Steel beams embedded in concrete are also sometimes used to distribute the weight over piles, but this is too expensive a method to be commonly used. Driftbolt – a short rod or square bar driven into holes bored in timber, for attaching adjacent sticks to each other or to piles; varies from 1 to 2 ft (300 x 600 mm) in length; often provided with a head or with a sharpened end; also called a drift or driftpin CONCRETE PILES Concrete piles, either plain or reinforced, possess many advantages over wooden piles and, in general, can be used in all places where wooden piles can be driven. Concrete piles are generally used where wooden piles would be subject to decay or deterioration by the action of marine worms. They are especially advantageous for foundations on land where the permanent ground water is at a considerable depth. Wooden piles must cut of under water as, when subjected to an atmosphere which is alternately wet and dry, they will decay. This is unnecessary with concrete piles, and foundations under such conditions need not start so low as would be the case if timber piles were used. In practice concrete piles are generally reinforced. Reinforced-concrete piles are of two general types: those molded in place and those molded before driving. Spacing for concrete piles usually from 2’ – 6” to 4’. Concrete piles are extended at least 4” into the concrete of the footing, and where a steel casing surrounds the pile, 3 to 4 in. of concrete is required between the top of the piles and the footing reinforcement, unless the casing is trimmed back at a distance, in which case the case reinforcement is allowed to lie directly upon the butts of the piles. PRE-CAST PILES These are usually moulded in a yard or at the site allowed to cure for 4 weeks before using. In driving, a pre-cast pile is provided with a cast-iron point, and a driving head is used in which a cushion of sand, rope or other material is placed between a driving block of wood and the concrete in order to prevent the crushing of the pile. Concrete piles are often sunk by means of water-jet. This method is made possibly by inserting an iron pipe in the center of the pile. CAST-IN-PLACE PILES Cast in place piles are constructed in the ground in the position they are to occupy, and are often reinforced. Practically all cast in place piles are covered by patents. Cast-in-place piles may be formed by any of the following methods: a. A hollow cylindrical steel tube usually furnished with a tight-fitting collapsible steel core or mandrel, is driven into the soil. The core is then collapsed and removed, and the steel shell filled with concrete. Thus there is a shell or form for every pile, e.g. McArthur piles, Raymond piles (this uses a No. 24 gauge shell in which a spiral of No. 3 wire is encased). This is also commonly called a cased pile. A steel tube is fitted at the bottom with a driving point and is driven into the ground to the required depth. Concrete is then poured into the hole thus formed as the steel tube is gradually withdrawn. The driving point may be either a conical cast-iron point that is left in place or a hinged cutting- edge called an alligator point which opens as the tube is withdrawn, e.g. Simplex piles. This is called an uncased pile. A steel pipe or shell is first driven into the ground. The steel driving core is then removed and the bottom of the shell is filled with concrete to a height of about 5 ft. from the bottom. Pressure is then applied to force out the concrete into the surrounding soil as the core is withdrawn. These are known as pedestal piles. STEEL PILES These are concrete-filled steel pipes which are made to bear on rock or hard pan. The pipes are generally 10 to 18 inches in diameter, having a thickness of 3/8 to 5/8 inches. The pipe is driven in sections with a steam-hammer and, as additional sections are required, these are attached to the driven section by means of a cast-iron or steel internal sleeve and re-driven. When the pipe has reached its bearing level it is cleaned out by blowing or dug out by means of augers or similar tools. The pipe is then pumped out and concreted. CAISSON FOUNDATIONS Caissons are cast-in-place, plain or reinforced concrete piers formed by boring with a large auger or excavating by hand a shaft in the earth to a suitable bearing stratum and filling the shaft with concrete. For this reason they are also referred to as drilled piles or piers.