Pile foundations

Pile Foundation

Pile foundation, is a deep foundation, is actually a long cylinder column made of reinforced concrete or steel which are used to support the structure and transfer the load at desired depth either by end bearing or skin friction. pile foundation comprises two components, The pile and The pile cap.  A pile cap supports structural column, wall, or slab, except and it bears on a single pile or group of piles.

Pile foundation offers solution to many difficult problems: Presence of a soft/ compressible layer of clay, High ground-water levels, Abandoned underground structures, and water bodies. Pile structure can bypass these problems and adequate foundation support can be obtained at any depth, without deep excavation, de-water, and install temporary sheeting and bracing. Though pile foundations offer simple solution to obtain deep foundation support it may be costly in some cases. Types of pile, total number and length of piles required, means of pile installation, and size of pile caps may significantly increase foundation costs.

Types of piles

Timber piles Timber piles, uses the trunks of trees. Use of Timber piles starts from Roman era, and it’s still most commonly used type of pile in the world. Length of timber pile varies from 40-60 ft (12-18 m).  Southern pine being the most common tree type of timber pile used in the United States. Normally the capacity of a timber piles is limited to 30 tons each. During pile driving in the stiff soil the danger of breaking the pile may happen at the pile butt (the head of the pile which receives the impact from pile-driving hammer). The damage can be reduced by limiting the driving force of the pile hammer. Timber piles needs to be pressure-treated with creosote to protect the wood from structural deterioration by dry rot or attack by termites. Treating timber piles with creosote enhances the life of the timber pile in excess of 40 years. In a marine environment, these timber piles are subjected to attack by various marine organisms, chemical treatment is not always a proven means of protection. Providing concrete encasement or installing rubber jackets around the timber pile are viable means of protection.

Concrete piles Cast-in-place Structural concrete are poured into a hollow shaft with predefined size and depth that is created in the ground. The hollow shaft may be cased or uncased. Fundamentally, the casing must be able to resist lateral pressure from the surrounding soil before it is filled with concrete. Very thin walled shells may not be strong enough to resist damage when driven into the ground. Thickness of the shell varies depending on the site condition. In most of the cases the casing is withdraw after filling up with concrete to reduce the cost. During the drilling process the borehole is continually filling with a bentonite slurry to keep the hole from collapsing. Concrete is pumped through a hollow pipe, known as a tremie pipe, that is lowered to the bottom of the hole. As the level of the concrete rises in the bored hole, the tremie pipe is gradually lifted out of the hole to allow the volume of concrete to displace the volume of bentonite slurry. Eventually, all bentonite slurry is replaced by concrete, and the tremie pipe is removed, resulting in a solid column of concrete embedded in the ground. The various types of cast-in-place piles differ in the shape, texture, and wall thickness of the casing; the method of casting the concrete in the hole; and the overall pile cost. Precast Precast concrete piles are reinforced concrete piles, cast prior to installation in a casting yard. Similar to timber piles, they must be reinforced to resist stresses in the pile due to driving. In addition, precast piles must be reinforced to prevent damage to the pile during handling prior to driving, especially when piles are stored, transported horizontally, and lifted into position. Prestressing of the steel reinforcement in the precast pile tends to reduce tension cracking during handling and driving, and provides an efficient way of withstanding bending stresses. The primary difficulty in using precast concrete piles is in extending the length of the pile or cutting the excess length off, when there is a variation in the anticipated pile length. However, the advantages of using precast concrete piles are their high load-carrying capacity and their outstanding resistance to deterioration above the water table under ordinary conditions. When exposed to salt water, the precast concrete piles are subject to attack of the steel reinforcement through cracks in the concrete, causing rust to develop and the concrete to spall. Use of high-density concrete and prestressing to minimize tensions cracks serve as the best measures of protection against reinforcing steel corrosion. See also: Concrete; Precast concrete; Reinforced concrete

Steel piles Steel is the only material used for piling that has a crushing strength comparable to that of hard rock. Available in a wide variety of sizes, steel piles are ideal when conditions call for hard driving, unusually great lengths, or high load capacity. Primarily, steel piles are either steel pipes, usually filled with concrete after being driven, or steel H-sections. Steel-pipe piles are similar to cased concrete piles, except that the steel-pipe casing is the primary load-carrying component, an therefore it is designed with a heavy wall thickness. When pipe piles are driven open-ended, they must be cleaned out before they are filled with concrete. Often, pipe piles are closed at the bottom with a closure plate or conical point to eliminate the task of cleaning out the pipe and to allow for inspection of the inside of the pile to check for possible damage after driving. Steel H-sections offer economic advantages not available in other types of piles. Manufactured as a rolled-steel section, the H-section or H-pile can be readily spliced in the field with full-strength welds, and they are available in extremely long lengths. H-piles do not require expensive field fabrication; they can be driven through unconsolidated soils containing timbers, boulders, and other debris with minimum difficulty; and they are designed to carry highly concentrated loads. Protection against marine organisms, termites, dry rot, spalling, and chipping are of no concern when using H-piles. See also: Structural steel Corrosion of steel due to exposure to salt-water spray, wave action, and sand abrasion in waterfront structures is a major concern. The most severe corrosion will occur within the splash zone that extends 2 ft (0.6 m) above and below the water level, where the steel is wetted with a thin film of water saturated with oxygen. By comparison, the area below the water level is oxygen-starved, and the rate of corrosion decreases rapidly with water depth. Similarly, steel piles driven into dry soils or soils that maintain a water table at some depth below the ground surface will not experience significant corrosion, since the amount of oxygen contained below the ground level is negligible. Cathodic protection is an effective method of preventing corrosion of unprotected steel exposed to seawater. Alternatives involve the use of protective coatings, including flame-sprayed aluminum and a blend of epoxy and polyamide resins. However, any protective coating will reduce corrosion as long as the coating lasts. Maintaining the coating in the tidal region can be expensive and, in many cases, virtually impossible. Another effective means of protecting the steel pile from corrosion, similar to preventing timber piles from deterioration, is by installing a concrete jacket around the pile extending 2 ft (0.6 m) below low water level and 2 ft (0.6 m) above high tide. See also: Corrosion; Metal coatings

Uses A pile foundation can be used to support any type of structure through any type of adverse soil conditions. Obviously, pile foundations are used to support marine structures and offshore platforms, since they are located over bodies of water. On land, pile foundations are used primarily in locations where poor soil condition exist. Under extreme conditions, all structures including utility lines are supported on piles and pile caps. Pile foundations have also been used as a means of underpinning. Bracket piles are used to extend the depth of foundation support for shallow footings adjacent to deep excavations. Piles are driven as close as possible to the edge of an existing footing. Pile caps, designed as cantilevered brackets, are installed at the top of the piles that extend beneath the existing footing. The space between the top of the bracket and the underside of the existing footing is wedged and dry-packed to create a positive load transfer from the existing footing to the bracket pile. Special care must be taken when driving piles near existing structures that are not supported on pile foundations. Because of the displacement of soil that occurs at the pile tip during pile driving, adjacent spread footings are susceptible to settlement. The magnitude of the soil displacement is a function of the shape and cross-sectional area of the pile tip. Closed-end pipe piles, timber piles, and precast concrete piles yield a relatively greater soil displacement than steel H-piles. If the soil displacement directly below the footing can be kept to a minimum, the magnitude of the settlement will be negligible. Experience has shown that the optimum pile to be driven adjacent to a soil bearing structure is the steel H-pile. Open-end pipe piles, despite their relatively small cross-sectional area, are not effective in minimizing soil displacement because they are circular in shape. As the open-end pipe pile is driven into the ground, the soil at the pile tip displaces radially away from the outside face of the pipe and converges radially inward inside the pipe. After driving the pile a certain distance, the converging soil inside the pipe becomes extremely dense and forms an earth plug at the tip of the pile. Unless the earth plug is cleaned out intermittently during the course of driving the pile, the pile will essentially behave as a closed-end pipe pile. Piles may also be installed by means of jacking. Jacked piles are used as a means of underpinning when job constraints prohibit the use of a pile-driving hammer. Short sections of open-end pipe are set up vertically below an existing footing and jacked down into the ground with a hydraulic jack installed between the top of the pipe and the bottom of the footing. Using the existing structure as a reaction, the hydraulic jack pushes the pipe downward. The next section of pipe is set on top of the previous pipe section and spliced by welding. The advantages of using an open-end pipe for jacking is the ability to remove an obstruction or excavate the soil inside the pipe to reduce the resistance by the soil to jacking. When the jacking operation is completed, the pile is wedged in place to ensure load transfer from the existing footing to the jacked pile.

Steel-sheet piling The practice of driving structural members into the ground had led to the development of interlocking steel-sheet piling construction. Steel-sheet piles are rolled-steel shapes with interlocking joints along their edges. They are produced in three standard shapes: straight web, arch web, and Z type; and they are available in a graduated series of weights and strengths. The interlocks, which must be sufficiently loose to permit free sliding during installation, are designed to provide strength and watertightness in both longitudinal and transverse directions. The Z-pile type has the highest bending resistance because of the favorable distribution of steel area away from the axis of bending. For this reason, Z-piles are most economical in the construction of permanent filled bulkheads, land walls, deeper braced cofferdams, and similar wall types that require high beam strength. The arch web piling is used for similar but generally lighter applications than Z-piling, such as in the construction of low-head cofferdams and shallow bulkheads or walls. The advantages of using arch web piling instead of Z-piling is its relative ease in handling and installation and its adaptability to a wide variety of field conditions. The straight web piling offers the least amount of bending strength, but the highest value of tension between the interlocks, which is the prime consideration in the construction of large, filled cellular-type structures. Most cellular structures are used as temporary cofferdams to aid in the construction of locks, dams, bridge piers, and land reclamation projects. Cellular cofferdams are also used in various types of permanent marine construction such as deep-water bulkheads, mooring or turning cells, artificial islands, and breakwaters