Deep Foundations

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notes on deep foundations, engineering, civil
  DEEP FOUNDATIONS PILE FOUNDATIONSCE 522 PILES  –  are structural members that are made of steel, concrete, or timber  PURPOSE OF PILE FOUNDATIONS: transmit a super structure load to deeper load bearing strata, withstand lateral, vertical, uplift load and to minimize the settlement. WHY PILES ARE NEEDED? 1.Top layers of soil are highly compressible for it to support structural loads through shallow foundations.2.Rock level is shallow enough for end bearing. Pile foundations provide a more economical design.3.Lateral forces are relatively prominent.4.In presence of expansive and collapsible soils at the site.5.Offshore structures6.Strong uplift forces on shallow foundations due to shallow water table can be partly transmitted to Piles.7.For structures near flowing water (bridge abutments, etc.) to avoid the problems due to erosion. CLASSIFICATION BASED ON MATERIALS OR COMPOSITION:  Steel PilesPipe pilesRolled steel H-section piles  Concrete PilesPre-cast PilesCast-in-situ PilesBored-in-situ piles  Timber Piles  Composite Piles  STEEL PILES  Usual length: 15 m –60 m  Usual Load: 300 kN–1200 kN  Advantage:1. Relatively less hassle during installation and easy to achieve cut-off level.2. High driving force may be used for fast installation3. Good to penetrate hard strata4. Load carrying capacity is high  Disadvantage:1. Relatively expensive2. Noise pollution during installation3. Corrosion4. Bend in piles while driving STEEL PILES  In many cases, the pipe piles are filled with concrete after they have been driven.  The allowable structural capacity for steel piles is: Q   = Asfs where:As = cross-sectional area of the steelfs = allowable stress of the steel ( ≈0.33 –0.5fy) CONCRETE PILES  Pre-cast Piles:Usual length: 10 m –15 mUsual Load : 300 kN–3000 kN  Pre-cast Pre-stressed PilesUsual length : 10m –45 mUsual Load: 7500 kN–8500 kN  Advantage:1. It can be easily combined with concrete superstructure2. Corrosion resistant3. It can bear hard driving  Disadvantage:1. Difficult to transport2. Difficult to achieve desired cut-off CONCRETE PILES  Cast-in-situ piles or cast in-place piles:a.) Cased cast-in place pilesUsual length: 5 m –15 mUsual Load: 200 kN–500 kNAdvantages:a. Relatively cheapb. Allow for inspection before pouring concretec. Easy to extendDisadvantages:a. Difficult to splice after concretingb. Thin casings may be damaged during driving  CONCRETE PILES  Allowable load: Q   = Asfs +Acfc where:As = cross-sectional area of the steelAc = cross-sectional area of the concretefs = allowable stress of the steelfc = allowable stress of the concrete CONCRETE PILES b.) Uncased cast-in-place concrete pilesUsual length: 5 m to 15 mUsual load: 300 kNto 500 kNAdvantages:a. Initially economicalb. Can be finished at any elevationDisadvantages:a. Voids may be created if concrete is placed rapidlyb. Difficult to splice after concretingc. In soft soils, the sides of the hole may cave in, squeezing the concrete CONCRETE PILES  Allowable load: Q   = Acfc where:Ac = cross-sectional area of the concretefc = allowable stress of the concrete COMPOSITE PILES  The upper and lower portions of composite piles are made of different materials.  Steel-and-concrete piles consist of a lower portion of steel and an upper portion of cast-in place concrete.  This type of pile is used when the length of the pile required for adequate bearing exceeds the capacity of simple cast-in-place concrete piles.  CLASSIFICATION OF PILES BASED ON THE NATURE OF THEIR PLACEMENT 1. Displacement Piles  Driven piles are displacement piles, because they move some soil laterally; hence, there is a tendency for densification of soil surrounding them.  Concrete piles and closed-ended pipe piles are high-displacement piles. 2. Non-displacement Piles  Bored piles are non-displacement piles because their placement causes very little change in the state of stress in the soil. TYPES OF PILES BASED ON THEIR FUNCTION 1. Point Bearing Piles  If soil-boring records establish the presence of bedrock or rocklike material at a site within a reasonable depth, piles can be extended to the rock surface.  In this case, the ultimate capacity of the piles depends entirely on the load-bearing capacity of the underlying material.  If, instead of bedrock, a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few meters into the hard stratum.The ultimate pile load may be expressed as : Q   = Q   +Q  where:Qp= load carried at the pile pointQs = load carried by skin friction developed at the side of the pile (caused by shearing resistance between the soil and the pile) 2. Friction Piles  When no layer of rock or rocklike material is present at a reasonable depth at a site, point bearing piles become very long and uneconomical.  In this type of subsoil, piles are driven through the softer material to specified depths. The resistance of these piles is derived from skin friction .  The lengths of friction piles depend on the shear strength of the soil, the applied load, and the pile size.
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