Title 21 Pile Foundations.pdf ✅

Geotech 09: Pile Foundations 1. Types of Piles Steel Piles Steel piles generally are either pipe piles or rolled steel H-section piles. Pipe piles can be driven into the ground with their ends open or closed. Wide-flange and I-section steel beams can also be used as piles. However, H-section piles are usually preferred because their web and flange thicknesses are equal.  Usual length: 15m to 60m  Usual load: 300 kN to 1200 kN  Advantages o Easy to handle with respect to cutoff and extension to the desired length o Can stand high driving stresses o Can penetrate hard layers such as dense gravel and soft rock o High load-carrying capacity  Disadvantages o Relatively high cost o High level of noise during pile driving o Subject to corrosion o H-piles may be damaged or deflected from the vertical during driving through hard layers or past major obstructions Concrete Piles May be divided into two basic categories: precast piles and cast-in-situ piles. Precast piles can be prepared by using ordinary reinforcement, and they can be square or octagonal in cross section. Reinforcement is provided to enable the pile to resist the bending moment developed during pickup and transportation, the vertical load and the bending moment caused by a lateral load. Pre-Cast Piles  Usual length: 10m to 15m  Usual load: 300 kN to 3000 kN  Advantages: o Can be subjected to hard driving o Corrosion resistant o Can be easily combined with a concrete superstructure  Disadvantages: o Difficult to achieve proper cutoff o Difficult to transport Cast-in-Place Piles  Usual length: 5m to 15m  Maximum length: 30m to 45m  Usual load: 200kN to 500kN  Approximate maximum load: 800kN  Advantages: o Relatively cheap
o Allow for inspection before pouring concrete o Easy to extend  Disadvantages: o Difficult to splice after concreting o Thin casings may be damaged during driving  Allowable load: Q௔௟௟ = A௦f௦ + A௖f௖ Where: A௦ = area of cross section of steel A௖ = area of cross section of concrete f௦ = allowable stress of steel f௖ = allowable stress of concrete Timber Piles Timber piles are tree trunks that have had their branches and bark carefully trimmed off. The maximum length of most timber piles is 10 to 20m. to quality for use as a pile, the timber should be straight, sound, and without any defects. Composite Piles The upper and lower portions of composite piles are made of different materials. 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. Forming proper joints between two dissimilar materials is difficult, and for that reason, composite piles are not widely used. 2. Estimating Pile Length Point Bearing Piles - The ultimate capacity of the piles depends entirely on the load-bearing capacity of the underlying material Ultimate Pile Load Q௨ = Q௣ + Q௦ Where: Q௣ = load carried at the pile point Q௦ = load carried by skin friction developed at the side of the pile (caused by shearing resistance between the soil and the pile) If Q௦ is very small Q௨ ≈ Q௣
Figure 1. Point Bearing Piles 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. - Since Q௣ is relatively small, Q௨ ≈ Q௦ - Most of their resistance is derived from skin friction Figure 2. Friction Piles