Dr. Tin Tun ( 10-8-2014 ) 4:15am 5) Suction Piles Suction piles are used underwater to secure floating platforms. Tubular piles are driven into the seabed (or more commonly dropped a few meters into a soft seabed) and then a pump sucks water out the top of the tubular, pulling the pile further down. The proportions of the pile (diameter to height) are dependent upon the soil type: Sand is difficult to penetrate but provides good holding capacity, so the height may be as short as half the diameter; Clays and muds are easy to penetrate but provide poor holding capacity, so the height may be as much as eight times the diameter. The open nature of gravel means that water would flow through the ground during installation, causing piping flow (where water boils up through weaker paths through the soil). Therefore suction piles cannot be used in gravel seabed’s. Once the pile is positioned using suction, the holding capacity is simply a function of the friction between the pile skin and the soil, along with the self-weight and weight of soil held within the pile. The suction plays no part in holding capacity because it relieves over time. The wall friction may increase slightly as pore pressure is relieved. One notable failure occurred (pullout) because there was poor contact between steel and soil, due to a combination of interal ring stiffeners and protective painting of the steel walls. 6) Adfreeze Piles In extreme latitudes where the ground is continuously frozen, adfreeze piles are used as the primary structural foundation method. Adfreeze piles derive their strength from the bond of the frozen ground around them to the surface of the pile. Typically the pile is installed in a pre-drilled hole 6-12 larger then the diameter of the pile. A slurry mixture of sand and water is then pumped into the hole to fill the space between the pile and the frozen ground. Once this slurry mixture freezes it is the shear strength between the frozen ground and the pile, or the adfreeze strength, which support the applied loads. Adfreeze pile foundations are particularly sensitive in conditions which cause the permafrost to melt. If a building is constructed improperly, it will heat the ground below resulting in a failure of the foundation system. Another ongoing concern for adfreeze pile foundations is climate change. As the climate warms, these foundations lose their strength and will eventually fail.The hierarchic chart representation given below can be used for a quick understanding of pile classification: Advantages and disadvantages of different pile material Wood piles: Advantages: The piles are easy to handle. Relatively inexpensive where timber is plentiful. Sections can be joined together and excess length easily removed. Disadvantages: The piles will rot above the ground water level. Have a limited bearing capacity. Can easily be damaged during driving by stones and boulders. The piles are difficult to splice and are attacked by marine borers in salt water. Prefabricated concrete piles (reinforced) and pre stressed concrete piles. (driven) affected by the ground water conditions. Do not corrode or rot. Are easy to splice. Relatively inexpensive. The quality of the concrete can be checked before driving. Stable in squeezing ground, for example, soft clays, silts and peats pile material can be inspected before piling. Can be re driven if affected by ground heave. Construction procedure unaffected by ground water. Can be driven in long lengths. Can be carried above ground level, for example, through water for marine structures. Can increase the relative density of a granular founding stratum. Disadvantages: Relatively difficult to cut. Displacement, heave, and disturbance of the soil during driving. Can be damaged during driving. Replacement piles may be required. Sometimes problems with noise and vibration. Cannot be driven with very large diameters or in condition of limited headroom. Driven and cast-in-place concrete piles Permanently cased (casing left in the ground)Temporarily cased or uncased (casing retrieved) Advantages: Can be inspected before casting can easily be cut or extended to the desired length. Relatively inexpensive. Low noise level. The piles can be cast before excavation. Pile lengths are readily adjustable. An enlarged base can be formed which can increase the relative density of a granular founding stratum leading to much higher end bearing capacity. Reinforcement is not determined by the effects of handling or driving stresses. Can be driven with closed end so excluding the effects of GW
Posted on: Sat, 09 Aug 2014 22:43:24 +0000
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