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In absence of a hard and rigid strata at an acceptable depth it may require a compensating footing to reduce the shear stress applied by the building load on the soil and yield acceptable values of settlement. Type II soil has lower shearing strength values which may result in rupture of soil if building is to be founded on ground level. A balancing raft may be implemented to reduce the settlement of soil.
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Type I soil are those which are safe under shear failure but yields higher settlement value especially differential settlement for the given load of building structure.
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Types of soil in which the floating foundation can be possible are classified as follows: Type I Thus a partially compensated footing may exist with the characteristics of minimum settlement and nearly equal weights. This concludes that for a fully compensated foundation to exist we must be able to maintain the groundwater table to its initial level which really a tough challenge. One of the reasons is that when excavating is continued below the groundwater table there may occur seepage of water which lowers the level of groundwater, thus effecting the effective strength parameter of the soil. Such a balance of weights is only approximate so it is really tough to obtain fully compensating footing. It is also known as a compensating foundation or balancing raft. It’s an analogy to the Archimedes’ principle of floatation has resulted in such a foundation being called a floating foundation, in which a foundation structure acts as a boat floating in the water. Weight of the building = Weight of the soil excavated including water Hence there is zero net pressure acting at the bottom of the foundation resulting in zero settlement of soil below. When the soil is so soft that not even friction piles will support the building load, these types of foundation are the final option and makes the building behave like a boat: obeying Archimedes' principle, buoyed up by the weight of the earth displaced in creating the foundation.This implies that the pressure/stress developed at the bottom of the foundation is approximately equal before and after the construction of the building structure. The earliest precursor to the modern version may be the concrete rafts developed for the building of Millbank Prison in 1815 by Robert Smirke.įor a floating raft foundation – or simply "floating foundation" – the foundation has a volume such that, if that volume filled with soil, it would be equal in weight to the total weight of the structure. It was a necessary innovation in the development of tall buildings in the wet soil of Chicago in the 19th century, when it was developed by John Wellborn Root who came up with the idea of interlacing the concrete slab with steel beams. JSTOR ( June 2019) ( Learn how and when to remove this template message)įloating raft is type of land-based foundation that protects against settlement and liquefaction of soft soil from seismic activity.Unsourced material may be challenged and removed.įind sources: "Floating raft system" – news Please help improve this article by adding citations to reliable sources.