13-04-2012, 03:03 PM
Bearing Capacity Of Shallow Foundation
BEARING CAPASITY OFSOIL.ppt (Size: 333 KB / Downloads: 543)
Bearing Capacity Of Shallow Foundation
A foundation is required for distributing the loads of the superstructure on a large area. * The foundation should be designed such that a) The soil below does not fail in shear & b) Settlement is within the safe limits.
Basic Definitions :
Ultimate Bearing Capacity (qu) : The ultimate bearing capacity is the gross pressure at the base of the foundation at which soil fails in shear. 2) Net ultimate Bearing Capacity (qnu) : It is the net increase in pressure at the base of foundation that cause shear failure of the soil.Thus, qnu = qu – γDf (ovrbruden pressure)
Net Safe Bearing Capacity (qns) :
It is the net soil pressure which can be safely applied to the soil considering only shear failure.
Modes of shear Failure :
Vesic (1973) classified shear failure of soil under a foundation base into three categories depending on the type of soil & location of foundation. 1) General Shear failure.2) Local Shear failure.3) Punching Shear failure
Local Shear failure -
* When load is equal to a certain value qu(1),
* The foundation movement is accompanied by sudden jerks.
* The failure surface gradually extend out wards from the foundation.
* The failure starts at localized spot beneath the foundation & migrates out
ward part by part gradually leading to ultimate failure.
* The shear strength of soil is not fully mobilized along planes & hence
failure planes are not defined clearly.
Effect of water table on Bearing Capacity :
* The equation for ultimate bearing capacity by Terzaghi has been developed based on assumption that water table is located at a great depth .* If the water table is located close to foundation ; the equation needs modification.
Consolidation Settlement :
* Compressibility of soil is the property of the soil due to which a decrease in volume occurs under compressive forces. * The compression of soils can occurs due to-A) Compression of solid particles & water in the voids.B) Compression & expulsion of air in the voids.C) Expulsion of water in the voids.