421 in granular soils the ultimate bearing capacity

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Unformatted text preview: is generally very large. However, for rafts placed at considerable depth ( for example basement rafts ), the possibility of punching mode of failure should be investigated. The influence of soil compressibility and related scale effects should also be assessed. 4.2.2 For rafts on cohesive soils stability against deep seated failures shall be analysed. 4.2.3 In cohesive soils, the effect of long term settlement due to consideration shall be taken into consideration. 4.3 Depth of Foundation less than 1 m. The depth of foundation shall generally be not *Code of practice for determination of bearing capacity of shallow foundation (first revision ). 5 ._ ^l..-._ -.- IS :295O(PartI)-1981 to 0 0 0 0 0 0 0 a d 0 0 0 0 0 cl 0 0 0 0 0 0 0 0 0 p Cl 0 cl 0 SECTION AA SECTION 1B Flat Plate Thickened Under Columns 1A Fiat Plate SECUON CC 1C Two-Way BB SECTION Beam and Slab ID 00 Flat Plate with Pedestals TYPES F RAFT FOUNDATIONS O FIG. 1 COMMON 6 ..-_I IS:29!5o(PartI)-1981 4.4 Sub-soil Water Pressure considered in the design. The uplift due to the sub-soil water shall be 4.4.1 All construction below the ground water level shall be checked for flotation. 4.5 General 4.5.1 Dimensional Parameters - The size and shape of the foundation adopted affect the magnitude of subgrade modulus and long term deformation of the supporting soil and this, in turn, influence the distribution of contact pressure. This aspect shall be taken into consideration in the analysis. 4.5.2 Eccentricity of the building and portion it coinciding resultant force. In pressure distribution of Loading - A raft generally occupies the entire area often it is not feasible and rather uneconomical to prothe centroid of the raft with the line of action of the such cases, the effect of the eccentricity on contact shall be taken into consideration. 4.5.3 Properties of the Supporting Soil - Distribution of contact pressure underneath a raft is affected by the physical characteristics of the soil supporting it. Considerations must be given to the increased contact pressure developed along the edges of the foundation on cohesive soils and the opposite effect on granular soils. Long term consolidation of deep soil layers shall be taken into account in the analysis. This may necessitate evaluation of contact pressure distribution both immediately after construction and after completion of the consolidation process. The design must be based on the worst conditions. 4.5.4 Rigidity of the Foundation - Rigidity of the foundation tends to iron out uneven deformations and thereby modifies the contact pressure High order of rigidity is characterized by large moments and distribution. relatively small, uniform settlements. A rigid foundation may also generate high secondary stresses in gtructural members. The effects of rigidity shall be taken into account in the analysis. 4.5.5 Rigidity of the Superstructure - Free response of the foundations to soil deformation is restricted by the rigidity of the superstructure. In the extreme case, a stiff structure may force a flexible foundation to behave as rigid. This aspect shall be consider...
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