Code of practice for design and construction of

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Unformatted text preview: ‡Code of practice for design and construction of foundations in soils: General requirements ( third revision ). 4 IS : 2950 (Part I) - 1981 j ) Information necessary to assess the possible effects of the new structure on the existing structures in the neighbourhood. k) Proximity of mines or major storage reservoirs to the site. 3.2 Parameters for the Analysis — These are obtained by averaging the parameters ( see 3.1 ) which can be determined only for relatively less number of points of the foundation soil. The accuracy with which the average values represent the actual conditions is of decisive importance for the final results. 4. DESIGN CONSIDERATIONS 4.1 Choice of Raft Type 4.1.1 For fairly small and uniform column spacing and when the supporting soil is not too compressible, a flat concrete slab having uniform thickness throughout (a true mat) is most suitable ( see Fig. 1A ). 4.1.2 The slab may be thickened under heavily loaded columns to provide adequate strength for shear and negative moment. Pedestals may also be provided in such cases ( see Fig. 1B ). 4.1.3 A slab and beam type of raft is likely to be more economical for large column spacing and unequal column loads, particularly when the supporting soil is very compressible ( see Fig. 1C ). 4.1.4 For very heavy structures, provision of cellular raft or rigid frames consisting of slabs and basement walls may be considered. 4.2 Allowable Bearing Pressure — The allowable bearing pressure shall be determined in accordance with IS : 6403-1981*. 4.2.1 In granular soils, the ultimate bearing capacity of rafts 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 — The depth of foundation shall generally be not less than 1 m. *Code of practice for determination of bearing capacity of shallow foundation ( first revision ). 5 IS : 2950 (Part I) - 1981 FIG. 1 COMMON TYPES OF RAFT FOUNDATIONS 6 IS : 2950 (Part I) - 1981 4.4 Sub-soil Water Pressure — The uplift due to the sub-soil water shall be considered in the design. 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 Loading — A raft generally occupies the entire area of the building and often it is not feasible and rather uneconomical to proportion it coinciding the centroid of the raft with the line of action of the resultant force. In such cases, the effect of the eccentricity on contact pressure distribution 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 conso...
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This note was uploaded on 03/14/2014 for the course CE 684 taught by Professor Prof.deepankarchoudhury during the Spring '13 term at IIT Bombay.

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