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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
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
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.
- Spring '13