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Unformatted text preview: ls, d Loading conditions preferably shown on a schematic plan indicating
design combination of loads transmitted to the foundation. 4 Environmental Factors - Information relating to geologic history of
the area, seismicity of the region, hydrological information indicating ground water conditions and its seasonal variations, climatic
factors like vulnerability of the site to sudden flooding by surface
run-off, erosion, etc. e) Geotechnical Information - Giving subsurface profile with stratification details ( see IS : 1892-1979t ), engineering properties of the founding strata, namely, index properties, effective shear parameters
determined under appropriate drainage conditions, compressibility
characteristics, swelling properties, results of field tests like static
and dynamic penetration tests, pressure meter tests, etc. f) Modulus of Elasticity and Modulus of Subgrade Reaction - Appendix A enumerates the methods of determination of modulus of
elasticity ( E, ) and Poisson’s ratio ( k ). The modulus of subgrade
reaction ( k ) may be determined in accordance with Appendix B. g) Limiting values of the angular distortion and differential settlement,
the superstructure h) can withstand ( see IS : 1904-1978$ ). A review of the performance
locality. of a similar structure, if any, in the *Glossary of tempt and symbols relating to soil engineering(first revision ).
tCode of practice for subsurfaceinvestigationsfor foundations (first revision ).
ICode of practice for structural safety of buildings : Shallowfoundations ( second
revision ). 4 IS : 2950 ( Part I ) - 1981 3 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
parameters ( see 3.1 ) which can be determined only for relatively
number of points of the foundation soil. The accuracy with which
average values represent the actual conditions is of decisive importance
the final results.
4. DESIGN the
for 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...
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