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
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
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
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
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
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
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
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.
extreme case, a stiff structure may force a flexible foundation to behave as
rigid. This aspect shall be consider...
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- Spring '13
- Geotechnical Engineering, New Delhi, Raft, Contact pressure