If the soil is right, the structure will be stable
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A structure’s stability depends on soil foundation. Hence all influences on the structure and the soil should be considered at the time of installation.
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The ultimate support for any structure is provided by the underlying earth or soil material and, therefore, the stability of the structure depends on it. Soil is much weaker than common materials of construction such as steel and concrete.
Hence a larger area or volume of soil is involved in satisfactorily transmitting and distributing the load. Therefore, the load carried by structural members such as walls will have to be distributed over a larger area, symmetrically, so that the soil below it is not overstressed or permits serious settlement of the structure. The spread foundation generally performs the function of distributing the load uniformly over a larger area.
Vital support
The type of structural foundation employed should be related to the soil. Thus, one should recognise that it is the “soil foundation system” that provides the support for the structure. The foundation is built and installed on the soil that exists at the site.
The stability of the structure depends on this “soil foundation system.” Hence all influences on the structure and the soil during the life span of the structure should be considered at the time of installation of the foundation.
In addition to the loading, the action of wind, earthquake, variability of water table, heat, frost, vibration caused by traffic, scour in case of adjacent flowing water are all factors that will affect the performance of the “soil foundation system.”
Types of foundations
Spread footing is basically a pad used to spread out loads from walls or columns over a large area of foundation soil. Square or rectangular pads are used to support structural columns. They can be one of the following four types:
Mass concrete block
Reinforced concrete with sloping upper face (tapered footing)
Plain concrete with spread footing
Stepped reinforced concrete.
The most common type preferred is the reinforced concrete with sloping upper face.
Strip foundation is normally provided for load-bearing walls and rows of columns so closely spaced that pad foundation would nearly touch. This is the common type of foundation for bearing wall buildings adopted up to four-storey height.
Wide strip foundation is necessary where the soil is weak. The overhang of the wide strip pad may be large if the footing is not gradually stepped. In such cases reinforced strip is required to prevent cracking due to bending.
Raft foundation is required on soils of low-bearing capacity or when heavy column loads cause overlapping of the pads. Raft foundation is useful in reducing differential settlement, especially when the soil nature varies over the area where the building is constructed.
Tapered footing
Apart from the structural design the following guidelines will ensure good performance of the most commonly employed tapered footing adopted for multi-storey flats:
Edge thickness of minimum 150 mm for footing on soils should be provided. Reinforcement should be designed for the load and moment requirement. Clear cover should be 75mm from corrosion prevention point of view.
Unless special calculations are made for the eccentricity, the footing should be symmetrical with respect to column/wall axis.
The unreinforced concrete ship foundation using a suitable concrete mix (50 kg of cement to 0.1 m3 of sand and 0.2 m3 of coarse aggregate) is very widely adopted to support load-bearing walls of dwelling houses, boundary walls and wall pads of the ground floor of framed structures.
Soil test
A mix as lean as the one mentioned above should not be used in aggressive soil environment. The width of foundation should be fixed based on the safe bearing pressure determined using soil test at site.
However, for normal conditions, the width of the foundation for load-bearing walls should be not less than 0.8m. The practical minimum depth of pad irrespective of the load should be 150mm.
The brick masonry wall should be provided with a damp proof course layer above ground level but below plinth.
The depth of footing should not be less than soccer depth or 1m.
A minimum thickness of pad of 150mm is required to enable the foundation to bridge over loose pockets in the soil and to resist longitudinal forces set up by thermal expansion and contraction and moisture movement of the footing walls.
A.R. SANTHAKUMAR
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