Thursday, November 15, 2012

Strip or Pad Foundation - Black Cotton Soils.

For medium loads, strip foundation (for walls) and pad foundation (for columns) may be provided, along with special design features discussed above. Fig. 3.31 shows some typical section of shallow footings sutable for black cotton and other expansive soils.


Section of Fig. 3.31 (a) is suitable when the soil, though expansive, ha little swelling pressure. A 60 cm thick layer of cohesionless sand is placed below the foundation concrete, and is compacted. Sand is also filled around the footing. When the soil swells, the sand grains would yield by moving up, thus relieving the swelling
pressure. When the soil shrinks, the sand layer would expand, but there will be no discontinuity in the soil support. Sand fill should also be used below flooring. Section of Fig. 3.31 (b) is suitable where the swelling pressures are relatively high. The alternate layers of mooram (or ballast) and sand act as a spring which can compress or expand along with the sub-soil movements. It will, thus absorb all the movements, thus keeping the footing free from these cffects. If the soil is soft and has poor bearing capacity, a 30 cm thick layer of ballast and mooram should first be rammed into the soil. Over the top of it, a min. of 30 cm thick layer of coarse grained sand may be placed. In all the three cases, the foundation concrete may be done in rigid cement concrete, and if possible, it may contain nominal reinforcement. Fig. 3.31 (d) shows a section which may be used for soils of high swelling pressure, and having high shrinkage properties. After compacting the base of the tranch, 25 to 30 cm wide strips of concrete, 25 to 30 cm thick, may first be laid and compacted. Afer the strip concrete is cured, the space between the two is filled with sand. The space between the two strips of concrete(i.e. width of sand fill) may be kept equal to width of the bottom caurse of masonry. On thc top of this, the foundation concrete layer, preferably of reinforced concrete is laid. The sides of the masonry footings is filled with sand as usual. In addition to this, 80 mm dia. pipes spaced at 1.5 to 2 m etc. are placed through masonry and concrete bed, so as to reach the bottom sand fill a shown, and sand is filled in the pipe. A plug may be placed on the top of the pipe, to facilitate the inspection from time to time, and to pour fresh sand if required.



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