How deep should a foundation of a loadbearing construction be?
For a ground plus one storey loadbearing house a strip foundation of 1′-6″ below ground level is sufficient. What is important is that soil and silt is removed and that the wall rests on murrum.This area is a hill slope zone almost rocky or dry degraded land with very little vegeation. Soil is whitish due to degradation of soft rock(murrum). Layer of black soil is only approximately 2′ after which we came across soft murrum.
Difference between soft murrum and hard murrum:
Soft murrum appeared to be whitish-yellow..distinctly different from the soil that we dig. Top soil if taken in your fist will crumble into powder. Or at the most of lumps of powder. Soft rocke/murrum when taken in a fist will break into flakes. The flakes cannot be broken easily by one hand or by just pressing on it. one would need two hands to crack it in the centre again into 2 flakes. This flake is not consolidated soil. It is disintegrated part of lower hard rock. This soft murrum is in the process of becoming soil. It is not soil coming together to form lumps.
Soft murrum is left exposed to sunlight or wera and tear will dry up quickly and break away into soil like powder/fine particles.Hence it is important to cover it or seal it up as soon as it is dug for a foundation. Thus the strenght of murrum remains intect.
Workers generally do a ठेपा ठेपी बांधकाम in stone over a base layer of pcc. this evens out the level. I was at this site where the big question was to whether we should go deeper upto 5′ below ground level so as to hit real hard rock. we had hit soft rock at 3′. the conclusion after a discussion with the structural engineer was that it is not necessary . soft murrum is hard enough for transferring and carrying load of a ground plus one loadbearing structure’s distributed load.
I write this with refernce to a site on katraj kondhwa road, Pune. The site is on a hill slope which is degraded, with very little vegetation and soil cover.