Having built houses in different parts of state of Kerala,I have come across different beliefs in different areas about foundation work for a building.
Kerala is yet to reach a situation were professionals are sought for design and construction of a building.Professional Architect and structural Engineer is considered to be ornamental requirements for those who build large costly houses.Most of the professionals of Kerala are also responsible to a great extent for this alienation from the majority of their possible clientele ,as they never tried to make themselves easily accessible to their possible clients.
People of Kerala have this peculiar behavior where they try to avoid making payments to professionals for their services.I have come across several people who ask “Why you want to take thousands of rupees for drawing some lines as plan of the house?”They will not bother about spending lakhs of rupees but spending few thousands will be extremely painful to them especially if it is design charges or consultation charges.For such services they expect the professional to be happy with whatever they may please to pay without specifying any specific fees or charges.
In different locations of Kerala people are following certain practices for doing foundation of a building without bothering to check with a qualified professional to see whether that procedure is the right ,technically correct way of doing foundation of a building.
In most of the places especially in southern Kerala people believe that excavation for foundation of a building should be at least six feet deep.Soil in most of these places are quite strong and in some places it is even hard laterite.There is no point in breaking this solid soil strata to have “deep excavation”.Technically depth of excavation do not matter much if you have a stable soil.Of course having wider excavation helps to increase strength of foundation.
After excavation in most of the places people use dry pack rubble foundation.Instead of doing rubble dry pack foundation carefully with good tight packing,rubble is simply thrown into the deep excavation area leaving lots of gap in between rubble pieces.This actually weakens foundation.In stable soil conditions you do not need more than two feet depth for excavation.It is better to make it about 45 cm so that foundation when done with careful dry packing of rubble will have about 15 cm projecting up.This 15 cm or slightly lower projection of foundation due to ups and downs of ground level can be used to raise ground level slightly higher.
Once the foundation dry packing of rubble is done fill the gaps of rubble work with crusher sand using water forced into the foundation as can be seen in the pictures below
Force fill crusher sand into rubble foundation till water will not go down any more thus making your foundation rubble work one monolithic water tight rubble work. You can then be sure of strength of your rubble foundation.
Above proposed ground level rubble foundation is called Basement.You can either do it again with dry pack rubble work(which I recommend)or Rubble work in cement mortar.In case you do daypack,point outside of basement with mortar to fill gaps between rubble pieces.Again fill inside of basement with red sand for filling and force fill this sand into basement rubble work using water forced inside as in the case of foundation this time from inside basement.Top of basement may be given a plastering with mortar.See pictures.