Every time we start a foundation work, we will fill “X” inches of sand (specifically river sand) and then “X” inches of PCC bed under every footing.
The purpose of this may differ due to the soil condition and the need. sometimes we skip this (rarely).
Sand Filling in Footing
Situation 1 – For uniformity
If the foundation soil has more clay and silt content with water, river sand is used to provide uniformity for the foundation bed (especially for PCC bed).
Situation 2 – To minimize the resistance
If the footing is placed directly on the rock bed, even a small sensible amount of vibration on the earth (during an earthquake) will make the structure collapse. Sand is used to minimizing that resistance
Situation 3 – To minimize the weak spots
Even though we use anti-termite in foundations, there may be some weak spots for termite entry. This is one of the reasons why we use sand (with anti-termite treatment) below the foundation
Manufactured Sand – We also use M-sand instead of river sand. The process of replacing sand in concrete with M-sand has been successfully tested. Comparing to river sand, M-sand is reliable and more economic.
Purpose of PCC bed
PCC bed has used for the same set of purposes, giving uniformity to the foundation bed and to prevent termite entry to the structure.
Think if the footing has been placed directly above the unstable sand, it may collapse due to weather condition. Soil may be expansive or become more unstable during rain times if so then the structure will collapse fully.
If there are any other reasons, let us know ?
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