The foundation is the one part of the house you can never fix later. Which type you need depends on three things: how strong the soil is, how much load each column brings down, and how close the columns sit to your plot boundary.
Start with a soil test
A soil investigation gives the safe bearing capacity (SBC) at different depths. It costs little compared with the foundation itself. Much of Patna and the Gangetic plain is alluvial soil with a high water table, and filled or low-lying plots are common. A test stops you from both over-designing and under-designing.
The main options
Isolated footings
These are individual square or rectangular pads under each column, and the standard choice for houses up to G+2 on reasonable soil. They are the most economical.
Combined and strap footings
Columns on the boundary line cannot have a footing that spreads onto the neighbour's land. The footing is either combined with the next column, or tied to it with a strap beam that balances the off-centre load. On narrow city plots this is very common.
Strip (wall) footings
A continuous footing under load-bearing walls. It is used for boundary walls, and for single-storey load-bearing buildings.
Raft foundation
A single thick slab under the whole building, used when soil is weak or footings would overlap. It costs more but spreads load evenly and handles uneven settlement better.
Pile foundation
Used when good soil is only found deep down, or on very soft filled ground. It needs specialist equipment and design.
Plan for the future floors now
If you may add a floor later, the foundation and columns must be designed for it today. Strengthening footings under an existing house is difficult and expensive. It is the most common regret we hear from owners who built "G+1 for now".
On-site essentials
- PCC bed (1:4:8) under every footing.
- 50 mm cover to the footing steel.
- A plinth beam tying the columns together.
- DPC at plinth level against rising damp.
