Depth and lateral extent of exploration

Depth of exploration

The boring must reach the soil that will be significantly stressed by the structure.

• Stress criterion: go to the depth where the added vertical stress $\Delta\sigma$ falls to about 10% of the foundation pressure $q$ (or 20% of the effective overburden stress at that depth).
• Thumb rules (IS 1892): isolated footing: about 1.5 times the width $B$ below the foundation level; closely spaced footings/raft: about 1.5 times the width of the loaded area; piles: 1.5 times the pile group width below the pile tip, or at least 5 m into hard stratum.
• Sowers' rule: $D = 3S^{0.7}$ for light steel/narrow concrete buildings and $D = 6S^{0.7}$ for heavy steel/wide concrete buildings ($S$ = number of storeys, $D$ in m).
• Borings must go through any fill, peat or soft layer to firm stratum, and at least 3 m into rock (or continue until bedrock is verified).
• Check the groundwater level.

Lateral extent (number and spacing of boreholes)

• Minimum 3 boreholes (not in line) for a small site; at least one at each corner and at the centre for large buildings.
• Reduce spacing if the strata change quickly.

Example

For a 5-storey RCC building, 20 m x 30 m plan on uniform clayey soil, with isolated footings of width $B$ about 2 m:

• Depth: Sowers' rule gives $D = 3 \times 5^{0.7} \approx 9.3$ m (light/medium building). The thumb rule $1.5B = 3$ m is much smaller, so the larger value governs; take about 10 m. If a raft were used, check the depth against 1.5 times the raft width as well.
• Spacing about 15 m, giving a 3 x 2 grid of 6 boreholes. Add one extra borehole where the soil is seen to vary, and one deep hole to check the rock/hard layer.