---
title: "Depth and lateral extent of exploration"
subject: "Foundation Engineering"
chapter: 2
key: "semester_5/foundation_engineering:depth-lateral-extent-exploration"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/depth-lateral-extent-exploration
site: BCT Engineering
---

# Depth and lateral extent of exploration

Foundation Engineering · Chapter 2: Soil Exploration

- Asked in: 2082 Bhadra, 4 marks; 2076 Asoj, 2+5 marks; 2078 Bhadra, 3 marks; 2079 Bhadra, 3 marks; 2075 Chaitra, 4 marks (asked 5 times)
- Video: [Depth of Exploration - Sub-Surface Investigation - Foundation Engineering GATE](https://www.youtube.com/watch?v=L5MV0i-32-0) — Ekeeda GATE & ESE
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/depth-lateral-extent-exploration
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-2

## Question

How would you decide the depth and lateral extent (number, spacing and depth of boreholes) of a subsurface/site investigation? Explain with a suitable example.

## Short answer

Depth is determined by stress criterion (Δσ ≈ 10% of q) or thumb rules: 1.5B for footings, about 1.5 times loaded width for rafts, and Sowers’ rule D=3S^0.7. Borehole spacing is 10–30 m for multistory buildings, with minimum three not in line. For a 5-storey RCC building, depth ~10 m and 6 boreholes at ~15 m spacing.

## Answer

#### 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)
| Structure | Spacing |
|---|---|
| Multi-storey buildings | 10-30 m |
| One-storey industrial | 20-60 m |
| Highways | 250-500 m (closer in variable soil) |
| Earth dams | 30-60 m |

- 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. By the stress criterion, boring is carried to the depth where the added vertical stress falls to about
   A. 50% of q
   B. 10% of the foundation pressure q
   C. 90% of q
   D. zero exactly
   Answer: B. Soil below this depth is not significantly stressed by the structure. (Key point: Stress criterion)

2. Sowers' rule for light buildings gives the boring depth as
   A. D = 3 S
   B. D = 3 S^0.7 (S = storeys, D in m)
   C. D = 0.7 S^3
   D. D = 6 S^0.7 for light buildings
   Answer: B. 3 S^0.7 is for light steel or narrow concrete buildings; 6 S^0.7 for heavy ones. (Key point: Sowers' rule)

3. Typical borehole spacing for multi-storey buildings is
   A. 250-500 m
   B. 1-2 m
   C. 10-30 m
   D. 100-200 m
   Answer: C. The 250-500 m range is for highways; buildings use 10-30 m. (Key point: Spacing)

4. What is the minimum number of boreholes for a small site?
   A. 1
   B. 2 in a line
   C. 6 in a line
   D. 3, not in a line
   Answer: D. At least 3 non-collinear boreholes are needed to define the strata plane. (Key point: Number of boreholes)

## Related in Foundation Engineering

- [Contents of site investigation report](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asite-investigation-report) — Foundation Engineering, chapter 2, asked 5×
- [Importance and stages of site investigation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aimportance-stages-site-investigation) — Foundation Engineering, chapter 2
- [Information from preliminary exploration](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aprelim-exploration-info) — Foundation Engineering, chapter 2
- [Investigation details for bridge foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abridge-foundation-investigation) — Foundation Engineering, chapter 2
- [Spacing for 4×5 pile group, block failure](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-20-piles-block-failure) — Foundation Engineering, chapter 8
- [Depth of well and sinking process](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-depth-sinking) — Foundation Engineering, chapter 9, asked 3×

## Related in other subjects

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- [Purposes of site investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Asite-investigation-purposes) — Engineering Geology II, chapter 4, asked 7×
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- [Engineering geological map and site investigation](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-geological-map-and-site) — Engineering Geology I, chapter 1, asked 3×