In soil mechanics, the number and disposition of bore holes are varied depending upon the ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 10/10/2023 (Morning)
Detailed Explanation
Boreholes are the main way to see what lies beneath a site. The engineer must decide how many to drill, where, and how deep.
Why “subsoil condition” Is Correct
If the strata are uniform and horizontal, a few widely spaced boreholes are enough. If the subsoil condition is erratic — layers that change thickness, lenses of soft clay, sloping rock — more boreholes at closer spacing are needed to map it. So the number and layout of boreholes depend mainly on the subsoil condition, together with the size and type of the structure.
| Type of project | Typical borehole spacing |
|---|---|
| Compact building on uniform soil | One at each corner and one at centre |
| Multi-storey buildings | About 10–30 m |
| Highways / long linear works | About 250–500 m |
| Dams, bridges | At every abutment and pier, closer where strata vary |
Only a guide: the spacing is reduced when the subsoil is erratic.
Why the Other Options Are Wrong
- A. earth surface: The ground surface shape matters for access, but the number of boreholes is set by what lies below.
- B. atmosphere: The atmosphere has nothing to do with subsurface investigation.
- D. underground water: Groundwater is recorded in boreholes but does not decide their number.
Key Point
Borehole number and spacing: governed by subsoil variability and the structure.
Key Concepts for Students
- Boring depth is usually taken to where the stress increase falls below about 10% of the applied pressure.
- Under a building, boreholes typically go to about 1.5 times the width of the loaded area.
- Trial pits suit shallow depths; boreholes go deeper.
Practice more: this is question 1 in Soil Exploration MCQs – Part 1. Try the whole set and check every answer instantly.