---
title: "Direct methods of sub-surface investigation"
subject: "Engineering Geology II"
chapter: 4
key: "semester_4/engineering_geology_ii:direct-subsurface-methods"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/direct-subsurface-methods
site: BCT Engineering
---

# Direct methods of sub-surface investigation

Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation

- Asked in: 2074 Bhadra, 3 marks; 2071 Bhadra, 3 marks (asked 2 times)
- Video: [Direct Methods of Soil Exploration | Trial Pits and CPT Methods    #soiltesting #siteinspection](https://www.youtube.com/watch?v=bcJZRBoDAb4) — Civil Engineering Fanatics
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/direct-subsurface-methods
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4

## Question

Describe the direct methods of sub-surface site investigation.

## Short answer

Direct sub-surface methods expose or sample ground: test pits, trenches, shafts, adits, and boreholes (auger, percussion, rotary core drilling). They yield soil/rock profiles, RQD, undisturbed samples, and permit in-situ tests like SPT and Lugeon, but are point-wise and costly.

## Answer

Direct sub-surface methods expose or sample the ground directly so that rock and soil can be seen, tested and logged.

1. **Test pits (trial pits):** open excavations, 1-3 m wide and up to 3-5 m deep; give a clear view of the soil profile, bed rock, water table; allow undisturbed block samples and in-situ tests. Used for shallow foundation and borrow areas.
2. **Trenches:** long narrow excavations across slopes, faults or landslides to expose structure and thickness of overburden.
3. **Shafts:** vertical excavations (up to 100 m) to examine rock at depth, used at dam sites and tunnels.
4. **Adits, drifts and exploratory tunnels:** horizontal tunnels into abutments of dams; allow geological mapping and in-situ tests such as shear, plate load and flat jack test on rock.
5. **Boring and drilling:**
   - *Auger boring* in soft soil;
   - *Wash boring* and *percussion boring* in soil;
   - *Rotary core drilling* in rock, giving cores for logging, RQD and lab tests; diamond bits used in hard rock.
   Samples taken: disturbed (SPT split spoon) and undisturbed (thin-walled Shelby tube).
6. **In-situ tests in boreholes:** standard penetration test, packer (Lugeon) permeability test, pressuremeter, borehole camera.
7. **Water-level observation** in holes.

Limits: these methods are costly and time-consuming and give information only at points, so they are supplemented with geophysics.

## Check yourself

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

1. Test pits are used for
   A. Deep rock drilling
   B. Dam abutment tunnels
   C. Geophysics
   D. Shallow foundations and borrow areas with a clear view of the soil profile
   Answer: D. They allow block samples. (Key point: Test pits)

2. Rotary core drilling gives
   A. Rock cores for logging, RQD and lab tests
   B. Only soil cuttings
   C. Satellite data
   D. Seismic velocity
   Answer: A. Diamond bits are used in hard rock. (Key point: Rotary core drilling)

3. Adits and drifts are used to
   A. Take aerial photos
   B. Map rock and do in-situ tests such as shear, plate load and flat jack
   C. Measure gravity
   D. Dewater
   Answer: B. They are horizontal tunnels into abutments. (Key point: Adits)

4. What is the limit of direct methods?
   A. Cheap and fast
   B. Cover large areas
   C. Costly, slow and give point information only
   D. Non-destructive
   Answer: C. They are supplemented by geophysics. (Key point: Limit)

## Close variants set in other exams

- [Surface versus sub-surface investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Asurface-vs-subsurface-investigation) — Engineering Geology II, chapter 4, asked 3×

## Related in Engineering Geology II

- [Indirect sub-surface methods: tunnel and dam](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aindirect-subsurface-tunnel-dam) — Engineering Geology II, chapter 4
- [Indirect methods of surface investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aindirect-surface-methods) — Engineering Geology II, chapter 4
- [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×
- [Types and methods of site investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Asite-investigation-methods) — Engineering Geology II, chapter 4, asked 7×
- [Detail stage of EG site investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Adetail-stage-investigation) — Engineering Geology II, chapter 4
- [Tunneling and site investigation process](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunneling-site-investigation-process) — Engineering Geology II, chapter 4

## Related in other subjects

- [Methods of surface and subsurface site investigation](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Amethods-of-surface-and-subsurface) — Engineering Geology I, chapter 1, asked 4×
- [Site investigation for dam and reservoir](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Asite-investigation-for-dam-and) — Engineering Geology I, chapter 1, asked 2×
- [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×
- [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×