--- title: "Methods of surface and subsurface site investigation" subject: "Engineering Geology I" chapter: 1 key: "semester_3/engineering_geology_i:methods-of-surface-and-subsurface" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/methods-of-surface-and-subsurface site: BCT Engineering --- # Methods of surface and subsurface site investigation Engineering Geology I · Chapter 1: Geology and Civil Engineering - Asked in: 2062 Baisakh (old course), 8 marks; 2061 Baisakh (old course), 8 marks; 2066 Bhadra (old course), 6 marks; 2063 Baisakh (old course), 3 marks (asked 4 times) - Video: [Surface and Subsurface investigations](https://www.youtube.com/watch?v=kC7jQ4kvbyk) — Hemantkumar Bhosale - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/methods-of-surface-and-subsurface - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-1 ## Question Describe the methods of surface and sub-surface investigations for the selection of a suitable site to construct civil engineering structures (describe the methods of site investigation in the different phases). ## Short answer Surface methods include geological mapping, aerial photo interpretation and geophysics (seismic, resistivity). Subsurface methods involve test pits, trenches, adits, core drilling, sampling and in-situ tests (SPT, Lugeon). Investigation proceeds in phases: desk study, reconnaissance, preliminary, detailed, construction. ## Answer Site investigation is the study of the geological and ground conditions at a proposed site so that a safe and economical structure can be built. It is done in stages (phases), and the methods are of two groups: surface and subsurface. #### Phases of investigation 1. **Desk study / reconnaissance**: collect existing maps, aerial photographs, satellite images, reports and records; walk over the area to see main features. 2. **Preliminary investigation**: geological mapping, a few boreholes and pits, and geophysical profiles to compare alternative sites or alignments. 3. **Detailed investigation**: closely spaced drilling, trenches, in-situ tests, laboratory tests and geophysics on the selected site to supply design parameters. 4. **Construction stage**: inspection of excavations and foundations, and checking of assumptions. #### A. Surface methods - **Geological mapping**: rock types, bedding, folds, faults, joints, springs and slope stability recorded on a topographic base, giving an engineering geological map. - **Aerial photographs and remote sensing**: show lineaments, landslides, drainage and rock boundaries. - **Geophysical methods** (done from the surface): - Seismic refraction gives depth of bedrock and rock quality. - Electrical resistivity gives depth of water table, bedrock and soil layers. - Others are gravity, magnetic and ground-penetrating radar. #### B. Subsurface methods - **Test pits and trenches**: shallow, direct view of soil and weathered rock; give undisturbed samples; trenches across faults and shear zones. - **Drifts and adits**: tunnels driven into a hillside to inspect rock at depth (dam abutments, tunnels). - **Boring and drilling**: - Auger boring for soft soils. - Wash boring and percussion boring for soils. - Rotary core drilling for rocks, giving continuous cores. - Borehole logs record strata, core recovery and RQD. - **Sampling**: disturbed and undisturbed samples for laboratory tests. - **In-situ tests**: standard penetration test, plate load test, permeability (Lugeon) test, and shear tests. - **Borehole geophysics and logging** for fractures and water. ```text Desk study -> Reconnaissance -> Preliminary -> Detailed -> Construction stage ``` The results are combined into a report with maps, sections and recommendations. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the correct order of the phases of site investigation? A. Detailed, preliminary, desk study, construction B. Construction, detailed, reconnaissance, preliminary C. Preliminary, construction, desk study, detailed D. Desk study/reconnaissance, preliminary, detailed, construction stage Answer: D. Investigation goes from general collection of data to detailed, then checks during construction. (Key point: Phases) 2. The purpose of preliminary investigation is mainly to: A. Compare alternative sites or alignments B. Supply final design parameters with closely spaced drilling C. Inspect completed foundations D. Collect only existing maps Answer: A. Preliminary work compares options; detailed work supplies design parameters. (Key point: Preliminary investigation) 3. Seismic refraction from the surface gives: A. Continuous solid rock cores B. Depth of bedrock and rock quality C. SPT N values D. Undisturbed soil samples Answer: B. Seismic refraction indicates bedrock depth and quality without drilling. (Key point: Surface geophysics) 4. Which method gives continuous cores of rock? A. Auger boring B. Wash boring C. Rotary core drilling D. Percussion boring in soils Answer: C. Rotary core drilling recovers continuous cores; the others suit soils or give cuttings. 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