Geological Site Investigation for Engineering Works Notes

Last reviewed 16 Sept 2026 · 9 min read

Objectives of site investigation

Determine the geology — rock and soil types, their distribution, thickness and depth to bedrock.
Identify geological structures — faults, folds, joints, shear zones, cavities.
Assess engineering properties of soils and rocks — strength, permeability, compressibility.
Establish groundwater conditions.
Identify hazards — landslides, seismicity, liquefaction, subsidence, karst.
Locate construction materials — aggregates, impervious and pervious soils, building stone.
Provide data for design, construction methods and cost estimates, and reduce unforeseen ground conditions.

Stages of investigation

Stage	Activities
1. Desk study	Collection of existing data — topographic maps (Survey of India), geological maps and reports (Geological Survey of India), aerial photographs, satellite imagery, previous investigation records, well logs, seismic and flood data
2. Site reconnaissance	Walk-over survey — outcrops, landforms, drainage, vegetation, springs, landslides, existing structures and their performance, access
3. Preliminary investigation	Geological mapping, a few boreholes/pits, geophysical surveys; compare alternative sites or alignments
4. Detailed investigation	Closely spaced boreholes, sampling, in-situ and laboratory tests, detailed mapping for the selected site and structure layout
5. Construction-stage investigation	Verification of ground conditions as excavation proceeds — foundation mapping, tunnel face logging, additional drilling
6. Post-construction monitoring	Instrumentation — piezometers, inclinometers, settlement gauges, seepage measurements

Remote sensing and aerial photographs

Aerial photographs (stereo pairs) reveal lineaments (possible faults, joints), landslides, drainage patterns, rock outcrops, alluvial fans and old channels.
Satellite imagery (multispectral) helps regional mapping of lithology, structures, land use and groundwater potential zones.
GIS integrates maps and data layers; LiDAR gives high-resolution terrain models.
Drainage patterns indicate geology — dendritic (uniform rocks), trellis (folded sedimentary rocks), rectangular (jointed/faulted rocks), radial (domes/volcanoes), annular (eroded domes).

Subsurface exploration

Direct methods

Method	Features / use
Test pits	Shallow (a few metres), allow direct visual examination and undisturbed block samples; cheap for shallow depths above water table
Trenches	Continuous exposure across contacts or faults (e.g. fault trenching to study active faults)
Drifts and adits	Horizontal tunnels into hillsides — dam abutments and tunnel sites
Shafts	Vertical excavations for deep inspection

Boring methods

Method	Principle	Suitability
Auger boring	Hand or power auger rotated into soil	Soft to stiff cohesive soils, shallow depths, above water table; disturbed samples
Wash boring	Chopping bit with water jet; cuttings washed up	Soils (not boulders/rock); fast; samples highly disturbed — used with separate samplers
Percussion (cable tool) boring	Repeated lifting and dropping of a heavy bit; slurry bailed out	Gravels, boulders, rock; disturbs material
Rotary drilling	Rotating bit with circulating fluid	Soils and rocks; fast
Rotary core drilling	Diamond/tungsten carbide core barrel (single, double tube, triple tube) recovers rock cores	Rock — cores for logging, RQD and testing

Core logging, core recovery and RQD

FormulaCore recovery and RQD

Core recovery ratio (CR)

C R = \frac{Total length of core recovered}{Length of core run} \times 100

Rock Quality Designation (Deere)

R Q D = \frac{\sum length of intact core pieces \geq 100 mm}{Length of core run} \times 100

(Measured along the core centre line on NX-size or similar cores; mechanical breaks from drilling are ignored.)

RQD (%)	Rock quality
< 25	Very poor
25–50	Poor
50–75	Fair
75–90	Good
90–100	Excellent

Core logs also record rock type, weathering grade, discontinuity spacing and condition, fracture index and water losses (packer/Lugeon tests for permeability of rock).

Geophysical methods (indirect)

Geophysical methods are rapid and economical for covering large areas, but must be correlated with boreholes.

Seismic refraction

Seismic waves generated (hammer/explosive) are detected by geophones; first arrival times are plotted against distance.
Wave velocity increases with rock density/soundness — loose soil low velocity, fresh rock high velocity; a velocity increase with depth is required.
Used for depth to bedrock, rippability of rock, rock quality.

FormulaTwo-layer refraction

Direct wave: $t = x / V_{1}$ ; refracted wave arrives first beyond the crossover distance $x_{c}$ .

h = \frac{x _{c}}{2} \frac{V _{2} - V _{1}}{V _{2} + V _{1}}

$h$ = depth to the interface; $V_{1}$ , $V_{2}$ = velocities in upper and lower layers ( $V_{2} > V_{1}$ ).

Electrical resistivity

Current passed through the ground between two current electrodes; potential difference measured between two potential electrodes.
Resistivity depends on rock type, porosity, water content and salinity — clays and saline water have low resistivity; dry sand, gravel and hard rock high.
Wenner array — four equally spaced electrodes (spacing $a$ ); Schlumberger array — potential electrodes closely spaced at the centre.
Vertical electrical sounding (VES) — increasing spacing to probe deeper layers; profiling — constant spacing along a line.
Widely used for groundwater exploration, depth to bedrock, locating clay/saline zones.

FormulaWenner apparent resistivity

ρ_{a} = 2 π a \frac{V}{I} = 2 π a R

Other methods

Ground penetrating radar (GPR) — shallow high-resolution imaging (utilities, cavities, pavement layers).
Magnetic surveys — magnetic minerals, dykes, buried objects.
Gravity surveys — density variations, cavities, basins.
Seismic reflection — deep structures (oil exploration).
Borehole geophysics — logging within boreholes; cross-hole seismic tests.

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