Soil Exploration, Sampling & In-situ Tests Notes

Last reviewed 16 Sept 2026 · 8 min read

Objectives

Soil exploration (site investigation) determines the subsurface conditions needed for safe, economical design:

sequence, thickness and extent of soil and rock strata;
ground water level and its variation;
engineering properties — strength, compressibility, permeability — from samples and in-situ tests;
suitable type and depth of foundation, bearing capacity and settlement;
construction problems (excavation support, dewatering, aggressive chemicals) and suitability of borrow materials.

Stages

Reconnaissance — desk study of geological maps, aerial images, previous records; site visit to see topography, drainage, existing structures and cracks, vegetation, cuttings, wells.
Preliminary exploration — a few borings and tests to establish general stratigraphy.
Detailed exploration — enough borings, sampling and tests for design; sometimes construction-stage checks.

Planning

Spacing of borings depends on variability and structure type; for example about 15–30 m apart for multi-storey buildings and closer for heavy or sensitive structures; along roads and pipelines at larger intervals (hundreds of metres).
Depth of exploration — until the stress increase from the structure becomes negligible (about 10% of the applied pressure or 20% of the effective overburden), commonly about 1.5 to 2 times the width of isolated footings or the loaded area; for pile foundations, well beyond the pile tips (at least about 1.5 times the pile-group width below the tips); into rock where rock is met, to prove it is bedrock and not a boulder (a few metres of coring).
Critical zones — expected compressible layers, weak seams, water table.

Methods of exploration

Method	Description	Use / limitation
Test pits / trenches	Open excavations	Direct visual inspection and block samples; shallow (a few m), above water table
Auger boring	Hand or power augers (post-hole, helical)	Cohesive soils above water table; disturbed samples
Wash boring	Casing driven, soil loosened by chisel bit and water jet, cuttings washed up	Quick in soils; poor identification (cuttings washed); samples taken by samplers at intervals
Rotary drilling	Rotating bit with drilling fluid (bentonite mud)	Soils and rock; rock coring with core barrels
Percussion drilling	Heavy bit repeatedly dropped	Gravels, boulders, hard strata
Geophysical methods	Seismic refraction (wave velocities), electrical resistivity	Rapid coverage of large areas, depth to rock, water table; need boring confirmation

Samples and samplers

Disturbed samples — structure destroyed but mineral composition and (if sealed) water content preserved; used for classification, index tests, compaction.
Undisturbed samples — structure and water content substantially preserved; needed for shear strength, consolidation and permeability tests. Truly undisturbed sampling is impossible; disturbance is minimised by design.

FormulaSampler design parameters

Area ratio (disturbance from wall thickness):

A_{r} = \frac{D _{o}^{2} - D _{i}^{2}}{D _{i}^{2}} \times 100%

Should not exceed about 10% for good undisturbed samples of soft clays (up to about 20% in stiff soils).

Inside clearance $C_{i} = \frac{D _{s} - D _{c}}{D _{c}} \times 100%$ — about 1–3% (reduces friction of the sample on the tube wall). Outside clearance $C_{o} = \frac{D _{w} - D _{t}}{D _{t}} \times 100%$ — about 0–2% (eases withdrawal). $D_{c}$ = cutting edge inner diameter, $D_{s}$ = inner diameter of tube, $D_{w}$ = outer diameter of cutting edge, $D_{t}$ = outer diameter of tube.

Recovery ratio $L_{r} = \frac{length of sample recovered}{length of penetration}$ — ideally close to 1.

Sampler	Use
Split-spoon (SPT) sampler	Disturbed samples during SPT
Thin-walled (Shelby) tube	Undisturbed samples in soft to medium clays
Piston sampler (stationary/fixed piston)	Very soft, sensitive clays and loose sands
Denison / double-tube core barrel	Stiff clays, dense sands
Core barrels (single, double, triple tube)	Rock cores

Rock Quality Designation (RQD): sum of lengths of core pieces ≥ 100 mm divided by total length of the core run × 100%. RQD < 25% very poor; 25–50 poor; 50–75 fair; 75–90 good; 90–100 excellent.

Standard Penetration Test (SPT)

Code ProvisionSPT (IS 2131) — procedure

Split-spoon sampler (outer diameter 50.8 mm) driven at the bottom of a clean borehole.
Hammer mass 63.5 kg, free fall 750 mm.
Driven 450 mm in three 150 mm stages; the first 150 mm is seating drive and ignored.
N-value = number of blows for the last 300 mm. Refusal is recorded when more than about 50 blows are needed for 150 mm (or no advance).

Corrections

Overburden pressure correction (granular soils) — N at shallow depth is too low because confining stress is low. A common form (Peck, Hanson & Thornburn):

N_{1} = C_{N} N, C_{N} = 0.77 lo g_{10} \frac{2000}{σ _{v}^{'}} (σ_{v}^{'} in kPa)

Dilatancy correction (very fine or silty sand below the water table, apply after overburden correction when the value exceeds 15):

N_{c} = 15 + \frac{1}{2} (N_{1} - 15)

Energy (hammer efficiency) correction to $N_{60}$ is used in modern practice.

Correlations

N (sands)	Relative density
0–4	Very loose
4–10	Loose
10–30	Medium
30–50	Dense
> 50	Very dense

N (clays)	Consistency
< 2	Very soft
2–4	Soft
4–8	Medium
8–15	Stiff
15–30	Very stiff
> 30	Hard

N also correlates with $ϕ$ of sands (roughly 28° at N = 5 to about 40° at N = 50) and is used in IS 6403 / IS 8009 bearing capacity and settlement charts.

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Soil Exploration, Sampling & In-situ Tests