Soil Mechanics / Ground investigation and ground profiles

Methods, sampling and groundwater

Intrusive and in-situ methods, sample quality, and measuring groundwater.

Lesson review: expert verification remains open

This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Corrections checked in this pass

Added walkovers and corrected claims about sampling and investigation-method exclusions.

Checks needed for project use

A geotechnical reviewer must check laboratory interpretation, classification conventions and the limits of drained, undrained and one-dimensional idealisations.
Method, locations, depths, sampling quality, access and safety require project-specific investigation design.

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

Read the lesson register and outstanding review notes

Why this matters

No single method sees everything. Choosing an investigation means matching each tool to the ground (soft clay, dense sand, rock) and to what you must measure (a profile, a strength, a groundwater level, an undisturbed sample). The art is assembling a small set of methods that together answer the project's questions.

Intrusive methods reach and recover the ground:

Trial pits — a machine-dug hole, cheap and giving a direct view of shallow ground and made ground, but limited in depth and below the water table.
Cable-percussion boreholes — the workhorse in soils, giving a borehole with samples and in-situ tests to moderate depth.
Rotary core drilling — cuts a continuous core, the method for rock and stiff strata.

In-situ tests measure the ground where it lies, avoiding sampling disturbance:

Standard penetration test (SPT) — a driven-sampler blow count, N, correlated with the density/strength of sands.
Cone penetration test (CPT) — a cone pushed at constant rate, giving a continuous, repeatable profile of resistance.
In-situ vane — measures the undrained shear strength of soft clay directly in the ground.

Geophysics (seismic, resistivity) profiles the ground non-intrusively between boreholes.

Samples come in grades of disturbance, and the grade limits what you can test:

Disturbed / bulk samples — the grains are jumbled but present; good for classification (grading, Atterberg limits) but not for strength or stiffness.
Undisturbed samples (thin-wall tube, or block) — the fabric and water content are preserved as far as possible; needed for meaningful strength, stiffness and consolidation tests.

And groundwater is measured, not guessed: a standpipe gives a water level, a piezometer the pore pressure at a chosen depth, monitored over time because levels vary seasonally.

Try it

Ground-investigation planner

Compare methods with the priorities assumed in this example. A method outside the suggested set is not automatically unsuitable; site access, sampling quality and the ground model determine the programme.

Scenario

How to choose

Always start with a desk study — it is the cheapest and shapes everything else.
Match the method to the ground (soft clay vs rock) and to what you must measure (strength, groundwater, samples).
High-quality sampling and appropriate in-situ tests depend on soil type and disturbance sensitivity. A vane can suit soft clay; rotary coring can recover rock. No sampling method gives a perfectly undisturbed specimen.

Select the methods this scenario needs, then press Check.

The SPT blow count needs one correction almost always applied: the hammer does not deliver the same energy on every rig, so N is normalised to a standard 60% energy ratio to make results comparable:

SPT energy normalisation

N_{60} = N\,\dfrac{E_r}{60}

What it calculates: the blow count corrected to a standard 60% energy ratio

$N_{60}$: energy-normalised blow count
N: measured blow count
E_r: the rig's energy ratio (%)

This assumes

Only the energy correction is applied here; further corrections (overburden, rod length, sampler) are standard-dependent and must be taken from the current standard.

In plain terms: Energy normalisation lets blow counts from different rigs be compared. It is the one correction you can state generally; anything beyond it should be verified against the current standard for the jurisdiction.

Practice

An SPT gives a measured blow count N = 24 on a rig with an energy ratio of 75%. What is the energy-normalised value N₆₀?

Summary

Match methods to the ground and to what must be measured; no one method does everything.
Trial pits (shallow), cable percussion (soils), rotary coring (rock); SPT/CPT/vane test in situ.
Disturbed/bulk samples suit classification; undisturbed samples are needed for strength and stiffness.
Measure groundwater with standpipes/piezometers over time; normalise SPT N to N₆₀ and verify any further correlation against the current standard.