Standard Penetration Test (SPT) Explained: Procedure, Use Cases, and Limits for Georgia Projects

Why SPT Still Dominates Site Investigation

The Standard Penetration Test is the most widely deployed in-situ test in geotechnical practice worldwide. It survives in a world that also has cone penetration testing, pressuremeters, and downhole geophysics for one stubborn reason: it is cheap, fast, available almost everywhere, and it pulls a soil sample to the surface on every blow. For Georgia, where rig fleets are mostly rotary and percussion-capable, SPT is also simply what the local supply chain delivers. A serious investigation in Tbilisi or Kutaisi today will almost always include an SPT profile.

That convenience comes with caveats. The test is operator-dependent, energy-sensitive, and unreliable in certain soils — yet decades of correlations let an experienced engineer extract bearing capacity, settlement, liquefaction triggering, and pile capacity from a column of N-values, provided the data is properly corrected. This article walks through the mechanics, the maths, and the field realities, with specific attention to how SPT behaves in Georgian ground conditions.

What the N-Value Actually Measures

The Standard Penetration Test produces a single number per depth interval: the N-value, also called the SPT blow count. The N-value is the number of hammer blows required to drive a standard split-spoon sampler 300 mm into the soil, after a 150 mm seating drive. The blows are counted in three increments of 150 mm each; the first 150 mm is the "seating drive" and is discarded; the sum of the second and third increments is the N-value.

A low N-value (say, N = 4) means the sampler advanced easily under the hammer — the soil is soft or loose. A high N-value (say, N = 50 in less than 300 mm, recorded as "refusal") means the sampler barely moved — the material is dense, very stiff, or rock-like. The number itself is dimensionless. Its value comes from the correlations built up across thousands of sites and decades of research, linking N to relative density in sands, undrained shear strength in clays, friction angle, modulus, and dynamic response.

Crucially, the N-value is not a direct soil property. It is a derived index. The same soil drilled by two rigs with two hammers will return different N-values. This is why correction factors exist, and why an SPT report that does not declare its hammer type and energy ratio is incomplete.

The ASTM D1586 Procedure, Step by Step

The reference procedure is ASTM D1586 — Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils. The equivalent European standard is EN ISO 22476-3. The two are operationally very close. The procedure:

Advance the borehole to the target depth using a method that does not disturb the soil below (rotary wash, hollow-stem auger, or cased borehole with clean bottom).

2. Lower the split-spoon sampler on drill rods to the bottom of the hole. The standard sampler has an outside diameter of 51 mm and an inside diameter of 35 mm, with a beveled drive shoe. 3. Drive the sampler using a 63.5 kg (140 lb) hammer falling freely through 760 mm (30 in). The drive proceeds in three 150 mm increments. 4. Record the blow count for each 150 mm increment. The N-value is the sum of the second and third increments. If 50 blows are recorded in any 150 mm before reaching the full 450 mm, the test is stopped and recorded as refusal. 5. Retrieve the sampler, open it, log the recovered soil, take samples for laboratory classification (water content, Atterberg limits, grain size, USCS classification per ASTM D2487), and seal jars or bags. 6. Repeat at typical intervals of 1.5 m through the depth of interest, or more frequently in transition zones and at engineering levels.

A good SPT log records, at minimum: depth, blow counts per increment, sampler recovery, hammer type, energy ratio if measured, drilling fluid level, groundwater observations, and a soil description per sample. The engineer reading the log six months later, possibly in another country, needs every one of those fields.

Energy, Depth, and Fines: The Corrections That Matter

The raw N-value is rarely used in design directly. Modern practice applies a chain of corrections that account for the actual energy delivered by the hammer, the depth of the test, and the soil composition. The most important ones:

Energy correction to N60 — The theoretical maximum energy of a 63.5 kg hammer falling 760 mm is about 475 J. Real hammers deliver less. N60 is the blow count normalized to 60% of theoretical energy: N60 = N × ER / 60, where ER is the measured energy ratio in %. Automatic trip hammers typically deliver ER ≈ 60-85%, safety hammers ≈ 60%, and donut hammers ≈ 45%. Without this correction, N-values from different rigs are not comparable.

Overburden correction to (N1)60 — In granular soils, the same relative density gives a higher N at greater depth because of confining stress. The correction (N1)60 = CN × N60, where CN is an overburden correction factor (Liao and Whitman, 1986: CN = (Pa/σ'v0)^0.5, capped at 1.7), normalizes the blow count to a reference effective vertical stress of 100 kPa.

Fines content correction — For liquefaction triggering analysis, the (N1)60 is further adjusted to (N1)60cs ("clean-sand equivalent") based on fines content, following procedures such as Youd et al. (2001) and Cetin et al. (2004). The correction increases the effective blow count to account for the additional liquefaction resistance contributed by plastic fines.

Skipping these corrections is a common error. A bearing capacity calculation on raw N gives a different answer than one on N60, and the difference can be the line between an oversized pile and a foundation that settles too much.

What You Can Do With an SPT Profile

Once you have a corrected N-value profile, the range of correlations is broad. The most commonly used in practice:

- Bearing capacity of shallow foundations — Meyerhof (1956) and later refinements give allowable bearing pressure on sand directly from N, calibrated against a tolerable settlement (typically 25 mm).

Settlement estimation — Burland & Burbidge (1985) provides a widely used SPT-based settlement method for shallow foundations on sand.

Friction angle in sand — Correlations from Peck, Hanson & Thornburn (1974) and Hatanaka & Uchida (1996) link (N1)60 to the effective friction angle φ', useful for retaining wall and slope checks.

Undrained shear strength in clay — Stroud (1974) and Terzaghi & Peck-style correlations link N to su, with the major caveat that SPT is unreliable in soft clays and the scatter is wide.

Liquefaction triggering — The Seed & Idriss (1971) simplified procedure, refined by Youd et al. (2001) and Cetin et al. (2004), uses (N1)60cs and the cyclic stress ratio (CSR) to estimate the factor of safety against triggering. For seismic-zone projects in Georgia, this is a core deliverable.

Pile capacity — Meyerhof's SPT-based pile capacity equations remain in active use for driven and bored piles. Skempton (1986) provides ageing guidance for old SPT data.

Where SPT Falls Short

For all its ubiquity, SPT has well-documented weaknesses any honest report should acknowledge:

- Operator and equipment dependence — Hammer type, rod length, sampler liner, borehole cleanliness, and driller technique all influence the result. Energy measurement reduces but does not eliminate this scatter.

Soft clays — In very soft to soft clays (N below 4), the test resolves poorly. Vane shear, CPTu, or laboratory triaxial on undisturbed Shelby samples (ASTM D1587) are more reliable.

Coarse gravels and cobbles — In Caucasian piedmont gravels and coarse alluvium along the Mtkvari and Rioni, the sampler often hits a cobble and registers artificial refusal. The N-value loses meaning.

Sample disturbance — The split-spoon is a disturbed-sample device. Lab tests requiring intact structure (consolidation, triaxial strength) need thin-walled tube samples per ASTM D1587, not SPT recovery.

Karst and voids — In karstified limestones of western Georgia, SPT cannot detect cavities reliably. Geophysics and targeted coring fill that gap.

SPT is a screening tool that produces a continuous profile and a sample on every blow. It is not a precision measurement of any single soil property. Treat it as such.

SPT in Georgia: Mtkvari Alluvium, Tbilisi Loess, Caucasus Weathered Rock

Georgian ground is varied, and SPT behaves differently across the major geological settings:

- Mtkvari alluvial valley (Tbilisi, Rustavi, Mtskheta) — Loose to medium dense fluvial sands and gravelly sands with interbedded silty clays. N-values often range from N = 8 to N = 25 in the upper 10 m. Groundwater is shallow in the lower terraces, which matters for liquefaction screening: Tbilisi sits in Eurocode 8 seismic zone II–III, and triggering analysis on saturated Mtkvari sands is a routine deliverable.

Tbilisi loess plateaus — Loess and loessial loams on the higher terraces. N-values are typically modest in the upper metres (N = 6 to 15) and the material is collapsible on wetting, which SPT does not measure directly. Double-oedometer testing on undisturbed samples is the proper complement.

Caucasus weathered rock and piedmont (Kutaisi, Borjomi, Kazbegi corridors) — Profiles transition from residual soil to weathered to fresh rock over short depths. SPT refusal signals the rock boundary, but rock characterization itself requires coring with RQD logging (ASTM D6032 / ISRM), not SPT.

Black Sea coast (Batumi, Poti) — Soft to very soft saturated alluvium and lagoonal deposits dominate. SPT is poor here; CPTu and field vane (ASTM D2573) are far more informative when budget allows.

When we run SPT for clients in Georgia the protocol we recommend is: automatic trip hammer, calibrated energy ratio, 1.5 m sampling interval, full N60 and (N1)60 correction, USCS classification on every recovered sample, and explicit flags on depths where SPT is unreliable.

For an end-to-end view of how investigations are scoped, executed, and reported, see our [soil investigation](/en/soil-investigation) page and the broader [geotechnical services](/en/geotechnical-services) overview.

Final Word

The Standard Penetration Test is not glamorous and it is not the most precise tool in the box. It is, however, the workhorse of geotechnical investigation in Georgia and across the Caucasus, and a properly corrected SPT profile remains the foundation of bearing capacity, settlement, liquefaction, and pile capacity analyses on the vast majority of projects we see. The difference between an SPT log that is useful and one that is not lies in the small details: hammer type, energy measurement, corrections applied, soils where the test is trusted, and soils where it is flagged as unreliable.

If you have a project where SPT data exists but you are not sure how to interpret it — or where investigation is still to be scoped — talk to our team. We will tell you exactly what the numbers do and do not support. Start with our [soil investigation](/en/soil-investigation) page.