{"id":1582,"date":"2026-10-28T11:55:15","date_gmt":"2026-10-28T08:55:15","guid":{"rendered":"https:\/\/setaf2018.com\/?p=1582"},"modified":"2026-10-28T12:04:16","modified_gmt":"2026-10-28T09:04:16","slug":"spt-corrections","status":"publish","type":"post","link":"https:\/\/setaf2018.com\/en\/post\/spt-corrections\/","title":{"rendered":"SPT Corrections: CN, N60 and (N1)60 Step by Step"},"content":{"rendered":"\n

An SPT correction converts the raw blow count recorded in a borehole into a value that can be compared with other boreholes, other equipment and the published correlations. The raw number is a measurement of a particular hammer, on a particular rod string, at a particular depth. The corrected number is what design uses.<\/p>\n\n\n\n

\"SPT<\/figure>\n\n\n\n

The distinction is routinely lost. A borehole log gives N, a correlation gives a friction angle from N, and the engineer applies one to the other without asking which N the correlation was built on. The correlations were derived from corrected values, and using a raw count in their place introduces an error that is systematic rather than random.<\/p>\n\n\n\n

The blow count is not a soil property. It is a record of the soil, the hammer and the rods together, and two of the three have to be removed before the third can be used.<\/strong><\/p>\n\n\n\n

What Does the Raw Blow Count Measure?<\/h2>\n\n\n\n

The number of hammer blows required to drive a standard split spoon sampler 300 mm into the soil, after an initial 150 mm seating drive that is discarded.<\/p>\n\n\n\n

The test is standardised in geometry: sampler dimensions, hammer mass of 63.5 kg and drop height of 760 mm are fixed. What is not standardised in practice is how much of the hammer’s potential energy actually reaches the sampler, and that is where the corrections begin.<\/p>\n\n\n\n

Why Is Energy Corrected First?<\/h2>\n\n\n\n

Because the delivered energy varies between rigs by a factor that dwarfs most other effects.<\/p>\n\n\n\n

A free-falling hammer should deliver its full potential energy to the rod string. In practice a rope-and-cathead system with two turns of rope on the drum delivers something in the region of 45 to 60 per cent, while an automatic trip hammer delivers a substantially higher and more repeatable fraction. The same soil tested with two rigs produces different blow counts for no reason connected with the ground.<\/p>\n\n\n\n

The convention is to normalise to 60 per cent of the theoretical energy, which is approximately what the historical correlations were based on:<\/p>\n\n\n\n

N\u2086\u2080 = N \u00b7 ( ER \/ 60 ) \u00b7 CB \u00b7 CS \u00b7 CR<\/strong><\/p>\n\n\n\n

where ER is the measured or assumed energy ratio of the hammer system as a percentage, and CB, CS and CR are the borehole diameter, sampler and rod length corrections.<\/p>\n\n\n\n

The energy ratio should come from a measurement on the rig being used. Where it is assumed from the hammer type, that assumption belongs in the report, because it is the single largest correction in the chain.<\/p>\n\n\n\n

What Are the Equipment Corrections?<\/h2>\n\n\n\n
Correction<\/th>What it accounts for<\/th>Typical direction<\/th><\/tr><\/thead>
CB, borehole diameter<\/td>Larger holes reduce confinement around the sampler<\/td>Slightly above 1.0 for large diameters<\/td><\/tr>
CS, sampler type<\/td>Samplers with and without a liner behave differently<\/td>Above 1.0 where the liner space is empty<\/td><\/tr>
CR, rod length<\/td>Short rod strings reflect the stress wave before full energy transfer<\/td>Below 1.0 for shallow tests, approaching 1.0 with depth<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The rod length correction is the one most often omitted and the one that matters most near the surface. A test at three metres depth with a short rod string can transfer appreciably less energy than the same test at fifteen metres, and the shallow layers are frequently the ones governing a shallow foundation.<\/p>\n\n\n\n

\"SPT<\/figure>\n\n\n\n

What Does the Overburden Correction Do?<\/h2>\n\n\n\n

It removes the effect of depth, so that the same soil at different depths gives the same number.<\/p>\n\n\n\n

A dense sand at twenty metres is confined by far more overburden than the same sand at two metres, and it resists penetration accordingly. Without correction, a uniform deposit appears to become denser with depth. The overburden correction normalises the blow count to a reference effective stress, conventionally one atmosphere:<\/p>\n\n\n\n

( N\u2081 )\u2086\u2080 = CN \u00b7 N\u2086\u2080<\/strong><\/p>\n\n\n\n

with a commonly used form<\/p>\n\n\n\n

CN = \u221a( Pa \/ \u03c3\u2032v )<\/strong><\/p>\n\n\n\n

where Pa is atmospheric pressure and \u03c3\u2032v the vertical effective stress at the test depth, both in the same units. The value is capped, typically at about 1.7, because the expression grows without limit as the effective stress approaches zero near the surface.<\/p>\n\n\n\n

Two things follow. The correction depends on effective stress, so it depends on the groundwater level and on the unit weights<\/a> used to build the stress profile: an error in either propagates into every corrected blow count below it. And the cap matters in the top few metres, where an uncapped CN would produce values with no physical meaning.<\/p>\n\n\n\n

Does the Order of the Corrections Matter?<\/h2>\n\n\n\n

Yes, and reversing it is a common error.<\/p>\n\n\n\n

Energy and equipment corrections are applied first, producing N\u2086\u2080. The overburden correction is then applied to that result, producing (N\u2081)\u2086\u2080. Applying CN to the raw N and then correcting for energy gives a different answer, because the cap on CN interacts with the value it is applied to.<\/p>\n\n\n\n

The notation carries the order. N\u2086\u2080 means energy-corrected only; (N\u2081)\u2086\u2080 means energy-corrected and then normalised to one atmosphere. A log reporting “corrected N” without saying which of the two is being reported is ambiguous, and the difference between them at depth is large.<\/p>\n\n\n\n

When Is the Fine Sand Correction Used?<\/h2>\n\n\n\n

In dense saturated fine sands and silts, where the measured value is inflated.<\/p>\n\n\n\n

In these materials, rapid driving generates pore pressure that cannot dissipate during the test, and the resulting dilatancy raises the blow count above what the soil’s density justifies. The traditional correction reduces values above 15:<\/p>\n\n\n\n

Ncorr = 15 + 0.5 \u00b7 ( N \u2212 15 )<\/strong><\/p>\n\n\n\n

It applies only to the material it was derived for. Applied to a clean sand or to a coarse deposit it removes strength the soil actually has, and the report should state where it was and was not applied.<\/p>\n\n\n\n

Which Corrected Value Does Each Application Need?<\/h2>\n\n\n\n
Application<\/th>Value required<\/th><\/tr><\/thead>
Correlations to friction angle and relative density<\/td>Usually (N\u2081)\u2086\u2080, occasionally N\u2086\u2080 depending on the source<\/td><\/tr>
Liquefaction triggering assessment<\/td>(N\u2081)\u2086\u2080 with a fines content adjustment<\/td><\/tr>
Bearing capacity from SPT in sand<\/td>The value specified by the particular method, often N\u2086\u2080<\/td><\/tr>
Settlement estimates from SPT<\/td>As defined by the method being used<\/td><\/tr>
Comparison between boreholes on one site<\/td>N\u2086\u2080 at minimum, (N\u2081)\u2086\u2080 where depths differ<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

There is no universally correct corrected value. Each published method was calibrated against a specific one, and the correlation used defines which is required. This is why the correlation between SPT N and friction angle<\/a> cannot be applied without first establishing what the correction state of the number is.<\/p>\n\n\n\n

Where Does the Correction Chain Break Down?<\/h2>\n\n\n\n

In materials the test was not designed for.<\/p>\n\n\n\n

In gravels and cobbles the blow count reflects particles striking the sampler shoe rather than the resistance of the mass, and no correction recovers a meaningful value. In weathered rock, refusal is reported as a partial penetration and extrapolating it to an equivalent N is an invention rather than a measurement. In soft clays the test has poor resolution, and undrained strength from a laboratory test or a vane is a better route than a correlation from a blow count of two or three.<\/p>\n\n\n\n

The corrections assume the test worked. Where it did not, correcting the number carefully produces a precise version of something that was never a measurement.<\/p>\n\n\n\n

What Should the Borehole Log Record?<\/h2>\n\n\n\n

Enough for the corrections to be checked.<\/p>\n\n\n\n

The raw blow counts for each 150 mm increment rather than only the summed N, so a seating anomaly is visible. The hammer type and the energy ratio, measured where possible. The borehole diameter, the sampler type and whether a liner was used. The rod length at each test. And the groundwater level with the date and method of measurement, since the overburden correction depends on it.<\/p>\n\n\n\n

A log carrying only depth and N forces every subsequent user to assume the corrections, and assumptions made downstream are never recorded.<\/p>\n\n\n\n

Common Errors in SPT Correction<\/h2>\n\n\n\n
Error<\/th>Consequence<\/th><\/tr><\/thead>
Applying a correlation to raw N<\/td>Systematic error in the direction the correlation was calibrated against<\/td><\/tr>
Reporting “corrected N” without stating N\u2086\u2080 or (N\u2081)\u2086\u2080<\/td>Result cannot be used or reproduced by anyone else<\/td><\/tr>
Applying CN before the energy correction<\/td>Cap interacts with the wrong value, changing the result<\/td><\/tr>
Omitting the rod length correction in shallow tests<\/td>Shallow layers, often the governing ones, overestimated<\/td><\/tr>
Using an assumed energy ratio without stating it<\/td>The largest correction in the chain is undocumented<\/td><\/tr>
Applying the fine sand correction outside dense saturated fine sand<\/td>Strength removed that the soil has<\/td><\/tr>
Correcting blow counts in gravel or at refusal<\/td>Precision applied to a number that was never a measurement<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Where SETAF2018 Fits<\/h2>\n\n\n\n

SETAF2018 applies the SPT correction chain inside the borehole record, so the corrected values used in design are derived from the entered field data rather than typed in separately.<\/p>\n\n\n\n

Each borehole carries its SPT records with depth and blow count, and the software computes CN, N\u2081, N\u2086\u2080 and (N\u2081)\u2086\u2080 from them together with the effective stress profile built from the layer unit weights and the groundwater conditions, including artesian pressure and capillarity. Because the stress profile and the blow counts live in the same soil profile<\/a>, a change to a water level or a unit weight updates every corrected value beneath it rather than leaving a stale number in a table.<\/p>\n\n\n\n

The corrected values then feed the methods that need them: SPT-based bearing capacity in granular soils within the foundation analysis<\/a>, the correlations used to derive strength and stiffness parameters, and liquefaction assessment<\/a> to TBDY 2018 Annex 16B with post-liquefaction settlement by Tokimatsu\u2013Seed. Menard pressuremeter profiles are carried alongside where that test is used instead.<\/p>\n\n\n\n

Calculation reports<\/a> print the correction expressions with the intermediate values, so a reviewer can see which energy ratio was assumed, where the cap on CN governed, and which corrected value each subsequent method consumed.<\/p>\n\n\n\n

Frequently Asked Questions<\/h2>\n\n\n\n

What is an SPT correction?<\/h3>\n\n\n\n

It is the adjustment of a raw standard penetration test blow count for the energy delivered by the hammer system, for the equipment used, and for the overburden stress at the test depth, so that the value can be compared between boreholes and used with published correlations.<\/p>\n\n\n\n

What is the difference between N60 and (N1)60?<\/h3>\n\n\n\n

N\u2086\u2080 is the blow count corrected to 60 per cent hammer energy together with the borehole, sampler and rod length corrections. (N\u2081)\u2086\u2080 is that value further normalised to a vertical effective stress of one atmosphere. Reporting one as the other changes the result significantly at depth.<\/p>\n\n\n\n

Why is the energy correction applied first?<\/h3>\n\n\n\n

Because the overburden correction is capped, and applying the cap to a value that has not yet been energy-corrected produces a different result. The convention is energy and equipment first to obtain N\u2086\u2080, then the overburden normalisation to obtain (N\u2081)\u2086\u2080.<\/p>\n\n\n\n

How is the overburden correction calculated?<\/h3>\n\n\n\n

From the vertical effective stress at the test depth relative to atmospheric pressure, commonly as the square root of their ratio, with an upper cap of about 1.7. Because it depends on effective stress, it depends directly on the groundwater level and the unit weights used.<\/p>\n\n\n\n

Does the rod length correction matter?<\/h3>\n\n\n\n

Yes, particularly in shallow tests. A short rod string reflects the stress wave before the full energy transfers to the sampler, so the correction is below unity near the surface and approaches unity with depth. Shallow layers are often the ones governing a shallow foundation.<\/p>\n\n\n\n

When is the fine sand correction applied?<\/h3>\n\n\n\n

Only in dense saturated fine sands and silts, where pore pressure generated during rapid driving inflates the blow count. Applying it to clean or coarse sands removes strength the soil actually has, so where it was applied should be stated in the log.<\/p>\n\n\n\n

Can SPT values be corrected in gravel?<\/h3>\n\n\n\n

Not meaningfully. In gravels and cobbles the blow count reflects particles striking the sampler shoe rather than the resistance of the soil mass, and no correction recovers a usable value. The same applies to refusal in weathered rock.<\/p>\n\n\n\n

What should a borehole log record for SPT?<\/h3>\n\n\n\n

The blow counts for each 150 mm increment rather than only the total, the hammer type and energy ratio, the borehole diameter, the sampler type and whether a liner was used, the rod length at each test, and the groundwater level with its date and measurement method.<\/p>\n\n\n\n\n