Take a measured Standard Penetration Test blow count and correct it for hammer efficiency, rod length and effective overburden stress to obtain N60 and (N1)60 for design. Move each parameter to see the correction factors, the final corrected N-value and the relative-density rating update in real time.
Parameters
Measured N-value N
Blow count to drive the sampler 30 cm
Effective overburden stress σ'v
kPa
Effective stress of the soil above at test depth
Hammer efficiency
%
Fraction of hammer energy delivered to the rods
Rod length
m
Short rods reflect energy and need a correction
Soil type
Used for the relative-density rating bands
Results
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Energy corr. factor C_E
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Rod-length corr. factor C_R
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Converted N₆₀
—
Overburden corr. factor C_N
—
Corrected N-value (N₁)₆₀
—
Relative density rating
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SPT setup — blow & penetration animation
The hammer drops onto the anvil and drives the sampler into the ground. The bars on the right show the measured N-value being transformed through the energy, rod and overburden corrections into the final (N₁)₆₀.
Overburden correction factor C_N vs effective overburden σ'v
N-value correction stages (measured N → N₆₀ → (N₁)₆₀)
The energy correction standardises the hammer to a 60% reference; the overburden correction normalises the effective overburden stress to a 100 kPa reference, removing the depth effect. N: measured N-value, E_hammer: hammer efficiency (%), C_R: rod-length correction factor, σ'v: effective overburden stress (kPa).
Energy correction factor C_E and the Liao & Whitman overburden correction factor C_N. C_N is capped at 1.7 to avoid blowing up at very shallow depths.
What is SPT N-Value Correction?
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I keep hearing about the "N-value" in site investigation. What is that number, really?
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Roughly speaking, it is "the hardness of the ground measured as a blow count". In the Standard Penetration Test, or SPT, a standard split-barrel sampler is placed at the bottom of a borehole, and a hammer of a fixed weight is dropped from a fixed height to drive it in. The number of blows needed to drive it 30 centimetres is the N-value. It is the most widely used soil investigation test in the world, and from a single N-value you can roughly judge density, bearing capacity, even how prone the ground is to liquefaction.
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If it is that handy, why not just use the N-value as measured? Why is a "correction" needed?
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That is exactly the catch. The raw, measured N-value changes quite a lot with things that have nothing to do with the soil. For instance, the very same sand at the same density gives a higher N-value the deeper you test it, because the weight of the soil above — the overburden — makes the sampler harder to drive. Equipment differences matter too. So you cannot directly compare an N-value from one borehole with another, or drop it straight into a design formula. You have to correct it first to put everything on a common footing.
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I see. So concretely, what kind of corrections do you apply?
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Two big ones. The first is the energy correction. Depending on the rig and hammer type, the fraction of the hammer's potential energy delivered into the rods ranges from 45 to 95 percent. Since the N-value is inversely related to the energy delivered, we convert everything to a common reference of 60 percent. That gives N60. Short rods reflect energy, so a rod-length correction is folded in at the same time. Move the "hammer efficiency" slider on the left and you will see the factor C_E change.
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And the other correction is the "depth" issue you mentioned earlier?
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Right — the overburden correction. To cancel out the extra N-value you get from depth, the effective overburden stress is normalised to a reference of 100 kPa, which is roughly 5 to 6 metres deep. The Liao & Whitman expression C_N = sqrt(100/sigma'v) is the standard choice. The result, with the depth effect removed too, is the fully corrected value (N1)60. Always use (N1)60 for liquefaction assessment and relative-density correlations. Judge with a raw N-value and you may rate the ground better than it really is and end up on the unsafe side.
Frequently Asked Questions
The measured N-value changes with factors that have nothing to do with the soil itself: the hammer type and release mechanism, the rod length, and the test depth (effective overburden stress). For example, the very same sand at the same density gives a higher N-value when it is deeper, because the greater confining pressure makes penetration harder. Before an N-value can be fed into a liquefaction assessment or a relative-density correlation, these effects must be removed, giving the corrected value (N1)60. Using a raw N-value in a design formula can seriously overestimate the ground.
N60 is the measured N-value converted to a common reference hammer-energy efficiency of 60%, with a rod-length correction applied as well. It is the value with the equipment differences removed. (N1)60 goes one step further: the overburden correction normalises the effective overburden stress to a reference of 100 kPa, removing the depth effect too. (N1)60 is the fully corrected value used for liquefaction assessment and relative-density correlations. This tool computes N60 first, then (N1)60.
The N-value is inversely related to the energy actually delivered to the rods. Different drilling rigs and hammer types deliver anywhere from 45% to 95% of the potential energy, so raw N-values cannot be compared between tests. Because most worldwide correlations are built around a common reference of 60% energy, the energy correction factor is C_E = hammer efficiency / 60. Modern automatic trip hammers tend to be efficient, so C_E often exceeds 1.
This tool uses the Liao & Whitman (1986) expression C_N = sqrt(100/sigma'v), where sigma'v is the effective overburden stress in kPa, normalised to a reference of 100 kPa (roughly 5 to 6 m depth). In shallow ground sigma'v is small and C_N exceeds 1; in deep ground C_N falls below 1. To prevent the factor from blowing up at very shallow depths it is capped at 1.7. C_N removes the depth-driven change in the measured N-value of sand at the same density.
Real-World Applications
Liquefaction assessment: For the liquefaction screening of sandy ground under earthquake loading, the resistance to cyclic shear stress is derived from the corrected blow count (N1)60. Because a raw N-value or even N60 still carries the apparent scatter caused by depth, the overburden-corrected (N1)60 must be used in the liquefaction-resistance correlation. When measured N-values differ between the shallow and deep parts of the same sand layer, it is usually the overburden, not a density difference — and the correction recovers the true density picture.
Bearing capacity and settlement of foundations: Empirical relations between the N-value and friction angle or subgrade reaction are widely used to estimate the capacity of shallow foundations and piles. Many of those correlations are built around the 60% energy reference, so N-values measured with rigs of differing efficiency must be energy-corrected before they are applied. Skip the correction and a high-efficiency rig will tend to underestimate the ground, leading to uneconomical designs.
Reviewing a site investigation report: When results from a single site investigated by several contractors or at different times are merged, a mix of rigs and hammer types is involved. Applying the energy and overburden corrections consistently — as this tool does — lets N-values from different boreholes be compared on the same footing, so the continuity of strata and the presence of weak layers can be judged correctly.
Relative density and compaction control: When evaluating the compaction state of reclaimed land or fill, the relative density is estimated from (N1)60. Using the depth-independent (N1)60 allows the degree of compaction near the surface and at depth to be compared fairly, giving an objective basis for deciding whether additional compaction or ground improvement is required.
Common Misconceptions and Pitfalls
The biggest pitfall is the order of the corrections and double-correcting. The overburden correction must always be applied to N60, the energy-corrected value. Multiplying a raw N-value by C_N straight away, or applying C_N a second time to a value that is already (N1)60, leaves the correction too strong or too weak. Depending on the liquefaction method, a fines-content correction may also be applied — and its input is likewise (N1)60. It is essential to know which stage you are at and to keep N, N60 and (N1)60 clearly distinguished in your records.
Next, using a single blanket value for hammer efficiency instead of measuring it. Hammer efficiency should properly be established for each rig by energy measurement. An automatic trip hammer is more efficient than a manual cathead-and-rope system, and even the same model varies with its state of maintenance. Leave the efficiency as an assumed value and a systematic error rides on N60. Because this tool lets you move the efficiency slider, you can check how much (N1)60 swings over the plausible efficiency range and keep a margin in your judgement.
Finally, the misconception that the overburden correction applies unconditionally to any soil and any depth. C_N = sqrt(100/sigma'v) is an empirical relation aimed mainly at sandy soils, and at very shallow depths the factor becomes too large, so a cap is set (1.7 in this tool). For cohesive soils the overburden-correction reasoning is different, and the effective overburden stress sigma'v itself changes substantially with the assumed groundwater table. Always enter the effective stress, not the total stress, and confirm that the correction matches the assumptions of the soil type and the method.
How to Use
Enter measured field N-value (blow count) in the nNum field, typically ranging 0–50 blows/30cm for cohesionless soils
Input hammer efficiency percentage (heNum), usually 60–90% depending on equipment type; common values are 70% for Chinese hammers and 85% for donut-type
Specify rod length in meters (rlNum) to account for wave energy losses; standard depths include 0–4m, 4–6m, 6–10m, and 10–30m intervals
Enter effective overburden pressure in kPa (svNum), calculated from soil unit weight and depth
The simulator calculates energy correction factor C_E, rod-length correction C_R, normalization to C_E=60% reference energy (N₆₀), overburden correction C_N, and final corrected N-value (N₁)₆₀ for relative density assessment
Worked Example
Sand deposit at 8m depth: measured field N=18 blows, hammer efficiency 75%, rod length 8m, unit weight 18 kN/m³ giving effective overburden σ_v'=144 kPa. Energy correction C_E=0.75/0.60=1.25; rod length correction C_R=0.80 (from 8m table); N₆₀=18×1.25×0.80=18 blows; overburden correction C_N=√(100/144)=0.833; final (N₁)₆₀=18×0.833=15 blows, indicating medium dense sand suitable for shallow foundations.
Practical Notes
Hammer efficiency varies by region and age of equipment; automatic-trip hammers typically deliver 60–80% efficiency while rope-and-pulley systems may only reach 50–70%
Rod length correction becomes critical below 10m depth; neglecting it can underestimate in-situ soil strength by 10–15%
Always normalize to N₆₀ reference standard before comparing across project sites or design codes (NAVFAC, Terzaghi correlations)
Corrected (N₁)₆₀ values above 35 indicate dense sand suitable for pile-bearing-capacity calculations; values below 10 suggest settlement concerns for footings