SPT N-value corrections: N60 and N1(60) explained

Every borehole log carries a column of blow counts, and almost every foundation decision leans on them. But the number written in the field is not a property of the soil — it is a property of the soil, the rig, the rods, the hole and the depth. Two crews can test the same sand an hour apart and record 12 and 22.

Corrections exist to remove the equipment from the measurement. This is what each one does, when it applies, and how a single field value moves as they are applied.

What the raw N-value actually measures

In a Standard Penetration Test, a 63.5 kg hammer falls 760 mm onto the drill rods, driving a split-barrel sampler into the base of the borehole. The sampler is driven 450 mm in three 150 mm increments. The blows for the first increment are discarded as seating; N is the sum of the blows for the second and third increments — the last 300 mm.

If 50 blows are reached before 300 mm of penetration, the test is stopped and reported as refusal, with the penetration recorded — for example 50/125 mm. Reporting that as “N = 50” quietly discards the fact that the sampler barely moved.

The energy problem

The hammer’s theoretical potential energy is fixed at 475 J. What reaches the sampler is not. Rope-and-cathead systems lose energy to rope friction and operator rhythm; automatic trip hammers release cleanly and consistently.

Hammer type	Typical energy ratio, ER	CE = ER / 60
Donut, rope and cathead	45 – 60 %	0.75 – 1.00
Safety, rope and cathead	55 – 70 %	0.92 – 1.17
Automatic trip	80 – 95 %	1.33 – 1.58

Two rigs on the same sand can therefore report blow counts differing by a factor near two, purely from hammer type. This is the single largest correction, and the one most often left unrecorded.

The four equipment corrections: N60

N60 normalises the field value to 60 % energy transfer:

N60 = N × CE × CB × CR × CS

C_E — energy ratio correction, ER/60, from the table above. Measure it where the result matters.

C_B — borehole diameter: 1.00 for 65–115 mm, 1.05 for 150 mm, 1.15 for 200 mm. A wider hole relieves lateral stress, so fewer blows are recorded.

C_R — rod length: 0.75 below 3 m, 0.85 for 3–4 m, 0.95 for 4–6 m, 1.00 above 10 m. Short rod strings reflect the stress wave back before the sampler has taken the full energy, so shallow tests over-read the resistance.

C_S — sampler type: 1.00 for a standard sampler with the liner in place, 1.10–1.30 where the liner is omitted from a sampler designed for one. A missing liner reduces friction, so fewer blows are needed.

Worked example

A field value of N = 18 at 4.5 m, driven by a safety hammer on rope and cathead with a measured energy ratio of 55 %, in a 150 mm borehole, sampler without liner:

CE = 55 / 60           = 0.92
CB = 150 mm            = 1.05
CR = 4.5 m rods        = 0.95
CS = no liner          = 1.20

N60 = 18 × 0.92 × 1.05 × 0.95 × 1.20 = 19.8  →  N60 ≈ 20

The corrections nearly cancelled here — but only by coincidence. With an automatic hammer (C_E = 1.40) the same field count becomes N60 ≈ 30, a different relative density and a different foundation.

Overburden correction: N1(60)

Sand gets stronger with confining pressure. The same sand at the same relative density gives higher blow counts at 15 m than at 2 m, simply because it is under more effective stress. To compare depths, N60 is normalised to a reference effective overburden pressure of one atmosphere (≈ 100 kPa):

N1(60) = N60 × CN            CN = √(Pa / σ'v)  ≤ 1.7

where σ’_v is the effective vertical stress at the test depth — so the groundwater level matters directly. C_N is capped at 1.7 because the relationship breaks down at very shallow depths.

The chart below shows the same borehole three ways: the field values, the energy-corrected N60, and the overburden-normalised N1(60). Hover any depth to read all three at once.

Field N N60 N1(60) Hover the plot to read values

Read the orange line: once overburden is removed, the deposit below 9 m is a single sand of roughly constant density. The grey field profile suggests it keeps getting denser with depth, which it does not.

Which value belongs in which calculation

You are doing	Use
Recording the test on the log	N (raw), with the increments and any refusal
Bearing capacity and settlement correlations	N60, unless the correlation states otherwise
Relative density, friction angle from sands	N1(60)
Liquefaction triggering	N1(60), then the fines-content adjustment
Comparing two boreholes on one site	N1(60)

The rule behind the table: use the value the correlation was derived with. A correlation published against N60 that is fed N1(60) will be wrong by the overburden factor, and nothing in the arithmetic will complain.

What to record so the corrections stay possible

Corrections can only be applied later if the field record supports them. On every log, capture:

• hammer type and, if measured, the energy ratio
• borehole diameter and drilling method
• rod length at each test, or the test depth and the string configuration
• whether the sampler had its liner
• the groundwater level and when it was read
• the three 150 mm increments separately, and full detail on any refusal

A blow count without this context can be recorded, but it cannot be corrected — and an uncorrectable N is a number you cannot defend in a design report.

In GeoEQ SPT Logs

The software records the three increments, computes N, and draws the profile against depth on the sheet. Energy-corrected columns — N60 and N1(60) as computed columns using a project-level energy ratio — are on the roadmap for version 1.5, along with the fines content already stored per sample.

Malo, R. C. (2026). SPT N-value corrections: N60 and N1(60) explained. GeoEQ. https://geoeq.dev/blog/spt-n-value-corrections/

@misc{malo2026spt,
  author       = {Ripon Chandra Malo},
  title        = {SPT N-value corrections: N60 and N1(60) explained},
  year         = {2026},
  howpublished = {GeoEQ},
  url          = {https://geoeq.dev/blog/spt-n-value-corrections/},
  urldate      = {2026-08-12}
}