What is SPT?

The Standard Penetration Test (SPT) to ASTM D1586 is an in-situ test of soil resistance, in which:

  1. A Split Spoon Sampler with an outside diameter of 50.8 mm is used
  2. It is driven by a 63.5 kg hammer falling from a height of 76 cm
  3. The sampler is driven a total of 45 cm (in 3 increments of 15 cm each), and blows are counted only for the last 30 cm
  4. N-Value = number of blows in the 2nd and 3rd 15 cm increments (the first 15 cm, the seating drive, is discarded)
📚 Basic principles

The higher the N-Value = the denser/stiffer the soil = the better its bearing capacity. N-Values apply to both sand (granular soil) and clay (cohesive soil), but are interpreted differently.

SPN rotary wash boring rig with a tripod, pulley and winch for lifting the SPT hammer during soil investigation on a project site, with drill rods, sampler, mud settling tank and water pump set up on site
SPN's soil boring rig used for SPT in the field — the tripod and winch lift the 63.5 kg steel hammer for a free fall from 76 cm in accordance with ASTM D1586, while the settling tank and pump circulate drilling mud to keep the borehole wall stable, a factor that directly affects the reliability of the N-value

Soil classification by N-Value

The tables below give the commonly used criteria for soil density/consistency (Terzaghi & Peck). For soil layer classification under Thai law, see Soil layer classification by SPT under Thai law

3D model of an SPT in a borehole with an example of blow count recording The left image is a 3D model of the site. A tripod rig stands over a borehole at the edge of the cut-away section. The crew pulls a rope around a cathead to lift a 63.5 kilogram steel hammer to a height of 76 centimeters and release it in free fall onto the anvil at the top of the drill rod. The section shows the drill rod reaching the bottom of the hole at the test depth, where the split spoon sampler is driven in three increments of 15 centimeters each. The right image is an example field driving record. At each depth, blow counts are recorded for three increments, and the N value is the sum of the second and third, for example 5, 7, 9 gives N equal to 16. If 50 blows are reached before the sampler has penetrated a full 15 centimeters, the result is recorded as 50 over the penetration achieved. Values in the table are hypothetical. 3D model — SPT and N-value recording Not to scale 76 cm drop 63.5 kg hammer Rope on cathead Anvil Rod head Split spoon 3 × 15 cm Clean hole base Test depth Field driving record Example · hypothetical Depth Inc. 1 Inc. 2 Inc. 3 N 1.5 1 1 2 3 3.0 1 1 1 2 4.5 0 1 1 2 6.0 1 1 2 3 7.5 1 2 2 4 9.0 2 3 4 7 10.5 3 5 6 11 12.0 5 7 9 16 13.5 18 32 50/10 > 50 N = Inc. 2 + Inc. 3 Highlighted: 7 + 9 = 16 (Inc. 1 not counted) 50/10 = 50 blows, only 10 cm Stop driving, record as Refusal
3D model — SPT in a borehole: the crew lifts the 63.5 kg hammer with a rope on a cathead for a 76 cm free fall onto the anvil, driving the split spoon at the bottom of the hole in 3 increments of 15 cm. On the right is an example field driving record: N = increment 2 + increment 3, with 50/penetration recorded at Refusal (hypothetical values)
How an SPT produces the N value, and SPT N-Value ranges for clay consistency and sand density classification The left image shows the SPT principle to ASTM D1586. A 63.5 kilogram hammer falls freely from 76 centimeters onto the anvil at the top of the drill rod, transmitting force to drive a split spoon sampler of 50.8 millimeter outside diameter at the bottom of the borehole in three increments of 15 centimeters each. The first increment is seating and is not counted; the N value is the number of blows in the second plus the third increment, or 30 centimeters of penetration. The right image shows N-value range bars. Clay is classed as soft, N up to 4; medium stiff, 4 to 8; stiff, 8 to 30; and hard soil, N greater than 30. Very soft clay is classified by undrained shear strength Su less than 12.5 kilopascals, not by N. Sand is classed as very loose, 0 to 4; loose, 5 to 10; medium dense, 11 to 30; dense, 31 to 50; and very dense, over 50. The range of N greater than 30 is the dense sand or hard soil layer commonly used for pile end bearing. The bottom strip summarizes that the higher the N, the stiffer or denser the soil; that for soft clay, Su from Unconfined Compression or Vane Shear should also be used; and that calculations use the corrected N60, not the raw N. The figures are guidance for understanding; for legal soil layer classification, refer to the text of the Ministerial Regulation. SPT N-Value: how it is obtained and what it tells you about the soil Not to scale Getting N (ASTM D1586) 63.5 kg hammer 76 cm free fall 1st 15 cm: seating (excl.) 2nd 15 cm 3rd 15 cm N N = blows, inc. 2 + 3 (30 cm) Split spoon OD 50.8 mm ↓ N ranges and soil type Common pile bearing layer Clay — consistency Soft N ≤ 4 Medium 4–8 Stiff 8–30 Hard N > 30 Sand — density Very loose 0–4 Loose 5–10 Medium dense 11–30 Dense 31–50 Very dense > 50 0 4 8 10 30 50 SPT N (blows / 30 cm) → Very soft clay is classified by Su < 12.5 kPa (not by N) N in clay varies widely → confirm with Su from UC / Vane Shear Guidance only — legal definitions follow the Ministerial Regulation 1 Higher N Stiffer / denser soil 2 Soft clay Check Su too, not N alone 3 Design uses N60 Not raw N
Left: N is obtained by driving the split spoon with a 63.5 kg hammer in 76 cm free fall, counting blows in the 2nd and 3rd 15 cm increments to ASTM D1586 · Right: N ranges and soil classification — Clay: soft N ≤ 4 · medium stiff 4–8 · stiff 8–30 · hard soil N > 30 | Sand: very loose 0–4 · loose 5–10 · medium dense 11–30 · dense 31–50 · very dense > 50 (very soft clay is classified by Su < 12.5 kPa)

Sand (Granular Soil)

← Scroll to see the full table →

N-ValueRelative densityCharacteristicsFriction angle φ
0-4Very LooseVery loose, settles easily< 30°
4-10LooseLoose, low bearing capacity30-35°
10-30Medium DenseMedium dense, adequate bearing35-40°
30-50DenseDense, good bearing40-45°
> 50Very DenseVery dense, suitable for pile bearing> 45°

Clay (Cohesive Soil)

← Scroll to see the full table →

N-ValueConsistencyCharacteristicsSu (kPa)
< 2Very SoftVery soft, squeezes between fingers< 12.5
2-4SoftSoft (Bangkok Clay)12.5-25.0
4-8Medium StiffFairly stiff25.0-50.0
8-15StiffStiff50.0-100.0
15-30Very StiffVery stiff, cannot be scratched with a fingernail100.0-200.0
> 30HardHard, rock-like> 200.0
⚠️ Caution for clay

Using N-Values in clay has major limitations, because the values vary widely with driving rate, moisture content and sampling method. Use the Unconfined Compression Test or Vane Shear Test to evaluate Su of soft clay rather than relying on N alone.

Soil profile from an actual 5-borehole investigation, BH-1 to BH-5, 15 meters deep, showing SPT-N values per layer beside each borehole, from N of 2 in soft to medium clay up to N of 57 and 90 per 7 inches in very dense sand, with groundwater levels of 2.00 to 2.50 meters and a legend classifying clay and sand layers
Soil profile from an actual SPN soil investigation of 5 boreholes — the numbers beside each borehole are the SPT-N values (blow/ft) per layer. Low N values (2–6) cluster in the soft to medium stiff clay layers, while high N values (30–57, up to 50/2″ and 90/7″) are in the dense to very dense sand layers. Reading N values alongside soil descriptions like this helps check whether the values are reasonable.

"Raw" N vs "corrected" N

The problem: raw N-Values cannot be compared directly

Because the N value measured in the field depends on many factors:

  • Hammer Efficiency — each hammer type has a different efficiency (Donut Hammer 45%, Safety Hammer 60%, Automatic 90%+)
  • Overburden Pressure — the test depth affects the N value
  • Borehole Diameter — the borehole size has a minor effect
  • Rod Length — rod length affects the energy delivered to the sampler
  • Sampler Type — with or without a liner

Corrections: N60 and N1-60

N60 = N value corrected to 60% Hammer Efficiency

N60 = N × (ER/60) × CB × CS × CR

ER  = Hammer Efficiency (45-90%)
CB  = Borehole Diameter Correction
CS  = Sampler Correction (1.0 with liner, 1.2 without)
CR  = Rod Length Correction (0.75 if rod < 4m, 1.0 if > 10m)

N1-60 = additionally corrected for Overburden Pressure (used for sand)

(N1)60 = N60 × CN

CN = √(Pa/σ'v)  (Liao & Whitman 1986)
Pa = 100 kPa (Atmospheric pressure)
σ'v = Effective Vertical Stress at the depth
💡 Which value to use, and when?

N60 is used for evaluating clay, bearing capacity calculations and liquefaction susceptibility assessment / (N1)60 is used for estimating the relative density of sand, friction angle and settlement calculations in sand

Using N in design

1. Estimating the friction angle (φ) of sand

Peck, Hanson, Thornburn (1974):

φ' ≈ 27.1 + 0.3 × (N1)60 - 0.00054 × (N1)60²    (degrees)

2. Estimating the undrained shear strength (Su) of clay

Hara et al. (1974):

Su ≈ 29 × N60^0.72    (kPa)

Or use the rule of thumb: Su (kPa) ≈ 6.7 × N for Bangkok Clay

3. Estimating the elastic modulus (Es) of sand

Es ≈ 500 × (N60 + 15)    (kPa) — clean sand, Bowles (1996)
Es ≈ 320 × (N60 + 15)    (kPa) — clayey sand, Bowles (1996)

4. Estimating pile foundation capacity

Meyerhof (1976) for piles in sand:

qp (pile tip)   = 40 × N60 × (Lb/D) ≤ 400 × N60   (kPa)
fs (pile shaft) = 2 × N60   (driven pile) / 1 × N60 (bored pile)   (kPa)

Lb = embedment depth in the bearing layer
D  = pile diameter

Note: qp has an upper limit of 400 × N60 kPa — once Lb/D is large enough, the value stays constant and no longer increases with embedment depth. fs = 2 × N60 applies to driven piles (displacement piles), while for bored piles approximately 1 × N60 is used.

Calculated values must be divided by a factor of safety and also checked against the building control law criteria — see Allowable pile capacity under the Ministerial Regulation and Summary of the 2023 foundation Ministerial Regulation

SPT limitations to watch out for

1. High variability

N values at the same location from separate borings may differ by 20-50% because of:

  • Drilling crew skill
  • How clean the borehole is kept
  • Equipment quality
  • Interpretation by the person counting

2. Unsuitable for some soil types

  • Gravel/rock: N-Values spike abnormally due to rock fragments — use Rock Coring
  • Very soft clay: N = 0 over several meters tells you nothing — use Vane Shear
  • Saturated fine sand/silty sand: N may be distorted by pore water pressure during driving — dense fine sand may give values higher than actual
SPN team performing a Field Vane Shear Test at borehole FV-1 on a riverbank; one technician turns the test rod to measure the undrained shear strength of soft clay while another records the field readings, with a whiteboard showing the project name and test date
SPN Field Vane Shear work on a riverbank — in soft clay where SPT gives N = 0 over several consecutive meters, the Vane Shear Test measures Su directly in the field, so it supplements locations where N cannot distinguish between soils

3. Depth limitations

With rods 30+ m long, more energy is lost through the rods, so the measured N value becomes higher than the actual soil resistance. The rod length correction (CR < 1.0) is used to reduce N when rods are shorter than ~10 m, per the Skempton (1986) table above, because short rods reflect the energy wave back quickly.

4. Samples are not intact

The Split Spoon Sampler heavily disturbs the sample, so lab tests such as Unconfined Compression may give lower-than-actual results. Where accuracy matters, use a Thin-Walled Tube (Shelby Tube) to take undisturbed samples (compare the two sampling methods in SS vs ST soil samples: what's the difference?)

SPT compared with other tests

← Scroll to see the full table →

TestBest forAdvantagesLimitations
SPTAll soil types except gravelLow cost, provides samplesHigh variability
CPTClay, sandContinuous, accurate dataCannot penetrate gravel
Vane ShearSoft clayMeasures Su directlySoft clay only
Plate BearingSurface soil/subgradeMeasures bearing directlyUpper layers only
PressuremeterAll typesHigh accuracyExpensive

Best practices

  1. Always use N60 or (N1)60 in calculations, never raw N
  2. Check the Hammer Efficiency of the contractor you hire — the Energy Ratio (ER) should be measured/reported per ASTM D4633 so N can be correctly corrected to N60
  3. Take supplementary samples such as Shelby Tubes in clay, and run lab tests to confirm
  4. Use N-Values together with other methods such as CPT or Vane Shear at critical locations
  5. Watch for anomalous data — isolated abnormal spikes may indicate a local boulder
  6. Always compare with the Boring Log to check that N values are consistent with the recorded soil description

Interpretation example in Bangkok Clay

Typical N values for a project in inner Bangkok, 0-25 m deep:

  • 0-2 m: N = 5-10 (Weathered Crust)
  • 2-15 m: N = 0-4 (Soft Bangkok Clay) → design with Su = 15-25 kPa
  • 15-21 m: N = 5-15 (Medium to Stiff Clay) → Su = 50-100 kPa
  • 21-25 m: N = 30-50+ (1st Sand Layer) → pile bearing layer
Actual Boring Log of borehole BH-1 in Ratchathewi, Bangkok, 30.45 meters deep with groundwater at 0.50 meters, plotting SPT-N against depth alongside Su from UC and Field Vane tests: soft to medium stiff clay with Su of about 1 to 3 t/m² from 0 to 14 meters, very stiff clay with N of 17 to 18 from 14 to 18.5 meters, dense clayey sand with N of 27 to 43, and Hard Clay with N of 40 to 75 below 21 meters
Actual Boring Log from an SPN soil investigation — borehole BH-1 in Ratchathewi, Bangkok (30.45 m deep · GWL 0.50 m), an example Bangkok Clay profile where N is almost meaningless in the soft clay from 0–14 m (Su from UC and FV is used instead), then climbs to N = 17–18 in the Very Stiff Clay and N = 40–75 in the hard layer at pile tip level. Learn to read every column in the Boring Log reading guide

Engineers use this data to design 21-25 m long piles with their tips in the dense sand layer, and calculate skin friction in the soft layers. Read more about Bangkok's soil profile in Bangkok soft clay and foundation work

Summary

The SPT N-Value is a powerful and economical tool for geotechnical engineering, but it must be used with an understanding of its limitations. Correcting to N60 or (N1)60 is essential, not optional.

For projects that demand high accuracy, combining SPT with other tests such as CPT, Vane Shear or lab tests gives more reliable results than relying on N-Values alone.

SPN Soil Engineering uses SPT equipment with calibrated Hammer Efficiency, and its crews are trained to ASTM D1586. We also offer supplementary CPT, Vane Shear and lab testing to give engineers complete data for design.