Theory: Boring Log
How the SPT is run, what N stands for, how it points to the compactness of sands and the consistency of clays, and when the boring may stop.
The test
In a percussion boring the hole advances meter by meter and, at each meter, a standard sampler is driven into its bottom by a 65 kg hammer falling 75 cm. The blows are counted for each of three successive 15 cm stretches, and the sampler brings up a disturbed sample of the soil to describe.
When the sampler falls short of a stretch, the blows and the penetration reached are written down, as 30/12, and the drive ends. Above the water table the hole usually advances with a helical auger (AU); below it, or when the auger stops advancing, by wash boring (WB).
Energy and N_{60}
Only part of the hammer's theoretical energy reaches the rods, and N is inversely proportional to it. International correlations usually take N referred to 60% of the free-fall energy.
Compactness and consistency
NBR 6484 ties ranges of N to the state of sands and sandy silts (compactness) and of clays and clayey silts (consistency). The log gives each layer the softest and the hardest state among its tests.
| N | Sands and sandy silts |
|---|---|
| ≤ 4 | Loose |
| 5 to 8 | Slightly dense |
| 9 to 18 | Medium dense |
| 19 to 40 | Dense |
| > 40 | Very dense |
| N | Clays and clayey silts |
|---|---|
| ≤ 2 | Very soft |
| 3 to 5 | Soft |
| 6 to 10 | Medium |
| 11 to 19 | Stiff |
| 20 to 30 | Very stiff |
| > 30 | Hard |
Stopping criteria
Unless the design says otherwise, NBR 6484 lets the boring end when, on successive meters, the ground proves strong enough:
- 30 blows for the first 15 cm, on 3 successive meters;
- 50 blows for the first 30 cm, on 4 successive meters;
- 50 blows for the full 45 cm, on 5 successive meters.
The page checks these criteria on the tests in the order they were entered. Check the edition of the standard in the contract, which also covers refusal of the auger, the wash boring and the sampler.
Limitations
- N depends on the rig, its energy and the operator; two tests in the same soil may differ widely.
- Compactness and consistency from the table are indicative; they don't replace laboratory tests or correlations calibrated for the site.
- In gravels the sampler may strike a particle and give a high N that doesn't stand for the layer.
References
- ABNT NBR 6484. Solo — Sondagem de simples reconhecimento com SPT — Método de ensaio. Rio de Janeiro: ABNT, 2020.
- Schnaid, F.; Odebrecht, E. Ensaios de Campo e suas Aplicações à Engenharia de Fundações. 2nd ed. São Paulo: Oficina de Textos, 2012.
- Décourt, L. The Standard Penetration Test: state-of-the-art report. In: International Conference on Soil Mechanics and Foundation Engineering, 12., 1989, Rio de Janeiro. Proceedings, v. 4, p. 2405–2416.
- Skempton, A. W. Standard penetration test procedures and the effects in sands of overburden pressure, relative density, particle size, ageing and overconsolidation. Géotechnique, v. 36, n. 3, p. 425–447, 1986.