Standard penetration test and corrections

Definition

The standard penetration test (SPT) is a dynamic in-situ test in which a standard split-spoon sampler is driven into the soil at the bottom of a borehole by a 63.5 kg hammer falling 750 mm. The N-value is the number of blows required for the last 300 mm of a 450 mm drive.

Procedure (IS 2131)

1. Advance a borehole (150 mm or less in diameter) to the test depth and clean the bottom.
2. Lower the split-spoon sampler (outer diameter 50.8 mm, inner diameter 35 mm) on rods to the bottom.
3. Drive it with the 63.5 kg hammer dropping freely 750 mm.
4. Count the blows for each 150 mm. The first 150 mm is the seating drive and is ignored.
5. The blows for the next two 150 mm intervals added together give $N$. If 50 blows are reached before the 300 mm is covered, stop and record it as refusal, together with the penetration achieved.
6. Withdraw the sampler, open it and take the disturbed sample.
7. Repeat at about every 1.5 m (or at each change of strata).

Uses of the result

• Relative density of sands and consistency of clays.
• Angle of friction $\phi$ and unconfined strength in empirical correlations.
• Bearing capacity and settlement of footings in sand (Terzaghi-Peck, Meyerhof charts).
• Pile capacity, liquefaction assessment and soil identification.

Nature of sample

A disturbed (but representative) sample. It is not suitable for strength or consolidation tests, but it is suitable for visual description, grain size and Atterberg limits.

Corrections

1. Overburden correction: for the same density, $N$ is higher at larger depth, so it is reduced to a standard 100 kPa effective stress:

$$N' = C_N N, \qquad C_N = 0.77\log_{10}\frac{2000}{\sigma_v'} \quad (\sigma_v' \text{ in kN/m}^2)$$

(Liao and Whitman: $C_N = \sqrt{100/\sigma_v'}$ is also used.) 2. Dilatancy (water table) correction: in saturated fine or silty sand, pore-pressure effects raise the apparent $N$ above 15, so

$$N'' = 15 + \tfrac{1}{2}(N' - 15) \quad (N' > 15)$$

3. Other corrections for the energy ratio (hammer, rod length, borehole diameter, sampler liner) are used in $(N_1)_{60}$ in modern practice.