Field tests and SPT corrections for sand

Common field tests

1. Standard penetration test (SPT)
2. Cone penetration tests (static cone, CPT; dynamic cone, DCPT)
3. Vane shear test (for soft clays)
4. Plate load test
5. Pressuremeter test
6. Field permeability (pumping) tests
7. In-situ density tests (core cutter, sand replacement)
8. Geophysical methods such as seismic refraction and electrical resistivity

Corrections to SPT $N$ in sand

1. Overburden pressure correction: since $N$ increases with depth for the same relative density,

$$N' = C_N N, \qquad C_N = 0.77\log_{10}\frac{2000}{\sigma_v'}\ (\sigma_v' \text{ in kN/m}^2,\ \sigma_v' \ge 25)$$

Here $\sigma_v'$ is the effective vertical stress at the test level. The standard reference stress is about 100 kN/m$^2$ ($C_N = 1$). 2. Dilatancy (saturation) correction: applicable for fine or silty sand below the water table, when $N' > 15$:

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

This allows for the negative pore pressure (dilation) in dense fine sand that stiffens it during driving, and for the positive pressures in loose sand; the formula corrects only above 15. 3. Energy, rod length, borehole and sampler corrections are applied in current practice to standardise the energy to 60%: $N_{60} = N\,\eta_H\eta_B\eta_S\eta_R/0.60$.