---
title: "Field tests and SPT corrections for sand"
subject: "Foundation Engineering"
chapter: 2
key: "semester_5/foundation_engineering:field-tests-spt-corrections"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/field-tests-spt-corrections
site: BCT Engineering
---

# Field tests and SPT corrections for sand

Foundation Engineering · Chapter 2: Soil Exploration

- Asked in: 2081 Baisakh, 5 marks; 2074 Chaitra, 1+4 marks (asked 2 times)
- Video: [How to Find Corrected N-value for Design  | Bearing Capacity Lec: 02  |  Geotech with Naqeeb](https://www.youtube.com/watch?v=Szzfywx52Co) — Geotech with Naqeeb
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/field-tests-spt-corrections
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-2

## Question

List the field tests commonly used in subsurface exploration. What are the corrections that must be applied to the SPT values for sand before they are used in design charts and empirical correlations?

## Short answer

Common field tests include SPT, CPT, vane shear, plate load, pressuremeter, permeability, in-situ density, and geophysical methods. SPT N-values in sand require overburden correction N′=C_N N (C_N=0.77 log10(2000/σ_v′)) and dilatancy correction N″=15+(N′−15)/2 for saturated fine sand when N′>15.

## Answer

#### 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%: $N_{60} = N\,\eta_H\eta_B\eta_S\eta_R/0.60$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which test is used in soft clays in the field to find undrained strength?
   A. Sand replacement test
   B. Vane shear test
   C. Core cutter test
   D. Electrical resistivity test
   Answer: B. The vane shear test is used for soft clays. (Key point: Field test)

2. CN = 0.77 log10(2000/sigma_v') equals 1 at about
   A. 10 kN/m^2
   B. 100 kN/m^2 effective stress
   C. 1000 kN/m^2
   D. 2000 kN/m^2
   Answer: B. The standard reference stress is about 100 kN/m^2. (Key point: Overburden correction)

3. The dilatancy correction is applied
   A. for clay at any N
   B. only when N' < 15
   C. for fine or silty sand below the water table when N' > 15
   D. for gravel in dry state
   Answer: C. It corrects only above 15, via 15 + 0.5(N' - 15). (Key point: Dilatancy correction)

## Related in Foundation Engineering

- [SPT corrected N-value, dense sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aspt-dilatancy-20m) — Foundation Engineering, chapter 2
- [Standard penetration test and corrections](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aspt-procedure-corrections) — Foundation Engineering, chapter 2, asked 3×
- [SPT corrected N-value, submerged sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aspt-corrected-6m-submerged) — Foundation Engineering, chapter 2
- [Drilling type and field tests, Kathmandu core](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Adrilling-field-tests-kathmandu) — Foundation Engineering, chapter 2
- [Information from preliminary exploration](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aprelim-exploration-info) — Foundation Engineering, chapter 2
- [Depth and lateral extent of exploration](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Adepth-lateral-extent-exploration) — Foundation Engineering, chapter 2, asked 5×

## Related in other subjects

- [Field tests for sand and clay](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afield-tests-sand-clay) — Soil Mechanics, chapter 3
- [Field capacity from applied water data](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afield-capacity-from-applied-water) — Irrigation and Drainage Engineering, chapter 2
- [Flexible pavement design with 8 CBR values](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-eight-cbr-340-cvd) — Transportation Engineering II, chapter 2
- [Field tests of bricks](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Afield-tests-of-bricks) — Civil Engineering Materials, chapter 3, asked 5×