Standard Penetration Test (SPT)
Definition
An in-situ test measuring the number of blows to drive a split-spoon sampler 300 mm into the soil at a given depth. The N-value is widely used to estimate soil strength, stiffness, and liquefaction susceptibility.
- Method
- Drive a split-spoon sampler 300 mm into soil, count blows
- Output
- N-value
- Used for
- Soil strength, stiffness, liquefaction susceptibility
- Field
- Geotechnical and foundation failure investigation
Common questions
How does the SPT N-value factor into a foundation failure investigation?+
Investigators compare the N-values recorded at design time or in nearby borings against actual foundation behaviour to check whether the soil was correctly characterised, since an underestimated N-value can mean the foundation was designed for stronger soil than was actually present.
What limits the reliability of an SPT result?+
Results are sensitive to hammer energy delivery, rod length, borehole condition, and groundwater presence, so uncorrected raw blow counts from different rigs or operators can differ even in identical soil, which is why standardized correction factors are applied before comparison.
Related terms
- Bearing Capacity
- The maximum stress a soil or rock can sustain beneath a foundation without shear failure. Expressed as ultimate bearing capacity (at failure)...
- Cone Penetration Test (CPT)
- A continuous in-situ test pushing an instrumented cone through the soil and measuring tip resistance and sleeve friction. Provides a nearly continuous...
- Differential Settlement
- Non-uniform vertical movement of different parts of a foundation, distorting the structure it supports. More damaging than uniform settlement because it induces...
- Liquefaction
- Loss of shear strength in saturated granular soil when earthquake-induced cyclic loading causes pore water pressure to equal the total stress, reducing...
- Piping
- Internal erosion in an embankment or foundation in which seepage flow progressively removes fine particles along a concentrated flow path until a...