The SPT test (standard penetration test) is the most widely used field test in soil investigation. It is simple, it gives a sample, and its result, the N-value, feeds into many foundation design methods.
SPT test procedure
- Drill the borehole to the test depth and clean the bottom.
- Lower the split-spoon sampler on the drill rods.
- Drive it using a 63.5 kg hammer falling freely through 750 mm.
- Count the blows for each of three 150 mm increments.
- Recover the sampler and keep the soil sample for logging and lab tests.
The first 150 mm is treated as seating drive because the soil at the bottom of the hole may be disturbed.
What is the N-value?
The N-value is the number of blows needed for the last 300 mm of penetration. For example, blows of 6, 9 and 11 give N = 9 + 11 = 20. Higher values generally mean denser or stiffer ground.
Corrections to the N-value
IS 2131 describes two corrections that apply mainly to cohesionless soil.
- Overburden correction: the same sand gives a higher N at depth because of the higher confining pressure, so the value is corrected to a standard pressure.
- Dilatancy correction: for fine or silty sand below the water table, a corrected value above 15 is further reduced using N'' = 15 + 0.5 (N' − 15).
How the result is used
- Estimating the density of sand and the consistency of clay
- Estimating bearing capacity and settlement of foundations
- Assessing liquefaction potential in seismic areas
- Choosing pile lengths and founding levels
Limitations
Results are affected by hammer energy, borehole cleaning and rod condition, and the test is less reliable in gravel and very soft clay. That is why SPT should be read together with lab results and, where needed, other field tests.
FAQ
What is the full form of SPT?
SPT stands for Standard Penetration Test.
At what depth interval is SPT done?
It is commonly done at regular intervals, often about 1.5 m, and at each change of layer, as set out in the investigation plan.
What does a high N-value mean?
It generally means dense sand or stiff clay. A refusal usually indicates very dense soil, boulders or rock.
Talk to our team
Need boreholes with SPT? Call +91 98718 04288, email contact@geowingseng.com or send your site details.
