DAY 11 – Standard Penetration Test (SPT): Basics, Procedure & Interpretation
The Standard Penetration Test (SPT) is one of the most widely used in-situ tests in geotechnical investigations. Almost every geotechnical report includes SPT values, and these numbers directly influence:
Foundation design
Bearing capacity
Settlement analysis
Soil strength classification
Understanding SPT properly is critical for every site geologist and geotechnical engineer.
1️⃣ What Is the Standard Penetration Test (SPT)?
SPT is a dynamic in-situ test carried out inside a borehole to measure the resistance of soil to penetration.
The resistance is expressed as an SPT N-value, which represents the number of hammer blows required to drive a standard sampler into the soil.
2️⃣ Purpose of the SPT Test
SPT is used to:
Assess soil density and consistency
Estimate shear strength parameters
Identify soil stratification
Correlate soil properties for design
Detect weak layers
3️⃣ Equipment Used in SPT
The standard SPT setup includes:
Split spoon sampler (50.8 mm outer diameter)
65 kg hammer
Free fall height of 750 mm
Drill rods
Borehole casing
⚠️ Any change in these standards affects the reliability of N-values.
4️⃣ SPT Test Procedure (Step-by-Step)
Drill borehole to the required depth
Clean the borehole bottom
Lower the split spoon sampler
Drop the hammer from 750 mm height
Record blows for each 150 mm penetration
First 150 mm → Seating blows (ignored)
Second 150 mm → Counted
Third 150 mm → Counted
🔢 SPT N-value = Blows for last 300 mm
5️⃣ Termination Criteria of SPT
The test is stopped if:
50 blows are required for any 150 mm
100 blows for full 300 mm
Sampler does not penetrate
This is recorded as refusal.
6️⃣ Interpretation of SPT N-Values
🔹 For Sandy Soils
N < 4 → Very loose
4–10 → Loose
10–30 → Medium dense
30–50 → Dense
50 → Very dense
🔹 For Clayey Soils
N < 2 → Very soft
2–4 → Soft
4–8 → Firm
8–15 → Stiff
15–30 → Very stiff
30 → Hard
7️⃣ Corrections Applied to SPT N-Values
Raw N-values must be corrected for:
Overburden pressure
Energy efficiency
Borehole diameter
Rod length
Sampler type
Corrected values provide more reliable design parameters.
8️⃣ Limitations of SPT
Not reliable in gravels and cobbles
Disturbed samples
Operator-dependent results
Affected by groundwater conditions
Despite limitations, SPT remains popular due to its simplicity and cost-effectiveness.
9️⃣ Importance of Accurate Logging
Site geologists must carefully log:
Depth of test
Blows per 150 mm
Groundwater presence
Soil description
Refusal conditions
Poor logging leads to wrong design decisions.
🔚 Final Takeaway
SPT is not just a number—it is a decision-making tool. Understanding its procedure, limitations, and interpretation ensures safe and economical geotechnical design.
Mastering SPT basics is a must-have skill for fresh graduates and site engineers.