Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is a common field test used in geotechnical engineering to determine the strength and properties of soil.

Objective of SPT

· Determine soil density and strength

· Evaluate bearing capacity

· Identify soil stratification

· Determine N-value for foundation design

Apparatus Required

· Split spoon sampler

· Drill rods

· 63.5 kg hammer

· Tripod arrangement

· Borehole equipment

Principle of SPT

A standard split spoon sampler is driven into soil using a 63.5 kg hammer dropped from a height of 750 mm.

Test Procedure

1. Drill borehole to required depth

2. Lower split spoon sampler into borehole

3. Drive sampler using hammer

4. Record blows for every 150 mm penetration

5. Ignore first 150 mm seating drive

6. Add blows of next 300 mm penetration

Formula for N-Value

N = N₂ + N₃

Where:

N₂ = Blow count for second 150 mm

N₃ = Blow count for third 150 mm

Example Calculation

Penetration	Blow Count
First 150 mm	8
Second 150 mm	12
Third 150 mm	15

First 150 mm is seating drive and is ignored.

N = 12 + 15

SPT N-Value = 27

Interpretation of N-Value

N-Value	Soil Condition
0 – 4	Very Loose
4 – 10	Loose
10 – 30	Medium Dense
30 – 50	Dense
> 50	Very Dense

Advantages

· Simple and economical

· Widely used method

· Provides soil samples

· Useful for foundation design

Limitations

· Disturbed soil samples

· Less accurate in gravel soils

· Results affected by groundwater

Standard Codes

IS 2131 – Method for Standard Penetration Test

ASTM D1586

Applications

· Building foundations

· Bridge foundations

· Road projects

· Soil investigation works