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Foundations of the Standard Penetration Test (SPT)

The Standard Penetration Test (SPT) is one of the most widely used in-situ soil investigation tests in geotechnical engineering. It is performed in a borehole to determine the relative density of sandy soils, consistency of cohesive soils, and approximate bearing capacity and settlement characteristics of foundations.

The SPT is particularly useful because it provides both a soil sample and a numerical resistance value called the SPT N-value.

1. Principle of SPT

The basic principle is to drive a standard split-spoon sampler into the soil using a standard hammer. The resistance offered by the soil to penetration is measured by counting the number of hammer blows required for a specified penetration.

A standard SPT hammer has a mass of approximately 63.5 kg and is allowed to fall through a height of about 750 mm.

The sampler is generally driven a total distance of 450 mm. The number of blows required for the first 150 mm is considered seating drive and is normally ignored.

The number of blows required for the final 300 mm is recorded as the SPT N-value.

SPT N-value

N = N₂ + N₃

Where:

• N = SPT penetration resistance

• N₂ = Number of blows for the second 150 mm

• N₃ = Number of blows for the final 150 mm

For example, if the blows are 8, 12 and 15 for successive 150 mm penetrations:

N = 12 + 15 = 27 blows

Thus, the SPT N-value is 27.

2. Procedure of SPT

The test is normally carried out at selected depths during boring.

1. The borehole is advanced to the required test depth.

2. The borehole is cleaned carefully.

3. The standard split-spoon sampler is placed at the bottom.

4. The sampler is driven 450 mm into the soil.

5. The number of blows required for each 150 mm penetration is recorded.

6. The first 150 mm is treated as seating penetration.

7. The blows for the remaining 300 mm are added to obtain the N-value.

8. The disturbed soil sample obtained inside the sampler is examined and classified.

3. Factors Affecting the SPT N-Value

The measured N-value can be influenced by several factors, including:

• Hammer energy and efficiency

• Borehole diameter

• Rod length

• Sampler dimensions

• Soil type

• Groundwater conditions

• Method of drilling

• Operator and equipment characteristics

Therefore, corrections are often required before using the N-value for engineering correlations.

4. Energy Correction

The measured N-value can be corrected to a standard energy level using:

N₆₀ = N × (Eᵣ / 60)

Where:

• N₆₀ = SPT value corrected to 60% hammer energy

• N = Measured SPT N-value

• Eᵣ = Actual hammer energy ratio (%)

For example, if N = 25 and the hammer energy ratio is 72%:

N₆₀ = 25 × (72/60) = 30

Therefore, the corrected value is N₆₀ = 30.

5. Importance in Foundation Engineering

SPT results are widely used for preliminary evaluation of shallow and deep foundations. The N-value can provide an indication of soil compactness and strength and can be correlated with:

• Allowable bearing pressure

• Relative density of sand

• Settlement

• Shear strength

• Pile capacity

• Liquefaction potential

For foundation design, however, SPT results should not be used alone. They should be interpreted along with soil classification, groundwater conditions, laboratory tests, geological information, and the proposed foundation type.

Key Point

The most important result of the Standard Penetration Test is the SPT N-value, which represents the number of hammer blows required to drive the sampler through the final 300 mm of penetration. It is a simple but valuable indicator of subsurface soil resistance and is extensively used in practical foundation investigations and geotechnical design.