Standard Penetration Test (SPT)

The standard penetration test is an on-site test that falls into the category of penetration test. Standard penetration tests are done in wells. This test measures the resistance of the soil layer to infiltration. Empirical infiltration correlations are derived between soil properties and infiltration resistance.

This test is very helpful in determining the relative density and shear strength angle of cohesive soil. It can also be used to determine the unlimited compressive strength of sticky soil.

Tools for Standard Penetration Test

The requirements to conduct SPT are:

1. Standard Split Spoon Sampler
2. Drop Hammer weighing 63.5kg
3. Guiding rod
4. Drilling Rig.
5. Driving head (anvil).

Procedure for Standard Penetration Test

The test is performed in a well equipped with a standard split spoon sampler. After drilling to the desired depth, remove the drilling tool and place the sampler in the well. The sampler is driven into the ground by a 63.5 kg mass hammer that drops to a height of 750 mm at a speed of 30 strokes per minute.

It complies with IS-2131: 1963. The number of hammer blows required to drive a depth of 150 mm is counted. It is also driven at 150 mm and strokes are counted. Similarly, the sampler is driven again at 150 mm and the stroke count is recorded. The number of strokes recorded in the first 150mm is not taken into account. The standard penetration number (N) is obtained by adding the number of strokes recorded at the last two 150 mm intervals. In other words, 

N = No: of blows required for 150mm penetration beyond seating drive of 150mm.

If the number of blows for 150mm drive exceeds 50, it is taken as refusal and the test is discontinued. The standard penetration number is corrected for dilatancy correction and overburden correction.

Corrections in Standard Penetration Test

Before the SPT values are used in empirical correlations and in design charts, the field ‘N’ value have to be corrected as per IS 2131 – 1981. The corrections are:

• Dilatancy Correction
• Overburden Pressure Correction

1. Dilatancy Correction

Subsurface fine sand and sludge of fine sand create pore water pressure that does not easily disappear. Pore water pressure increases soil resistance and thus increases infiltration (N). Terzaghi and Peck (1967) recommend the following corrections for fine sludge sand when the observed value N is greater than 15. Fixed number of intrusions,

N_C = 15 + 0.5 (N_R -15)

Here, NR is the registered value and NC is the modified value. If NR is 15 or less, then Nc = NR2. Overload Pressure Compensation Several studies have shown that the value of penetration resistance or N depends on the overload pressure.

If there are two grain soils with the same relative density, the soil with the higher limiting pressure will show a higher "N" value. As the depth of the soil increases, so does the limiting pressure. Therefore, the value of "N" at shallow depth and deep depth is underestimated and overestimated, respectively. Therefore, with this in mind, the "N" value obtained from the test is corrected to the standard effective boost pressure. The correction value of "N" is

Nc = C_N N

Here C_{N is} the correction factor for the overburden pressure.

Precautions taken for Standard Penetration Test

1. The split spoon sampler should be in good condition. Cutting shoes should not be worn
2. The height of the drop should be 750mm. Changing this will affect the value of "N".
3. The drill rod used must be in standard condition. No curved drill rod is used.
4. The bottom of the well should be cleaned before the test.

Advantages of Standard Penetration Test

The advantages of standard penetration test are:

1. Testing is easy and cheap
2. This test provides a visual inspection, a classification test, and a representative sample of water content.
3. The actual behavior of the soil is obtained by the SPT value
4. This method helps to infiltrate dense layers and fillers.
5. This test can be applied to a variety of soil conditions.

Disadvantages of Standard Penetration Test

The limitations of standard penetration tests are:

1. Results will vary due to mechanical or operator fluctuations or drilling turbulence.
2. Testing is expensive and time consuming.
3. The sample taken for testing is corrupted.
4. SPT test results cannot be displayed
5. Limited application of SPT to gravel, rocks and sticky soils