Standard Penetration Test (SPT)
The standard penetration test is an in-situ test that is coming under the category of penetrometer tests. The standard penetration tests are carried out in borehole. The test will measure the resistance of the soil strata to the penetration undergone. A penetration emphirical correlation is derived between the soil properties and the penetration resistance.
The test is extremely useful for determining the relative density and the angle of shearing resistance of cohesionless soils. It can also be used to determine the unconfined compressive strength of cohesive soils.
Tools for Standard Penetration Test
The requirements to conduct SPT are:
- Standard Split Spoon Sampler
- Drop Hammer weighing 63.5kg
- Guiding rod
- Drilling Rig.
- Driving head (anvil).
Procedure for Standard Penetration Test
The test is conducted in a bore hole by means of a standard split spoon sampler.Once the drilling is done to the desired depth, the drilling tool is removed and the sampler is placed inside the bore hole.
By means of a drop hammer of 63.5kg mass falling through a height of 750mm at the rate of 30 blows per minute, the sampler is driven into the soil. This is as per IS -2131:1963.
The number of blows of hammer required to drive a depth of 150mm is counted. Further it is driven by 150 mm and the blows are counted.
Similarly, the sampler is once again further driven by 150mm and the number of blows recorded. The number of blows recorded for the first 150mm not taken into consideration.. The number of blows recorded for last two 150mm intervals are added to give the standard penetration number (N). 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
Silty fine sands and fine sands below the water table develop pore water pressure which is not easily dissipated. The pore pressure increases the resistance of the soil and hence the penetration number (N).
Terzaghi and Peck (1967) recommend the following correction in the case of silty fine sands when the observed value is N exceeds 15.
The corrected penetration number,
NC = 15 + 0.5 (NR -15)
Where NR is the recorded value and NC is the corrected value.
If NR less than or equal to 15, then Nc = NR
2. Overburden Pressure Correction
From several investigations, it is proven that the penetration resistance or the value of N is dependent on the overburden pressure. If there are two granular soils with relative density same, higher ‘N’ value will be shown by the soil with higher confining pressure.
With the increase in the depth of the soil, the confining pressure also increases. So the value of ‘N’ at shallow depth and larger depths are underestimated and overestimated respectively.
Hence, to account this the value of ‘N’ obtained from the test are corrected to a standard effective overburden pressure.
The corrected value of ‘N’ is
Nc = CN N
Here CN is the correction factor for the overburden pressure.
Precautions taken for Standard Penetration Test
- Split spoon sampler must be in good condition.
- The cutting shoe must be free from wear and tear
- The height of fall must be 750mm. Any change from this will affect the ‘N’ value.
- The drill rods used must be in standard condition. Bent drill rods are not used.
- Before conducting the test, the bottom of the borehole must be cleaned.
Advantages of Standard Penetration Test
The advantages of standard penetration test are:
- The test is simple and economical
- The test provides representative samples for visual inspection, classification tests and for moisture content.
- Actual soil behaviour is obtained through SPT values
- The method helps to penetrate dense layers and fills
- Test can be applied for variety of soil conditions
Disadvantages of Standard Penetration Test
The limitations of standard penetration tests are:
- The results will vary due to any mechanical or operator variability or drilling disturbances.
- Test is costly and time consuming.
- The samples retrieved for testing is disturbed.
- The test results from SPT cannot be reproduced
- The application of SPT in gravels, cobbles and cohesive soils are limited