Standard Penetration Test | SPT Test | Overburden Pressure Correction | Dilatancy Correction
For the investigation of the subsurfaces of the soil, we performed different types of field tests such as Plate Load Test, Standard Penetration Test (SPT Test), Dynamic Cone Penetration (DCPT Test), Static Cone Penetration Test (SCPT Test).
In this blog article, we are going to study the Standard Penetration Test (SPT Test).
Standard Penetration Test | SPT Test
The Standard Penetration Test is performed in the clean borehole of size 100mm to 150mm.
Standard Penetration Test is an in-situ test that is performed especially for cohesionless soils.
The IS code 2131-1981 is used for the Standard Penetration Test.
The standard penetration test is useful to measure the angle of shearing resistance and relative density of cohesionless soils.
Standard Penetration Test Equipment
Here is the list of apparatus that is used while performing the standard penetration test.
- Standard Split-Spoon Sampler– Use for collecting the sample
- Drilling Tools– Use for borehole drilling.
- Hammer Of 63.5 kg – Use for providing the blows.
- Guide Rod–
- Driving Head Or Anvil– Impact block
Standard Penetration Test Procedure | SPT Procedure
1- First we have to create a borehole by using drilling tools of sizes 100 mm to 150 mm.
2- For the soft clay or the loose clay we have to provide casing (casing is the big diameter of a hollow pipe) in the borehole so the borehole is not collapsed.
3- After getting the desired depth of the borehole the drilling tools are removed.
4- After removing the drilling tools, we put the Standard Split-Spoon Sampler into the borehole and lowered it to the bottom of the borehole.
5- The standard split spoon sampler is first allowed to sink by its own weight then the number of blows is applied.
5- Now we drop the hammer of 63.5 kg weight from a height of 75 cm at the rate of 30 blows per minute according to the IS code 2131-1963.
6- The number of hammer blows up to penetration of 150mm depth is recorded and the further number of blows is recorded for two more 150 mm depth penetration.
7- The number of blows required in the first 150 mm penetration of the split-spoon sampler is ignored for the seating drive.
8- The number of blows recorded for the last two 150 mm intervals is added which gives the standard penetration number (N).
Note-
Now the question is what is SPT value, SPT number, or standard penetration test n value? The standard penetration test n value, SPT Value, or SPT Number is the total number of blows required in the last 300mm penetration of the sampler.
Now the second question arises can the value of SPT become zero? The answer is Yes the SPT value becomes zero when the entire sample sinks into the soil due to its self-weight only. In such a case, no hammer blows are required.
The observed value of SPT from the field needs some corrections. Let’s discuss the correction applied in the standard penetration test.
Correction In Standard Penetration Test
Two corrections are required on the value of the SPT which is obtained from the field.
- Overburden Pressure Correction
- Dilatancy Correction Or Watertable Correction Or Fine Correction
The water table correction is always applied after the overburden correction.
Overburden Pressure Correction
The overburden correction in SPT is mainly required in cohesionless soil or granular soil.
As we all know that effective overburden pressure or confining pressure increases when the depth increases. Due to this high confining pressure at higher depth SPT Number or SPT Value is overestimated.
But when the confining pressure is low in loose soil or at a smaller depth then the SPT Number is under-estimated.
Therefore, to magnify the SPT number the overburden correction is required.
The Overburden Pressure Correction formula in SPT is N1=No( 350 / 70 + Overbudne Pressure).
Here N1 is the corrected value after overburden correction, No is the observed value.
Dilatancy Correction | Watertable Correction | Fine Correction
The dilatancy correction or water table correction is applied in the fine sand and silt when the water table is present.
When the water table is at or above the test level, then the dilatancy or water table correction shall be applied.
If the water table is below the test level then this test is not required.
Due to the dynamic action of the hammer pore pressure resistance is developed which causes the SPT number to be overestimated.
Therefore, to reduce SPT- value, the water table or dilatancy correction is required.
The dilatancy Correction formula in SPT is N2=15 + 1/2 ( N1-15 ).
In the dilatancy correction formula, we have to take the value of N1 > 15 always.
If N1< 15 then this correction is not required.
Here N2 is the final corrected value after water table correction and N1 is the corrected value after overburden correction.
Advantages And Disadvantages Of Standard Penetration Test
| Advantages Of SPT Test | Disadvantages Of SPT Test |
| The process of the STP test is simple to perform. | There is a need for some correction in the value of SPT. |
| This test gives the angle of shearing resistance and relative density of cohesionless soils. | The Test is rejected when the number of blows for 150 mm penetration of the sampler exceeds 50. |
| We collect the soil sample by the Split-Spoon Sampler. | It is time-consuming become it needs a borehole. |
| We can penetrate the dense layer of the soil. | We collect the soil sample with the Split-Spoon Sampler. |
Frequently Asked Questions(FAQ)
The standard penetration test measures the relative density and angle of shearing resistance of the cohesionless soils.
The standard penetration test gives the SPT Value or SPT Number which is the total number of blows required in the last 300mm penetration of the sampler.
The SPT test is costly and time-consuming, and the result is very due to drilling disturbance.
The in situ of soil means soil that is still in its original condition.
The Standard Split-Spoon sampler is used in the standard penetration test.
First, we drill a borehole and lower the Split-spoon sampler in the borehole. After that, we release the hammer having a weight of 63.5 kg from a height of 75 cm.
Now we note the number of blows for 150 mm penetration three times. The number of blows for the first 150 mm penetration reading is ignored.
The number of blows for the last two readings of 150 mm penetration of sampler is added and this number is called SPT Value or SPT Number.
The standard penetration test is done to find out the shearing resistance and relative density of cohesionless soils.
On one condition, we do reject our SPT test result when the number of blows for 150 mm penetration of the sampler exceeds 50.
The standard penetration test is given in the IS code 2131-1981.
The high SPT N value means the sampler passes through high confining pressure or high effective overburden pressure in such conditions the number of blows is increased.
The SPT refusal is taken when the number of blows for 150 mm penetration exceeds 50.
The dilatancy correction is used in the case of fine sand and silt when the water table is present and overburden pressure correction is used in the case of cohesionless soil or granular soil.
We can calculate the overburden correction by applying the formula N1=No( 350 / 70 + Overbudne Pressure). Here N1 is the corrected value after overburden correction, No is the observed value.
The dilatancy correction is applied in SPT to reduce the overestimated value of SPT.
The overburden correction is applied first, after the overburden correction we go for water table correction.
The N value is the number of blows needed for the penetration of the spoon sampler up to a depth of 30 cm in the SPT test.
In the SPT test, the first 15cm penetration of the split spoon sampler is known as the seating drive.
In the SPT test, after the first 15 cm, the 30cm penetration of the split spoon sampler into the soil is known as actual penetration.
