Due to its simplicity, the standard penetration test is a widely used soil test.
In construction, we mainly conduct it for designing civil structures.
In this post, you’ll learn everything about the standard penetration test that you need to know as a construction professional.
What Is The Standard Penetration Test?
The Standard Penetration Test (SPT) is an in-situ dynamic penetration test.
It is conducted to determine the geotechnical engineering properties of subsurface soils, especially for cohesionless soil.
What Is The Use Of Standard Penetration Test?
The Standard Penetration Test (SPT) is useful in finding out –
- Angel of shearing resistance of cohesionless soils.
- The relative density of cohesionless soils
- Unconfined compressive strength of cohesive soils.
Purposes of the Standard Penetration Test
The test is conducted mainly for two purposes-
- Collect soil samples for identification purposes.
- Measure the penetration resistance of soils which is used for geotechnical design purposes.
Tools Required For Conducting SPT
You’ll need the following five tools for the test.
You can use any drilling equipment that provides a reasonably clean borehole, which is at least 5mm larger than the Split Spoon Sampler.
Split Spoon Sampler
This is a thick-walled sample tube.
The outside diameter of it is 50.8 mm and the inside diameter is 35 mm with a length of around 650 mm (26 inches).
This tool is consists of driving shoes, a steel tube, coupling, a check valve, and venting ports.
When assembled it looks like this:
This tool is used to collect a disturbed sample of soil.
Slide Hammer Or Drop Hammer
This tool is used to drive the sampler into the borehole. The mass of it is 63.5 kg (140 lb).
It is used to stop the hammer on a certain point.
The Guiding rod just guides the hammer to the anvil.
As you now know about the tools, let’s see the…
The Procedure Of Standard Penetration Test
The standard penetration test is really simple. You just need to go through the following five steps:
Step #1: Dig A Bore Hole
The first step of this test is to dig a borehole with the boring equipment.
Here you need to know how long the borehole depth should be.
Last time, I conducted a soil penetration test in a plot. I’ve been told to run the test every meter up to 10 meters depth.
So I needed to dig a one-meter depth borehole to run the test and again a one-meter depth borehole and so on till I reach 10 meters depth in total.
That means I needed to run the test 10 times in a borehole.
So first decide how long the borehole should be and dig till you reach your desired depth.
Step #2: Place The Sampler
- Once the drilling is done to the desired depth, remove the drilling tools from the borehole and clean all the disturbed-materials.
- Now assemble sampler with the drilling rod and lower into the borehole.
- Make sure the sampler is rested on the undisturbed soil.
Step #3: Assemble Equipment
Once you lower the sampler, now it’s time to get ready for the test.
Do the followings:
- Assemble the Hammer, Anvil and guiding rod.
- Mark the drill rod in three successive 150 mm (6 inches) increments to observe penetration.
Step #4: Run The Test
- Now drive the sampler into the ground at the bottom of the borehole by blows from the slide hammer with a mass of 63.5 kg falling through a distance of 750 mm (30 in) at the rate of 30 blows per minute.
- Count the number of blows required to drive depth of 150 mm (6 in).
- Similarly, drive the sampler further and count the blows needed to penetrate the second and third 150 mm (6 in).
The sum of the number of blows required for the second and third 150mm (6 in.) of penetration is termed the “standard penetration resistance” or the “N-value“. In cases where 50 blows are insufficient to advance it through a 150 mm (6 in.) interval the penetration after 50 blows are recorded.
If the sampler is driven less than 450 mm (total), then the “N-value” shall be for the 300 mm of penetration (if less than 300 mm is penetrated, the logs should state the number of blows and the depth penetrated).
If the number of blows for 150 mm drive exceeds 50, it is considered as a refusal and the test is discontinued.
The entire sampler may sometimes sink under its own weight when a very soft sub-soil stratum is encountered. Under such situations, it may not be necessary to give any blow to the sampler and “N-value” should be indicated as zero.
The Test shall be made at every change in the stratum or at intervals of not more than 1.5 meter whichever is less. The test may be made at lesser intervals if necessary or specified.
Step #5: Collect Soil Sample
Now it’s time to collect samples of soils.
- Remove the sampler from the borehole and bring it to the surface.
- Separate it from the drilling rod and remove any obvious contamination from the ends or sides and drain excess water.
- Carefully scrape or slice along one side to expose fresh material and any stratification.
- Record the length, composition, color, stratification, and condition of the sample.
- Remove the sample from the sampler and wrap or seal it in a plastic bag.
This is somehow the procedure of the standard penetration test.
What Do You Need To Inspect During The Test?
Inspect the following things for standard penetration test:
- The drill rods should be straight and in good condition.
- Make sure the split spoon sampler is in good condition and the cutting shoe must be free from tear and wear.
- The drop hammer should be of the right weight
- Make sure the slide hammer falling height is 750 mm exact and the fall should be frictionless and vertical. Any changes in this will affect the “N-value“.
- Make sure the bottom of the borehole is cleaned before conducting the test.
- Ensure the slide hammer falling rate is 30 blows per minute.
Factors Affecting “N-Value”
Some factors can affect the field “N-value”. Those are:
- Inadequate cleaning of the borehole.
- Not setting the split spoon sampler on the undisturbed soil.
- Friction during falling the weight.
- Not using the standard weight of the hammer.
- Not using a guide rod.
- Using drill rods heavier than standard.
Before using the SPT values in empirical correlations and in design charts, the field “N-value” is corrected.
As construction professionals, our responsibility is to make sure the standard penetration test is being performed as per standard procedure and the field “N-value” is as accurate as possible.