What Is SPT Test?
The Standard Penetration Test (SPT Test) is one type of in-situ soil test and It is conducted to determine the geotechnical engineering properties of subsurface soils, especially for cohesionless soil.
SPT Test is most widely used to check various parameters and properties of soil on the construction site. For any building foundation, design and construction soil testing are essential.
The test is extremely useful for determining the bearing capacity, density, and angle of shearing resistance of any soil. It can be used to determine the properties of cohesive and cohesionless soil. The Standard Penetration Test ASTM D1586-11
By performing an SPT test we can find out,
- Angel of shearing resistance of cohesionless soils.
- The relative density of cohesionless soils.
- Unconfined compressive strength of cohesive soils.
Purposes of SPT Test
The SPT test is conducted mainly for two purposes,
- Soil sample to identify the type and nature of the soil at various depths.
- To determine the penetration resistance of soil at various depths this is used for geotechnical design purposes.
Equipment Required For SPT Test
Following is the equipment used for the SPT test,
1. Drilling Equipment
We can use any drilling equipment which provides a good and clean borehole, which is at least 5mm larger than the Split Spoon Sampler.
2. Split Spoon Sampler
This is a thick-walled sample tube. The split spoon sampler has an outer diameter of it is 50.8 mm and an inside diameter is 35 mm with a length of around 650 mm (26 inches).
This tool has an assembly of a steel tube, coupling, driving shoes, a check valve, and venting ports. The split spoon sampler collects only disturbed soil samples.
3. Drop Hammer
This tool is used to drive the sampler into the borehole. Its mass of it is 63.5 kg (140 lb).
4. Driving Head (Anvil)
It is used to stop the hammer at a certain point.
5. Guiding Rod
The Guiding rod just guides the hammer to the anvil.
Standard Penetration Test Procedure
The SPT test procedure on site is as follows
1. Dig Starting Bore Hole
First, dig the starting borehole with boring equipment.
Before conducting the test, we have to decide the total depth of penetration bore and depth interval to carry the test and collect the sample the same.
Let’s take borehole depth up to 10m. (Generally, the depth of the bore is up to the groundwater table or up to hard strata below the ground surface.)
We can decide the intermediate test depth of every 1m. So, SPT is conducted at every one meter of boring below ground level and soil samples are collected at the same depth.
2. Assemble the Sampler
Once the boring of the hole is done up to desired depth (1 m depth we decided) remove the drilling tools from the borehole and clean all the disturbing materials.
After that fit the soil sampler named as split spoon sampler with the drilling rod and lower it into the borehole.
Now, the rest split spoon sampler was attracted with a drilling rod at bottom of the drilled borehole of undistributed soil.
3. Assemble Equipment
As we place the sampler rest on the bottom of the borehole, it’s time to conduct the SPT test.
Keep ready test equipment the Hammer, Anvil, and guiding rod and assemble them with each other properly.
Mark the distance of 150mm on the drilling rod to observe penetration details.
4. Conduct SPT Test
Firstly drive the drop hammer on the bottom of a borehole with 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.
Now, Count the number of blows required to reach or drive a depth of 150 mm (6 in).
Similarly, again drive the sampler in soil and count the blows needed to penetrate the second and third 150 mm (6 in).
In this test, the total sum of the number of blows required to drive the sampler 150mm (6 in.) of penetration is termed the “standard penetration resistance” or the “N-value“.
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, then the report should specify the number of blows and the depth penetrated).
If a number of blows are required to drive the sampler to a depth of 150 mm excess of the value 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 meters whichever is less. The test may be made at lesser intervals if necessary or specified.
5. Soil Sample Collection
Now, collect the soil sample from the borehole,
- Take out a spilled sampler from the borehole.
- Carefully, separate the split spoon sampler from the drilling rod and remove any excess soil from the bottom, also drain water if it contains it.
- Slowly open one side of the split spoon sampler to expose fresh material and any stratification.
- Record the length, composition, color, stratification, and condition of the sample.
- Remove the soil specimen from the split spoon sampler and pack it in a plastic bag.
How to Find Bearing Capacity of Soil From SPT Test Result
The following are the step to find the bearing capacity of soil from SPT Values or standard penetration test calculation.
Step: 1 Take Average of Penetration Value
Make necessary correction SPT values and take an average of corrected SPT values from the base level of the foundation to a depth equal to 2 times the width of the foundation.
Step: 2 Find our Angle of Shearing Resistance
Find out the angle of shearing resistance by correlating the above SPT values from a given graph.
Step: 3 Find out Effective Surcharge
Find out the value of the effective surcharge by multiplying the effective unit weight of soil with the depth of the foundation i.e.
q = ϒ*Df
q = Effective surcharge at foundation base level, in kgf/cm2
ϒ = Unit weight of soil, in kgf/cm3
Df = Depth of foundation, in cm
Step: 4 Find out the Bearing Capacity Factor
Calculate the corresponding values of bearing capacity factors (i.e. Nq & Nϒ) by correlating the values of the angle of shearing resistance from the table given below. For any intermediate value of ‘ϕ’, make linear interpolation.
Step: 5 Find Out the Shape Factor
Calculate shape factors (i.e. sq & sϒ) using the formula given below.
B = Width of foundation, in cm
L = Length of foundation, in cm
Step: 6 Find Out the Depth Factor
Calculate depth factors (i.e. dq & dϒ) using the following formula.
dq=dϒ=1 (for ϕ < 100)
dq = dϒ = 1+0.1(Df/B)(Nϕ)1/2 (for ϕ>100)
Nϕ is calculated using the following formula
Nϕ = tan2[(π/4)+(ϕ/2)]
Step: 7 Calculate the Inclination Factor
Calculate inclination factors (i.e. iq & iϒ) using the formula given below
α = Load inclination to the vertical in degrees
ϕ = Angles of shearing resistance in degrees
Step: 8 Calculate the Correction Factor
Find out the correction factor for the location of the water table using the following formula
W’ = 0.5+0.5[Dw/(Df+B)]
W’ = Water table correction factor
Dw = Depth of water table, in cm
Df = Depth of foundation, in cm
Step: 9 Calculate the Bearing Capacity of Soil
Calculate net ultimate bearing capacity by using the below equation.
qd = It is Net ultimate bearing capacity of the foundation (kgf/cm2)
q = Effective surcharge at the base level of foundation, in kgf/cm2 (Refer step-3)
Nq & Nϒ = Bearing capacity factors (Refer step-4)
sq & sϒ = Shape factors (Refer step-5)
dq & dϒ = Depth factors (Refer step-6)
iq & iϒ = Inclination factors (Refer step-7)
W’ = Correction factor for the location of the water table (Refer step-8)
B = Width of foundation, in cm
ϒ = Foundation soil bulk unit weight, in kgf/cm3.
Standard Penetration Test Lab Manual
Free download the standard penetration test lab manual from the below link,
SPT Test Report
Free download standard penetration test lab report or spt test report format from below link,
What is the Standard Penetration Test (SPT)?
The Standard Penetration Test (SPT) is a common in-situ test used to determine the geotechnical engineering properties of soil.
What information can be obtained from the Standard Penetration Test (SPT)?
The SPT provides information about the soil’s resistance to penetration, which is related to its strength and density.
How is the Standard Penetration Test (SPT) performed?
The SPT is performed by driving a split-barrel sampler into the soil using a standard number of blows from a hammer.
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