Standard Penetration Test

SPT Test | Soil Penetration Tests | Standard Penetration Test Procedure | SPT Test Report

What Is SPT Test?

The Standard Penetration Test (SPT) 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 e1105


Singnificance Soil Penetration Test

By performing a 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 Geotechnical 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 penetration resistance of soil at various depths this is used for geotechnical design purposes.

Standard Penetration Test Equipment

Following are 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 the inside diameter is 35 mm with a length of around 650 mm (26 inches).

SPT Test | Standard Penetration Test | Standard Penetration Test Procedure | SPT Test Report
Split Spoon Sampler

This tool has an assembly of a steel tube, coupling, driving shoes, a check valve, and venting ports.  Split spoon sampler collects only disturbed soil samples.

3. Drop Hammer

This tool is used to drive the sampler into the borehole. The mass of it is 63.5 kg (140 lb).

4. Driving Head (Anvil)

It is used to stop the hammer on a certain point.

5. Guiding Rod

The Guiding rod just guides the hammer to the anvil.


Standard Penetration Test Procedure

Spt Test procedure on site 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, depth of bore is up to groundwater table or up to hard strata below 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 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 disturbed-materials.

After that fit the soil sampler named as split spoon sampler with the drilling rod and lower into the borehole.

Now, rest split spoon sampler attracted with drilling rod at bottom of the drilled borehole of undistributed soil.


3. Assemble Equipment

As we place 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 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.

Now, Count the numbers of blow required to reach or drive depth of 150 mm (6 in).

Similarly, again drive sampler in soil and count the blows needed to penetrate the second and third 150 mm (6 in).

SPT Test | Standard Penetration Test | Standard Penetration Test Procedure | SPT Test Report
Standard Penetration Test (spt test)

In this test, the total sum of a number of blows required to drive 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 report should specify the number of blows and the depth penetrated).

If a number of blows required to drive the sampler to a depth of 150 mm excess 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 meter 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,

SPT Test | Standard Penetration Test | Standard Penetration Test Procedure | SPT Test Report
Spilt Spoon Sampler
  • Take out a spilt 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.
  • Slowly open the 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 soil specimen from 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 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 sharing resistance by correlating the above SPT values from a given graph.

SPT Test | Standard Penetration Test | Standard Penetration Test Procedure | SPT Test Report
Angle of Shearing Resistance

Step: 3 Find out Effective Surcharge

Find out the value of effective surcharge by multiplying the effective unit weight of soil with the depth of the foundation i.e.

q = ϒ*Df

Where,

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 Bearing Capacity Factor

Calculate out the corresponding values of bearing capacity factors (i.e. Nq & Nϒ) by correlating the values of angle of shearing resistance from the table given below. For any intermediate value of ‘ϕ’, make linear interpolation.

SPT Test | Standard Penetration Test | Standard Penetration Test Procedure | SPT Test Report
Bearing Capacity Factor

 Step: 5 Find Out Shape Factor

Calculate shape factors (i.e. sq & sϒ) using the formula given below.

standard penetration test
Shape Factor

Where,

B = Width of foundation, in cm

L = Length of foundation, in cm


Step: 6 Find Out 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 Inclination Factor

Calculate inclination factors (i.e. iq & iϒ) using the formula given below

SPT Test | Soil Penetration Tests | Standard Penetration Test Procedure | SPT Test Report
Inclination Factor

Where,

α = Load inclination to the vertical in degrees

ϕ = Angles of shearing resistance in degrees


Step: 8 Calculate Correction Factor

Find out correction factor for the location of the water table using the following formula

W’ = 0.5+0.5[Dw/(Df+B)]

Where,

W’ = Water table correction factor

Dw = Depth of water table, in cm

Df = Depth of foundation, in cm


Step: 9 Calculate Bearing Capacity of Soil:

Calculate net ultimate bearing capacity by using the below equation.

SPT Test | Soil Penetration Tests | Standard Penetration Test Procedure | SPT Test Report
Bearing Capacity of Soil

Where,

qd = It is Net ultimate bearing capacity of foundation (kgf/cm2)

q = Effective surcharge at 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 standard penetration test lab manual from below link,

SPT Test Report

Free download standard penetration test lab report or spt test report format from below link,

FAQs: SPT Test

Soil Penetration Test

The Standard Penetration Test (SPT) 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.

Standard Penetration Test Equipment

SPT Test required following equipment,
Drilling Equipment
Split Spoon Sampler
Drop Hammer
Driving Head (Anvil)
Guiding Rod

Standard Penetration Test Procedure

SPT test procedure as follows,
1. Dig Starting Bore Hole
up to 1m first..
2. Assemble the Sampler: Fit the soil sampler named as split spoon sampler with the drilling rod and lower it into the borehole.
3. Assemble Equipment: Keep ready test equipment the Hammer, Anvil and guiding rod and assemble them with each other properly.
4. Conduct SPT Test: Firstly drive the drop hammer on the bottom of a 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 numbers of blow required to reach or drive depth of 150 mm (6 in).
5. Soil Sample Collection: Now, collect the soil sample from the borehole,

What Is n Value

The number of blows required by hammer to dig 30 cm depth in SPT test is known as N- Value of Soil. The measured N-value is also called standard penetration resistance of the soil.

SPT Test IS Code

The standard penetration test conducted as per IS -2131:1963

Watch Video: Standard Penetration Test Procedure

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