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**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).

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).

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,

- 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.

**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.

### **Step: 5 Find Out Shape Factor**

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

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

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.

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 Soi**l. 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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