Standard Penetration Test – Complete Guide with Videos and Free Report

Standard Penetration Test – We will discuss standard penetration test procedure and standard penetration test values. Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils | Download Free SPT ASTM D1586-11 pdf

The shearing resistance of cohesion less soils and density, as well as the strength of stiff cohesive soils, are determined using Standard Penetration Test, a low-cost and simple testing procedure widely used in geotechnical investigations.


A standard penetration test consists of drilling a borehole to the desired sampling depth. The testing point is attached to the drill rod of a split-spoon sampler. A hammer weighing 140 pounds (63.5 kilogrammes) is repeatedly dropped from a height of 30 inches (76 cm), driving the sampler into the ground to a depth of 6 inches (15 cm).

A notebook keeps track of the number of required blows. The test is repeated two more times until a total penetration of 18 inches (45 cm) has been achieved. The total number of blows required to penetrate the remaining 30 cm depth and the number of blows required to penetrate the first 15 cm are referred to as “seating drive,” and the standard penetration resistance, or N-value, is calculated. If the N-value is greater than 50, the test is called a “refusal,” and it is then repeated.

The Standard Penetration Test (SPT) is a soil property testing method that is commonly used to determine the geotechnical engineering properties of subsurface soils.

The relative density of soils and approximate shear strength parameters can be estimated using this inexpensive and simple test.


Standard Penetration Test (SPT)

Description And Procedure of Standard Penetration Test (SPT)

This entails driving a standard thick-walled sample tube into the ground at the bottom of a hole with standard distance and weight blows from a slide hammer. The sample tube is driven 15 cm into the ground, and the number of blows required to penetrate the tube every 6 inches (15 cm) up to a depth of 18 inches (45 cm) is recorded. The total number of blows required for the third and second 6 inches is the sum of the total number of blows required. The standard penetration test blow count value, also known as the “N-value” or “standard penetration resistance,” is used to report penetration.

Provides The N-value is an indicator of the subsurface soil’s relative density, and it is used in empirical geotechnical correlation to calculate the approximate shear strength properties of soils.

Standard penetration test N3 [Blows/0.3 m – 1 ft]Soi packingRelative Density [%]Friction angle [°]
< 4Very loose< 20< 30
4 -10Loose20 – 4030 – 35
10 – 30Compact40 – 6035 – 40
30 – 50Dense60 – 8040 – 45
> 50Very Dense> 80> 45

Standard Penetration Test Tools

Standard Penetration Test - Complete Guide with Videos and Free Report 1
Standard Penetration Test

What are the tool used in SPT? This is same as standard penetration test apparatus. Following standard penetration test equipment will be required for a successful Standard Penetration Test.

  1. A Drill Rod For Extending The Test To Deeper Depths
  2. A Light Duty Rope To Operate Sand Bailer
  3. A Straight Edge (50 Cm)
  4. Casing Clamps
  5. Casing Couplings
  6. Casing Pipes
  7. Chain Pulley Block
  8. Drilling Rig.
  9. Driving Head (Anvil).
  10. Drop Hammer 65 Kg (140 Lbs ).
  11. Extension Rods.
  12. Guide Pipe Assembly
  13. Guiding Rod.
  14. Heavy Duty Auger Extension Rods
  15. Heavy Duty Helical Auger
  16. Heavy Duty Post Hole Auger (100 Mm To 150 Mm Diameter)
  17. Measuring Tapes
  18. Pulley
  19. Rope (About 15 M Long & Strong Enough To Lift 63.5 Kg Load Repeatedly)
  20. Sand Bailer
  21. Split Spoon (Tube).
  22. Standard Split Spoon Sampler
  23. Tool Box
  24. Tripod
  25. Tripod Head With Hook

Standard Penetration Test Theory

Standard Penetration Test (SPT) Full Length Explaining Video

The number of blows required to penetrate the soil to a depth of 12 inches. The ‘N’ value is measured in blows penetration and is commonly referred to as the ‘N’ value.

The standard penetration test is widely used to determine a soil’s bearing capacity at a specific depth. This test can often be used to estimate the consistency of clayey soils.

When the sampler (split spoon) is lowered to the bottom of the borehole and the drill tools are removed, the hole is extended to a predetermined depth.


The sampler is driven into the soil with hammer blows to the top of the drill rod. The hammer’s standard weight is 65 kg (140 lbs), and the number of blows required to penetrate a spoon at three intervals of 15 cm (6 inches) is recorded.

Now we will learn, how to conduct a standard penetration test.

Standard Penetration Test Procedure

  1. Place the tripod over the test hole and put the unit together.
  2. Place the spoon in the hole and let it rest there.
  3. Drive the spoon with 75 cm (30 inch) hammer blows until 45 cm (18 inch) has been penetrated or 100 blows have been applied.
  4. Keep track of the number of blows required to penetrate 15 cm (6 inches). The first 6 inches are referred to as the seating drive.
  5. The penetration resistance value N of the soil is calculated using the number of blows required for the second and third 15 cm (6 inch) of drive added.

Observations and Calculations

The penetration resistance value, N of the soil is ______________ blows. You can find using below formula to find n-value of standard penetration test.

Formula: q= 3.5 (N-3) [B+0.3/2B]Rw2Rd.

Where,

q= Allowable net pressure in T/m2.
N = Standard penetration number.
B = Breadth of footing in meters.
Rw2 = Water reduction factor= 0.5 [ 1+Z/B] .
Rd = Depth factor = l+D/B C or  C= 2.0.
Z = Depth of water table below foundation level.

N-Value of standard penetration test – “.

Precautions

  1. When conducting a standard penetration test in gravelly soils, the driving shoe is replaced with a solid 60-degree cone.
  2. Due to the loosening of the soil below the water table, care should be taken to avoid water entering the borehole through the bottom.
  3. Water should be added as needed to keep the borehole’s water table level stable.
  4. If the test is conducted in very silty sand and fine sand below the water table, the measured “N” value should be corrected for the increased resistance and excess water pressure created during the inability to lose immediately.
  5. The sides of the borehole can be supported with mud drilling.
  6. When the sample penetrates less than 25 mm under 50 blows, stop dropping weight.

Free SPT ASTM D1586-11 pDF

Standard Penetration Test (SPT) FAQ

These are some basic, one liner, Quiz type questions on standard penetration test.

SPT stands for

SPT stands for “Standard Penetration Test.”

What is the minimum depth to start SPT in borewell as per standard?

1

Weight of hammer used in Standard Penetration Test (SPT)

140 lbs or 65 Kg | The standard weight of the hammer is 65 kg(140 lbs).

Standard Penetration Test (SPT) is carried out up to such penetration

45 cm or 100 blows | The number of blows needed for the tube to penetrate every 6 inches (15 cm) up to a depth of 18 inches (45 cm) is recorded.

Maximum Blow are applied in Standard Penetration Test (SPT)

100 blows

Er. Md. Shahin Akhter

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Md Shahin Akhter is a Civil Engineer from MIT and has more than 5 years of experience in Civil Engineering and Construction Department. Md Shahin Akhter is Civil Engineer in Nagar Parishad (Municipal Council), Supaul. He is Webmaster and Founder at ProCivilEngineer.com Civil Engineering Website.