STANDARD PENETRATION TEST (SPT) – THEORY

Standard Penetration Test Setup
Standard Penetration Test Setup
  • This test involves the measurement of the resistance to penetration of a sampling spoon under dynamic loading
  • The resistance is empirically correlated with some of the engineering properties of soil such as density index, consistency, angle of internal friction, bearing capacity etc.
  • This test is useful for general exploration of erratic soil profile for finding depth to bed rock or hard stratum and to have an approximate indication of the strength and other properties of soil, particularly the cohesionless soil, from which it is difficult to obtain undisturbed samples.
  • The importance of this test is that, even though empirical, the soil design of foundations in sand is mostly based on the N-value. N value serves as the basic parameter for geotechnical design in sand.
  • In this test, a thick wall standard split spoon sampler, 50.8 mm outer diameter and 35 mm inner diameter, is driven into the undisturbed soil at the bottom of the bore hole under the blows of a 63.5 kg drive weight with 75 cm free fall. The minimum open length of the sampler should be 60 cm. the number of blows required to drive the sampler 30 cm beyond the seating drive of 15 cm, is termed as the penetration resistance N.

RELATION BETWEEN SPT (N) VALUE WITH DIFFERENT SOIL PROPERTIES IN CASE OF COHESION LESS SOIL.

SPT VALUE (N)

COMPACTNESS

ANGLE OF FRICTION (φ)

UNIT WEIGHT (γ) in T/m3

0 – 4

Very loose

< 28

1.1 – 1.8

4 – 10

Loose

28 – 30

1.4 – 2.0

10 – 30

Medium

30 – 36

1.7 – 2.2

30 – 50

Dense

36 – 41

1.7 – 2.3

> 50

Very dense

> 41

2.0 – 2.3

RELATION BETWEEN SPT (N) VALUE WITH DIFFERENT SOIL PROPERTIES IN CASE OF COHESIVE SOIL.

SPT VALUE (N)

COMPACTNESS

UCS (qu) in kPa

0 – 4

Very soft to soft

25

4 – 8

Soft to Medium

50

8 – 16

Medium to Stiff

100

16 – 32

Stiff to very stiff

200

>32 Very stiff to Hard

400

Also Read: How to Perform Standard Penetration Test of Soil On Site?

Also Read: How to Calculate Bearing Capacity of Soil From Standard Penetration Test?

Also Read: 20 Factors Affecting Standard Penetration Test Result

Also Read: 9 Precautions to Take While Performing Standard Penetration Test

Also Read: How to Calculate Pile Load Capacity Using SPT or N Values?

Also Read: Corrections Applied to SPT Values

 

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