STANDARD PENETRATION TEST (SPT) – THEORY

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