SPT

1. In-situ Testing
 When it is difficult to obtain
“undisturbed” samples
 Cohesionless soils, Sensitive clays
 In-situ Test Methods
– Standard Penetration Test (SPT)
– Cone Penetration Test (CPT)
– Vane Shear Test (VST)
– Texas Cone Penetration Test (TCP)

5. Standard Penetration Test (SPT)
 The standard penetration test is carried out
in a borehole, while DCPT and SCPT are
carried without a borehole.
 All the three tests measure the resistance of
the soil strata to penetration by a
penetrometer.
 Useful empirical correlations between
penetration resistance and soil properties
are available for use in foundation design.

6. Standard Penetration Test (SPT)
 It is an insitu test especially for cohesionless
soils.
 It gives idea about relative Density (ID)
 Conducted in a bore hole using standard split
spoon sampler.
 SPT value (N) Means, the number of blows
required for 30 cm penetration.
 Cohesion and Angle of internal friction values
of soil depends on the N-number.

7. Standard Penetration Test (SPT)
 63.5 kg Hammer
 76 cm free fall
 Drive sampler over 45 cm
 Record no. of blows per each 15 cm
penetration
 SPT blow count=blows for 2nd 15 cm
penetration + blows for 3rd 15 cm
penetration

8. Standard Penetration Test (SPT)
 The test is reported as ‘refusal’ and the test is
halted if 50 blows are required for any 150 mm
penetration.
 Importance of N-Value:
 The N Value indicates the relative density of the
cohesionless soil and the unconfined compressive
strength of the cohesive soil. If the soil is
compact or stiff, the penetration number is high.
In general , the greater the N-value , the greater
is the angle of shearing resistance.
 The consistency and the unconfined compressive
strength of the cohesive soils can be
approximately determined from SPT Number N.

9. Standard Split Spoon Sampler
 Thick wall (0.25in) cylinder
 Sampling tube is split along the length
 Hammered into the ground

10. Standard Split
Spoon
Sampler

13. Standard Penetration Test (SPT)

14. Types of SPT Hammers

15. SPT:
Automatic
Trip Hammer

16. Corrections to SPT blow Counts
Factors affecting SPT blow count:
 Hammer Efficiency (See Table 4.3)
 Borehole diameter (See Table 4.4)
 Type of sampler (See Table 4.4)
 Rod length (See Table 4.4)

17. SPT Correction Factors
60
.
0
N
C
C
C
E
N R
S
B
m
60 
 hammer efficiency (Em) …. Table 4.3
 bore hole diameter (CB)…….Table 4.4.
 sampler correction (CS) ……Table 4.4
 rod length (CR) ………Table 4.4

18. Corrections for N-Value
Overburden Pressure Correction
Dilatancy Correction

19. Corrections for N-Value
Overburden Pressure Correction
 In granular soils , the overburden pressure affects the
penetration resistance.
 If the two-soils having same relative density but different
confining pressures are tested, the one with higher
confining pressure gives a high penetration number.
 Thus N-Value depends upon confining pressure and
which in turn increases with depth.
 Higher the confining pressure , higher will be the N-
Value.
 N- Value for soils at shallow depth is underestimated and
that at greater depth is over estimated.

20. Corrections for N-Value
Overburden Pressure Correction
For Uniformity, the N-Values obtained from field
tests under different effective overburden pressure
are collected to a standard effective overburden
pressure.

21. Corrections for N-Value
 Overburden Pressure Correction

22. Corrections for N-Value
 Dilatancy Correction
Dilatancy Correction is to be applied
when N’ obtained after the overburden
correction exceeds 15 in saturated fine
sand and silts.

23. Corrections for N-Value

24.  N’ > 15 is an indication of a dense sand .
 In such a soil, the fast rate of application of
shear through the blows of a drop hammer is
likely to induce negative pore water pressure
in a saturated fine sand under undrained
condition of loading.
 Consequently, a transient increase in shear
resistance will occur, leading to a SPT value
higher than the actual one.

27. Use of SPT Data
 To Determine Relative Density, Dr
 To determine 
 To determine C