introduction and overview of SPT test as per IS code

1. PRESENTATION ON STANDARD
PENERTATION TEST
Presented by—
JOYDEEP ATTA(19CE4202)

2. INTRODUCTION
The standard penetration test (SPT) is an in-situ dynamic penetration test designed
to provide information on the geotechnical engineering properties of soil. This test is
the most frequently used subsurface exploration drilling test performed worldwide.
Most
commonly
used in situ
test
Especially for
cohesion less soil
which is very hard
to sample.

3. Useful to
find
• Relative density of soil
• Angle of shearing resistance of cohesion less soil
• Unconfined compression strength of the cohesive soil
• Liquefaction condition

4. SPT test process
(since1902)

5. INSTRUMENTS
 DRILLING EQUIPMENT FOR BOREHOLES
 SPLIT – SPOON SAMPLER
 DRIVE – WEIGHT ASSEMBLY
 CATHEAD
 HAMMER (SAFETY HAMMER, DONUT HAMMER, AUTOMATIC
HAMMER)

6. DRILLING EQUIPMENT FOR BOREHOLES
Any drilling equipment is
acceptable that provides a
reasonably clean hole, which
is at least 5 mm larger than
that sampler or sampling rods,
and less than 170 diameter.

7. SPLIT – SPOON SAMPLER
• It is a sampler for obtaining a disturbed sample
of soil .
• It is consists of—
• Driving shoe: made of steel about 75 mm long
• Steel tube: 450 mm long, split longitudinally in
two halves.
• Coupling: 150 mm long provided at top
• Check valve
• 4 venting ports: 10mm diameter

8. DRIVE – WEIGHT ASSEMBLY
• Hammer of 63.5 kg.
• A driving head .
• A guide permitting a free fall of 0.76 m
and over lift capability of at least 100
mm.

9. CATHEAD
• Operating at approximately 100
rpm.
• Equipped with suitable rope and
overhead sheave for lifting drive
weight.

10. HAMMER (SAFETY HAMMER)
• Closed system.
• Delivers approximately 60% of the
maximum free fall.
• Highly variable energy transfer.

11. HAMMER (DONUT HAMMER)
• Open system.
• Delivers approximately 45% of the
maximum free fall energy.
• Highly variable energy transfer.

12. HAMMER (AUTOMATIC HAMMER)
• Safest system.
• Delivers approximately 95-100% of
the maximum free fall energy.
• Consistent and effective energy
transfer.

13. PROCEDURE (IS:2131-1963)
 1. Erect the tripod over the test hole and assemble the unit.
 2. Allow the spoon to rest on the bottom of the hole.
 3. Drive the spoon with blows from the hammer falling 75 cm (30 inches), until
either 45 cm (18 inches) have been penetrated or 100 blows have been applied.
 4. Record the number of blows required to effect each 15 cm (6 inches) of
penetration. The first 6 inches is considered as seating drive.
 5. The number of blows required for the second and third 15 cm (6 inches) of
drive added is recorded as the penetration resistance value N of the soil.

14. Some correction on SPT value
 1. Overburden pressure correction
a. GIBBS and HLTZ`S Correction
b. PECK, HANSEN and THORNBURN`S correction
c. PECK and BAZARAA`S correction
 2. Dilatancy correction
After applying the overburden pressure correction in the
corrected SPT value we have to apply dilatancy correction and
it gives ultimate corrected SPT value.

15. OVERBURDEN PRESSURE CORRECTION
 In granular soil, overburden pressure affects the penetration resistance.
 If two soils, having same relative density but different confining pressure are
tested, the one with a higher confining pressure gives a higher penetration number
as the confining pressure in cohesion less soils increases with depth, the
penetration number for soils at shallow depths is underestimated and that at
greater depth is overestimated.
 For uniformity, the N value obtained from field tests under different effective
overburden pressure are corrected to a standard effective overburden pressure.

16. GIBBS and HLTZ`S Correction(1957)
 Applicable for σ* ≤ 280 KN/ 𝑚2
and for dry or moist clean sand.
NC = NR X
350
σ∗+70
NC = corrected N value
NR = observed N value
σ* = Effective overburden pressure in Kpa
 Ratio (NC/NR) should be lie between 0.45 to 2.0
 If the ratio greater than 2.0 , NC should be divided by 2.0 to obtain the design
value used in finding the bearing capacity of soil.

17. PECK and BAZARAA`S correction
 One of most commonly used corrections
NC =
4 𝑁𝑅
1+0.0418σ∗
σ* > 71.8KN/𝑚2
NC =
4 𝑁𝑅
3.25+0.0104σ∗
σ* < 71.8KN/𝑚2
NC =NR σ* = 71.8KN/𝑚2

18. PECK, HANSEN and THORNBURN`S correction
 They give the chat for correction on N values to an
effective overburden pressure of 96 KN/𝑚2.
 According to them for σ* ≥ 24 KN/𝑚2
.
NC = 0.77 NR log (
1905
σ∗
)

19. DILATANCY CORRECTION
 Silty fine sands and sine sand below the water table develop pore pressure which
is not easily dissipated.
 Pore pressure increases the resistance of the soil thus, penetration number (N) also
increases.
 This correction is applied when observed value of N exceeds 15.

20. DILATANCY CORRECTION
 TERZAGHI and PECK(1967) recommended the following correction—
N(C)act =15 +
1
2
(NC - 15)
Where,
 N(c)act – corrected penetration number
 Nc – recorded value after overburden correction
If, N(c)act ≤ 15; N(c)act = Nc .

21. FACTORS AFFECTING N- VALUE
FACTORS COMMENTS
Attitude of operators Blow counts for the same soil using the same rig can vary,
depending on who is operating the rig, and perhaps the mood
of operator and time of drilling.
Not using a good tip on the
sampling spoon
If the tip is damaged and reduces the opening or increases the
end area the N value can be increased.
Weight does not strike the
drive cap concentrically
Impact energy reduce, reduce N value.
Plugged casing High N value may be recorded for loose sand when sampling
below groundwater table. Hydrostatic pressure can cause sand
to rise within the casing.

22. FACTORS COMMENTS
Inadequate cleaning of borehole SPT is only partially made in organic soil.
Sludge may be trapped in the sampler and
compressed as the sampler is driven,
increasing the blow count.(also effect sample
recovery)
Use of drill rods heaver than standard With heaver rods more energy is absorbed by
the rods causing and increase N value
Drilling method Drilling technique may result in different N
value for same soil.

23. CORRELATIONS BETWEEN SPT AND SOIL PROPERTIES
 Relative density (SAND) & Consistency (CLAY).
 Effective stress friction angle.
 Unconfined compressive strength (CLAY).
 End bearing capacity of a pile & bearing capacity factor.

24. RELATIVE DENSITY & CONSISTENCY
SAND CLAY
N SPT Value Relative Density N SPT Value consistency
0-4 Very Loose 0-2 Very Soft
4-10 Loose 2-4 Soft
10-30 Medium 4-8 Medium
30-50 Dense 8-15 Stiff
> 50 Very Dense 15-30 Very Stiff
> 30 Hard

25. UNCONFINED COMPRESSION STRENGTH OF CLAY
N SPT Value Consistency Compressive Strength (KPa)
0-2 Very Soft <25
2-4 Soft 25-50
4-8 Medium 50-100
8-15 Stiff 100-200
15-30 Very Stiff 200-400
> 30 Hard >400

26. EFFECTIVE STRESS FRICTION ANGLE

27. BEARING CAPACITY
 Bowles’ equations:
Qa =
(N/2.5)
K
; B ≤ 4
=
(N/4)[(B+1)/B]
2
𝐾
; B > 4
 K = 1+0.33(D/B) ≤ 1.33
 Qa = allowable bearing capacity
 B= footing width
 D= depth from ground level to bottom of footing in feet.

28. BEARING CAPACITY OF PILE FOUNDATION
 From SPT N value we can calculate only end bearing resistance of a pile
foundation.
Qpu = 40 N (
𝐿
𝐷
) KN/𝑚2
which is not less than 400N KN/𝑚2
N = Without overburden correction
L= Length of pile
D= Diameter of pile

29. BEARING CAPACITY FACTOR

30. SPT & LIQUEFACTION
 Numerous case histories of soil liquefaction during past earthquakes are available
with SPT N-values. The method based on this history can reflect actual soil
behaviour during earthquakes, which cannot be simulated in the laboratory.
 According to the survey results of the code compilation group in China, the
liquefaction phenomenon does not exist when the soil depth is more than 20 m.
 And according to the Code for Seismic Design of Buildings (GB 50011-2010
(Standardization Administration of China)), the SPT-based method should be
adopted for the soil within 20 m from ground when the degree of saturated sand or
silt needs further liquidation evaluation.
 So the maximum depth of calculation in this paper is set to 20 m.

31. .
 The critical value of the SPT (Ncr) for liquidation evaluation can be calculated using the
following equation:
 where ds = is the penetration depth (m), dw = is the depth of the groundwater level
(m), ρc= is the percentage of clay (%), and β = is the regulation factor.
 If the measured value of the standard penetration test (N) is greater than Ncr, then no
liquefaction occurs; liquidation occurs if N is less than Ncr.

32. ADVANTAGES
 Relatively quick and simple to perform.
 Provides useful index of relative strength and compressibility of the soil.
 Able to penetrate dense layers, gravel, and fill.
 Numerous case histories of soil liquefaction during past earthquakes are available
with SPT N-values. The method based on this history can reflect actual soil
behaviour during earthquakes, which cannot be simulated in the laboratory.

33.  The SPT is an in situ test that reflects soil density, soil fabric, stress and strain
history effects, and horizontal effective stress, all of which are known to influence
the liquefaction resistance but are difficult to obtain with undisturbed samples.
 The SPT equipment is rugged, and the test can be performed in a wide range of
soil conditions.
 There are numerous correlations for predicting engineering properties with a good
degree of confidence.

34. DISADVANTAGES
 The SPT does not typically provide continuous data, therefore important data such
as weak seams may be missed.
 Limited applicability to cohesive soils, gravels, cobbles boulders.
 Somewhat slower than other sample methods due to sample retrieval.
 In addition to overburden pressure and relative density the SPT N-value is also a
function of soil type, particle size, and age and stress history of the deposit.
 Samples that are obtained from the SPT are disturbed.
 The greatest disadvantage to SPTs is the lack of reproducibility of the test results.

35.  Due to considerable differences in apparatus and procedure, significant variability
of measured penetration resistance can occur. The basic problems to consider are
change in effective stress at the bottom of the borehole, dynamic energy reaching
the sampler, sampler design, interval of impact, penetration resistance count.
 Drilling disturbance, mechanical variability, and operator variability all can cause
a significant variation in test results.
 Another disadvantage to SPTs is that progress is slower than other in place tests
because of incremental drilling, testing, and sample retrieval, and SPTs may be
more expensive than other in place tests.

36. THANK YOU