Theory and Practice

1. Approach to Standard Penetration Test
To find Bearing Capacity of Soils
Presented By : Amardeep Singh

2. STANDRAD PENETRATION TEST
Standard penetration test conducted by
means of the split spoon, furnishes data
about resistance of the soils to
penetration which can be used to
evaluate standard strength data, such as
N values (number of blows per 30 cm of
penetration using standard split spoon) of
the soil. Methods of calculation of bearing
capacity of soils based on N values are
covered in IS : 6403-1981*.

3. Requirement
• To find the bearing capacity we need c and Φ phi values of the soil
SPT is a measure to obtain the undisturbed soil samples from varying
depths and thus we can find cohesion (c) and angle of friction (Φ) by
Triaxial method.
• However there are also other methods for finding bearing capacity of
the soils e.g.
• Plate load test – cannot be done at varying depths IS 1888
• Vane Shear, Cone penetrometer etc. IS 4968
• RQD – for rocky strata IS 12070
• Each method has its limitations also

4. Apparatus for SPT
• Tripod Stand
• Standard Split Spoon Sampler conforming to IS : 9640-1980 –
consisting of three parts :
• Driving shoe, about 75 mm long
• Steel tube 450 mm long which can split in two parts having inner dia as 38
mm and outer dia of 50 mm
• Coupling at the top of tube about 150 mm long
• Guide pipe
• Drill Rods
• Drop hammer weights 63.5 kg and Driving head of 1.5 kg

5. Drilling Equipment
• The equipment used shall provide a clean borehole, 100 to 150 mm in
diameter, for insertion of the sampler to ensure that the penetration
test is performed on undisturbed soil and shall permit driving of the
split spoon sampler to obtain penetration record ( N value ) and the
soil sample min 10 cm in length
Drive Weight Assembly
• The drive weight assembly shall consisting of a driving head and a
63.5 kg dead weight with 75 cm free fall. It shall be ensured that the
energy of the falling weight is not reduced by friction between the
drive weight and the guides or between rope and winch drum.

6. Procedure of SPT
• The bore hole is drilled to 1.5 mtr depth;
• The drilling tools are removed and sampler is lowered to bottom of hole;
• Three markings @150 mm interval are made on sampler rod from ground
level;
• The sampler is driven into the soil by hammering through 63.5 kg weight
from 75 cm height on 1.5 kg driving head fixed on sampler rod;
• The no of blows required to penetrate each 150 mm are counted. The first
150 mm penetration blow nos are neglected and blows of last two
penetrations of 300mm are recorded as N value at observed depth.
• Like wise at every 1.5 mtr depth the N values are recorded.
• Disturbed samples are also taken where soil strata changes.
• Depth of encountered water table is also recorded.

7. Obtaining the Samples
• Tests shall be made at every
change in stratum or at
intervals of not more than
1.5 m whichever is less. Tests
may be made at lesser
intervals if specified or
considered necessary. The
intervals be increased to 3 m
if in between vane shear test
is performed.

8. Corrections
The N values observed during testing are not utilized directly for overburden
pressure these values are corrected to account of
• Due to Overburden-The N value for cohesion less soil shall be corrected for
overburden as
Nc = Nr*0.77*log10(2000/σ), where Nc is corrected N value, Nr is recorded N value
and σ is effective overburden pressure
Or as per Fig 1 of IS 2131
• Due to Dilatancy - The N value obtained shall be corrected for dilatancy if the
stratum consists of fine sand and silt below water table
for values of N greater than 15, as under
Nc = 15+0.5(Nr – 15 ), where Nr is recorded N value
If Nr is less than 15 then Nr=Nc

9. CALCULATION FOR CORRECTED SPT (N) as per IS 2131
DEPTH
OF
SAMPLE
BULK DENSITY
(T/CUM) SUB
DENSITY
OVERBURDEN
PRESSURE
(T/SQM)
OVERBURDEN CORRECTION
FACTOR
OBSERVED SPT
N VALUE
CORRECTED SPT(N')
VALUE (FOR
OVERBURDEN)
DILATENCY
CORRECTION(N'') for N'
more than 15
FINAL SPT VALUE
AFTER DILATENCY
CORRECTION (N'')
ROUNDED
UP
DEPTH Y(T/CUM)
1.5 0.7 1.97 2.955 1.36 6 8.16 8.16 8
3 5.91 1.18 8 9.44 9.44 9
4.5 8.865 1.03 9 9.27 9.27 9
6 11.82 0.95 10 9.5 9.5 10
7.5 7.0-25.0 1.9 14.775 0.88 15 13.2 13.20 13
9 17.1 0.82 17 13.94 13.94 14
10.5 19.95 0.77 18 13.86 13.86 14
12 22.8 0.73 18 13.14 13.14 13
13.5 25.65 0.7 20 14 14 14
15 28.5 0.67 23 15.41 15.41 15
16.5 31.35 0.64 25 16 16 16
18 34.2 0.61 28 17.08 16.04 16.04 16
19.5 37.05 0.58 29 16.82 15.91 15.91 16
21 39.9 0.55 29 15.95 15.475 15.475 15
22.5 42.75 0.52 26 13.52 13.52 14
24 45.6 0.5 26 13 13 13

10. METHOD OF SBC CALCULATION IS 6403 1981
Perform standard penetration test as per standard procedure given in IS-
2131. Standard penetration test must be done at every 150 cm in vertical
direction.
Decide the depth, width and length of foundation for initial calculation. This
is a trial and error process. In the first attempt, you can never get the exact
size of foundation which will satisfy all of your needs.
Apply necessary corrections to the standard penetration test values.
Calculate the cumulative average value of corrected SPT values from the
base level of foundation to a depth equal to 2 times the width of foundation.
Correlate the above cumulative average SPT value with the fig given below to
find out the corresponding angle of shearing resistance (ϕ), however (ϕ) can
also be determined in triaxial tests.

12. • Calculate effective surcharge at the base level of foundation by
multiplying the effective unit weight of soil with the depth of the
foundation i.e.
q = ϒ*Df
Where,
q = Effective surcharge at the base level of foundation, in kgf/cm2
ϒ = Unit weight of soil, in kgf/cm3
Df = Depth of foundation, in cm
• For the angle of shearing resistance value (ϕ) as calculated, find out
the corresponding values of bearing capacity factors (i.e. Nq & Nϒ)
from the table given below. For the intermediate values of ‘ϕ’, make
linear interpolation.

14. Calculate shape factors (i.e. sq & sϒ) using formula given below.
Shape Factors
Where,
B = Width of foundation, in cm
L = Length of foundation, in cm
Calculate depth factors (i.e. dq & dϒ) using following formula.
dq=dϒ=1 (for ϕ < 100)
dq = dϒ = 1+0.1(Df/B)(Nϕ)1/2 (for ϕ>100)
Nϕ is calculated using following formula
Nϕ = tan2[(π/4)+(ϕ/2)]

15. • Calculate inclination factors (i.e. iq & iϒ) using the formula given
below

16. • Where,
α = Inclination of the load to the vertical in degrees
ϕ = Angles of shearing resistance in degrees
Calculate the correction factor for location of water table using the
following formula
W’ = 0.5+0.5[Dw/(Df+B)]
Where,
W’ = Correction factor for location of water table
Dw = Depth of water table, in cm
Df = Depth of foundation, in cm

17. • Using the equation given below calculate the net ultimate bearing capacity.
• Where,
qd = Net ultimate bearing capacity of foundation, kgf/cm2
q = Effective surcharge at base level of foundation, in kgf/cm2
Nq & Nϒ = Bearing capacity factors
sq & sϒ = Shape factors
dq & dϒ = Depth factors
iq & iϒ = Inclination factors
W’ = Correction factor for location of water table
B = Width of foundation, in cm
ϒ = Bulk unit weight of foundation soil, in kgf/cm3.