In Geotechnical Engineering one
of the most preferred field test to carry out Soil Investigations is Standard
Penetration Test commonly known as SPT. In this test, the strength of soil
beneath ground surface is determined in terms of number of blows of hammer
(63.5 kg) required to penetrate the split spoon sampler into the ground by 30
cm at regular intervals of 0.75 m or 1.5 m upto the significant depth. The
apparatus used for SPT is:
- · Tripod Stand
- · Hammer of 63.5 kg
- · Rope
- · Split Spoon Sampler
- · Auger
- · Boring apparatus
- · Knife
- · Air Tight Containers
- · Chalk
- · Measuring Tape
- · A-Rods
The SPT is performed as per IS:2131 in
following steps:
Step 1.
Make a borehole upto 1.5 m
Step 2.
Place the Tripod Stand over the
borehole such that the hammer should be directly above the borehole
Step 3.
Insert the Split Spoon Sampler in
the borehole with the help of attaching A-Rods.
Step 4.
Mark three readings on the top
A-Rod with the help of chalk at intervals
of 15 cm above the ground surface
Step 5.
Allow the 63.5 kg to fall freely
from 75 cm height.
Step 6.
The number of blows of hammer
required for first 15 cm penetration is known as “Seating Drive” and is not
counted.
Step 7.
The number of blows of hammer
required for the next 30 cm penetration is noted and is termed as ‘N’value.
(Note: If
number of blows for any 15 cm penetration exceeds 50 or for 30 cm penetration
exceeds 100 then SPT is stopped and termed as refusal that means hard surface
is encountered and Sampler is not advanced further to protect it from damage)
Step 8.
Take out the sampler by striking
the hammer in upward direction.
Step 9.
From the sampler take out the 20
cm entrapped soil sample carefully from center of tube and preserve it in air
tight container mentioning the depth and borehole from where sample is taken.
Step 10.
Repeat Steps 1 to 9 at regular
interval of 1.5 m below ground surface or 0.75 m if soil strata changes
abruptly and complete the observation table given below.
S.No.
|
Depth (m)
|
‘N’ value
|
Density
(kN/m3)
|
Total Stress ‘σ’ (kN/m2)
|
Effective Stress ‘σ`’ (kN/m2)
|
Overburden Correction ‘Cn’
|
Corrected ‘N’ Value N`
|
Nc after Dilatency Correction
|
·
Density is calculated from Mass
of soil sample in air tight container and determining internal diameter of
sampler tube to know the volume of soil sample.
·
Overburden Correction is
determined by the formula:
Cn=0.77 log10(2000⁄(σ`)where σ` is in kN/m2
Cn=0.77 log10(2000⁄(σ`)where σ` is in kN/m2
·
Corrected N value ‘N`’ = Cn
× N
·
Dilateny Correction is applied
only if following conditions are satisfied:
o
Soil is Cohesionless and fully
saturated
o
N value is more than 15
Step 11.
Once table is completed the
average value of Nc is calculated by taking weighted average i.e. Nav
·
For determining Nav
count number of N values in table
·
Multiply the numbers to each Nc
in decreasing order i.e. Multiply 1 with last Nc, 2 with second last
Nc, 3 with third last Nc and so on.
·
Take weighted average by adding
all the values calculated above and dividing the result with the sum of all the
numbers that are multiplied with each Nc.
Precautions:
Ø Make
sure the A-Rods are vertical while the hammer is impacted.
Ø Do not
advance the sampler in the ground without making a borehole because it can damage
the cutting edge of sampler.
Ø Legs of
tripod stand should be anchored in the ground.
Ø Note
down the readings carefully by observing the markings on the top A rod.
Result:
The results of SPT are reported in the format
provided in Appendix of IS:2131 which includes the Borehole log i.e. drawing of
a borehole showing SPT Nc Values at different levels as well as type
of soil encountered at different levels below ground surface along with the
name of site and borehole at the top.
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