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The standard penetration test is an in-situ test used to determine surface information on land and offshore. It is mainly applicable to obtain the bearing capacity of the soil.
This test will calculate the opposition of the soil strata to the penetration experienced by the soil. A penetration-observed correlation is derived between the soil properties and the penetration opposition.
This test is majorly helpful for calculating the relative density and the angle of shearing resistance of cohesionless soils. It can also be employed to specify the unconfined compressive strength of cohesive soils.
To obtain the bearing capacity of the soil
2. Apparatus Required
Tools required to conduct SPT are as follows:
a. Standard Split Spoon Sampler
b. Drop Hammer weighing 63.5 kg
c. Guiding Rod Drilling Rig
d. Driving Head (anvil).
f. Extension Rods
3. Procedure For Standard Penetration Test
This test is achieved in a borehole by the use of a standard split spoon sampler. Once the drilling is accomplished to the preferred depth, the drilling tool is terminated and the sampler is positioned inside the borehole.
By use of a drop hammer of 63.5 kg mass dropping through a height of 750 mm at the rate of 30 blows per minute, the sampler is pushed into the soil. This is as per IS -2131:1963. The number of blows of the hammer needed to drive a depth of 150 mm is counted.
Further, it is pushed by 150 mm and the blows are counted. Similarly, the sampler is once again further driven by 150 mm and the number of blows was noted.
The number of blows recorded for the first 150 mm was not taken into consideration.
The number of blows recorded for the last two 150 mm intervals is added to give the standard penetration number (N).
In other words,
N = No. of blows required for 150mm penetration beyond the seating drive of 150 mm.
If the no. of blows for a 150 mm drive surpasses 50, it is taken as rejection and the test is terminated. The standard penetration no. is rectified for dilatancy correction and overburden correction.
Corrections in Standard Penetration Test Before the SPT values are used in empirical correlations and in design charts, the field ‘N’ value has to be corrected as per IS 2131 – 1981.
The corrections are:
a. Dilatancy Correction
b. Overburden Pressure Correction
1. Dilatancy Correction
Silty fine sands and fine sands below the water table produce pore water pressure which is not efficiently scattered. The pore pressure improves the resistance of the soil and hence the penetration number (N).
Terzaghi and Peck (1967) suggest the following correction in the case of silty fine sands when the practical value is N exceeds 15.
The corrected penetration number,
NC = 15 + 0.5 (NR – 15)
NR is the recorded value
NC is the corrected value.
In case NR is less than or equal to 15,
Nc = NR
2. Overburden Pressure Correction
From many studies and research, it is confirmed that the penetration resistance or the value of N relies on the overburden pressure.
If there are two granular soils with the relative density identical, a higher ‘N’ value will be presented by the soil with higher confining pressure. With the rise in the depth of the soil, the confining pressure also rises.
So the value of ‘N’ at shallow depth and larger depths are minimized and overvalued respectively.
Hence, to account for this the value of ‘N’ acquired from the test is corrected to a standard effective overburden pressure.
The corrected value of ‘N’ is,
Nc = CN N
The penetration resistance value, N of the soil is _______________ blow.
q= 3.5 (N-3) [B+0.3/2B]Rw2Rd
q= Allowable net pressure in T/m2
N = Standard penetration number
B = Breadth of footing in meters
Rw2 = Water reduction factor= 0.5[1+Z/B]
Rd = Depth factor = l+D/B C or C= 2.0
Z = Depth of water table below the foundation level
The precautions that need to be carried while performing precautions are as follows:
1. If the test is to be taken out in gravely soils, then the driving shoe is returned by the solid 60º cone.
2. Below the water table, care should be taken to neglect the entry of water through the bottom of the bore due to the loosening of soil happens.
3. Water should be added as necessary to maintain the water table level in the borehole.
4. If the test is taken out in very fine sand or silty sand below the water table the measured “N” value if higher than 15, should be corrected for the improved resistance to excess water pressure set up during driving and incapable to lose quickly.
5. Drilling mud can be helpful to support the sides of the hole.
6. Stop dropping weight, when the sample penetrates less than 25 mm under 50 blows.
6. Advantages of Standard Penetration Test
The advantages of a standard penetration test are:
a. The test is very simple.
b. This test gives representative samples for visual examination, classification tests, and moisture content.
c. True soil behavior is acquired through SPT values.
d. This test allows the penetration of dense layers and fills.
e. The test can be used for different types of soil conditions.
7. Disadvantages of Standard Penetration Test
The limitations of standard penetration tests are:
a. The outcomes will alter due to any mechanical or operator variability or drilling shakes.
b. The use of SPT in gravels, cobbles, and cohesive soils is limited.
c. The samples recovered for testing are disturbed samples.
d. The test outcomes from SPT cannot be reproduced.
e. This test is costly and more time-consuming.
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Read Also: Safe Bearing Capacity of the Soil
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