Standard Penetration Test:
Standard penetration tests have been carried in each bore as per IS 2131: 1981. The split spoon sampler holding on the bottom of the borehole is allowed to sink under its weight, then the split spoon sampler is seated 15 cm with the blows of the hammer falling through 750mm height. The driving arrangement consists of a driving head and a 63.50 kg weight. After that, a split spoon sampler is finally driven by 30cm. The number of blows required to drive every 15cm penetration is noted, the first 15cm of drive recognised as seating drive. The total blows needed for the second and third 15cm penetration is termed as a penetration resistance, N value. The N-values for each borehole is given in bore logs data.
The test stages:
- The first stage, you have to penetrate the sample by 15cm. Then you have to count the number of blows required to penetrate that soil sample sampler into the soil by 15cm.
- Then next stage, you have to penetrate that sampler into the soil again by 15cm.
- The third stage again, you have to penetrate the soil sampler into the soil by 15cm. So, in three stages, a total of 45cm penetration is required. And at each stage, you have to count the number of blows that are needed to penetrate that 15cm.
First 15cm penetration number is generally ignored due to the disturbance exist at the bottom of the drill hole. And, the summation of the next two stages penetration value, which is 30cm penetration is designated at as standard penetration number.
Checks during the test:
- It is to be ensured that the energy of the falling hammer is not decreased by friction between the hammer and the guides or between the ropes.
- The rod to which the sampler is connected for driving is straight, tightly joined and correctly aligned.
Conditions of refusal:
- If there is fifty blows are required for any 15cm increment, then it is called the refusal.
- If there is a hundred blows are obtained for required 30cm penetration, then it is called the refusal.
- Ten successive blows produce no advance. And, if there are any three conditions observed in the field, then we can say this is the refusal of the test.
- Hammer efficiency correction(N’70)
- Overburden correction(CN)
- Correction for hammer efficiency(n1)
- Correction for drill rod length(n2)
- Correction for the sampler(n3)
- Correction for borehole diameter(n4)
Hammer efficiency correction:
The SPT is standardised into some energy ratio. The energy ratio, which is expressed in percentage, the ratio between the actual hammer energy to sampler and input energy. Ultimately the input energy will be the weight of the hammer multiply by the height of free fall. So, if we know the weight of the hammer and the height of free fall, then we can determine the input energy of that particular hammer. Actually, in the field, this input energy may not be transferred to driven that soil sample into the test condition. So, we have to apply some correction considering the standard energy ratio as 70. Now in the field, we generally express the N value as N’70 after applying all the above corrections.
N’70= CN x N x n1x n2 x n3 x n4
Overburden pressure plays a vital role when determining the SPT for the granular type of soil. The effective overburden pressure is less near the ground surface. And, when we go into the deeper strata, then the effective overburden pressure is very high. That’s why there is some correction due to this less and more effective overburden pressure in different soil strata. The CN is the correction due to effective overburden pressure.
Correction factor n1 for hammer efficiency:
It represents the ratio of Er and Erb; where Er is the energy ratio, and Erb is the standard energy ratio. Now depending upon the type of hammer, this Er value varies. Generally, there are two types of hammers used for SPT. One is a safety hammer, and the other one is the donut hammer. For donut hammer, Er value varies from 45 to 67. For safety hammer, this Er value varies 70 to 80 for the rope pulley or cat head system and 80 to 100 for a trip or automatic.
Suppose if energy ratio at any condition is 80 and the standard energy ratio value is 70, then the efficiency, correction due to hammer is 80 divided by 70; that is 1.15. So, in this fashion, we can correct this N value at any energy ratio condition depending upon the type of hammer we are using.
Correction factor n2 for drill rod length:
- Rod length greater than 10m, the correction factor is 1
- Rod length 6-10m, the correction factor 0.95
- Rod length 4-6m, the correction factor 0.85
- Rod length 0-4m, the correction factor 0.75
Correction factor n3 for the sampler:
- Without a liner, it is 1.0
- Within liner for dense sand and clay, it is 0.80
- Within liner for loose sand, it is 0.90
Correction factor n4 for borehole diameter :
- Hole diameter 60-120mm, the correction factor 1.0
- Hole diameter 150mm, the correction factor 1.05
- Hole diameter 200mm, the correction factor 1.15
- The correction factor is equal to 1.00 for all diameters for hollow-stem augers, where the SPT is taken through the stem
Laboratory tests for collected soil sample:
The laboratory tests on soil samples have been started shortly after receiving the sample in the laboratory. Following laboratory tests are conducted to determine the physical and engineering properties of undisturbed and disturbed soil samples.
1. Dry density and moisture content (IS 2720 part-2 & 29)
2. Particle size analysis- (IS 2720 part-4 1985)
3. Atterberg’s limit – (IS 2720 part-51985)
4. Specific gravity – (IS 2720 part -3/sec2 1980)
5. Shear test – (IS 2720 part-11 1986)
6. Consolidation test-(IS 2720 part-15 1986)
7. Test for free swell and Swell pressure -(IS 2720 part-40 1977, part-41 1977)
All the tests are carried out as per the respective Indian code.